linux-kernel-module-cheat/README.adoc

= Linux Kernel Module Cheat
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The perfect emulation setup to study and develop the <<linux-kernel>> v5.0, kernel modules, <<qemu-buildroot-setup,QEMU>>, <<gem5-buildroot-setup,gem5>> and x86_64, ARMv7 and ARMv8 <<userland-assembly,userland>> and <<baremetal-setup,baremetal>> assembly. <<gdb>> and <<kgdb>> just work. Powered by <<about-the-qemu-buildroot-setup,Buildroot>> and <<about-the-baremetal-setup,crosstool-NG>>.  Highly automated. Thoroughly documented. Automated <<test-this-repo,tests>>. "Tested" in an Ubuntu 18.04 host.

TL;DR: <<qemu-buildroot-setup-getting-started>>

toc::[]

== Getting started

Each child section describes a possible different setup for this repo.

If you don't know which one to go for, start with <<qemu-buildroot-setup-getting-started>>.

Design goals of this project are documented at: <<design-goals>>.

=== QEMU Buildroot setup

==== QEMU Buildroot setup getting started

This setup has been mostly tested on Ubuntu. For other host operating systems see: <<supported-hosts>>. For greater stability, consider using the <<release-procedure,latest release>> instead of master: https://github.com/cirosantilli/linux-kernel-module-cheat/releases

Reserve 12Gb of disk and run:

....
git clone https://github.com/cirosantilli/linux-kernel-module-cheat
cd linux-kernel-module-cheat
./build --download-dependencies qemu-buildroot
./run
....

You don't need to clone recursively even though we have `.git` submodules: `download-dependencies` fetches just the submodules that you need for this build to save time.

If something goes wrong, see: <<common-build-issues>> and use our issue tracker: https://github.com/cirosantilli/linux-kernel-module-cheat/issues

The initial build will take a while (30 minutes to 2 hours) to clone and build, see <<benchmark-builds>> for more details.

If you don't want to wait, you could also try the following faster but much more limited methods:

* <<prebuilt>>
* <<host>>

but you will soon find that they are simply not enough if you anywhere near serious about systems programming.

After `./run`, QEMU opens up leaving you in the <<lkmc_home,`/lkmc/` directory>>, and you can start playing with the kernel modules inside the simulated system:

....
insmod hello.ko
insmod hello2.ko
rmmod hello
rmmod hello2
....

This should print to the screen:

....
hello init
hello2 init
hello cleanup
hello2 cleanup
....

which are `printk` messages from `init` and `cleanup` methods of those modules.

Sources:

* link:kernel_modules/hello.c[]
* link:kernel_modules/hello2.c[]

Quit QEMU with:

....
Ctrl-A X
....

See also: <<quit-qemu-from-text-mode>>.

All available modules can be found in the link:kernel_modules[] directory.

It is super easy to build for different <<cpu-architecture,CPU architectures>>, just use the `--arch` option:

....
./build --arch aarch64 --download-dependencies qemu-buildroot
./run --arch aarch64
....

To avoid typing `--arch aarch64` many times, you can set the default arch as explained at: <<default-command-line-arguments>>

I now urge you to read the following sections which contain widely applicable information:

* <<run-command-after-boot>>
* <<clean-the-build>>
* <<build-the-documentation>>
* Linux kernel
** <<printk>>
** <<kernel-command-line-parameters>>

Once you use <<gdb>> and <<tmux>>, your terminal will look a bit like this:

....
[    1.451857] input: AT Translated Set 2 keyboard as /devices/platform/i8042/s1│loading @0xffffffffc0000000: ../kernel_modules-1.0//timer.ko
[    1.454310] ledtrig-cpu: registered to indicate activity on CPUs             │(gdb) b lkmc_timer_callback
[    1.455621] usbcore: registered new interface driver usbhid                  │Breakpoint 1 at 0xffffffffc0000000: file /home/ciro/bak/git/linux-kernel-module
[    1.455811] usbhid: USB HID core driver                                      │-cheat/out/x86_64/buildroot/build/kernel_modules-1.0/./timer.c, line 28.
[    1.462044] NET: Registered protocol family 10                               │(gdb) c
[    1.467911] Segment Routing with IPv6                                        │Continuing.
[    1.468407] sit: IPv6, IPv4 and MPLS over IPv4 tunneling driver              │
[    1.470859] NET: Registered protocol family 17                               │Breakpoint 1, lkmc_timer_callback (data=0xffffffffc0002000 <mytimer>)
[    1.472017] 9pnet: Installing 9P2000 support                                 │    at /linux-kernel-module-cheat//out/x86_64/buildroot/build/
[    1.475461] sched_clock: Marking stable (1473574872, 0)->(1554017593, -80442)│kernel_modules-1.0/./timer.c:28
[    1.479419] ALSA device list:                                                │28      {
[    1.479567]   No soundcards found.                                           │(gdb) c
[    1.619187] ata2.00: ATAPI: QEMU DVD-ROM, 2.5+, max UDMA/100                 │Continuing.
[    1.622954] ata2.00: configured for MWDMA2                                   │
[    1.644048] scsi 1:0:0:0: CD-ROM            QEMU     QEMU DVD-ROM     2.5+ P5│Breakpoint 1, lkmc_timer_callback (data=0xffffffffc0002000 <mytimer>)
[    1.741966] tsc: Refined TSC clocksource calibration: 2904.010 MHz           │    at /linux-kernel-module-cheat//out/x86_64/buildroot/build/
[    1.742796] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x29dc0f4s│kernel_modules-1.0/./timer.c:28
[    1.743648] clocksource: Switched to clocksource tsc                         │28      {
[    2.072945] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8043│(gdb) bt
[    2.078641] EXT4-fs (vda): couldn't mount as ext3 due to feature incompatibis│#0  lkmc_timer_callback (data=0xffffffffc0002000 <mytimer>)
[    2.080350] EXT4-fs (vda): mounting ext2 file system using the ext4 subsystem│    at /linux-kernel-module-cheat//out/x86_64/buildroot/build/
[    2.088978] EXT4-fs (vda): mounted filesystem without journal. Opts: (null)  │kernel_modules-1.0/./timer.c:28
[    2.089872] VFS: Mounted root (ext2 filesystem) readonly on device 254:0.    │#1  0xffffffff810ab494 in call_timer_fn (timer=0xffffffffc0002000 <mytimer>,
[    2.097168] devtmpfs: mounted                                                │    fn=0xffffffffc0000000 <lkmc_timer_callback>) at kernel/time/timer.c:1326
[    2.126472] Freeing unused kernel memory: 1264K                              │#2  0xffffffff810ab71f in expire_timers (head=<optimized out>,
[    2.126706] Write protecting the kernel read-only data: 16384k               │    base=<optimized out>) at kernel/time/timer.c:1363
[    2.129388] Freeing unused kernel memory: 2024K                              │#3  __run_timers (base=<optimized out>) at kernel/time/timer.c:1666
[    2.139370] Freeing unused kernel memory: 1284K                              │#4  run_timer_softirq (h=<optimized out>) at kernel/time/timer.c:1692
[    2.246231] EXT4-fs (vda): warning: mounting unchecked fs, running e2fsck isd│#5  0xffffffff81a000cc in __do_softirq () at kernel/softirq.c:285
[    2.259574] EXT4-fs (vda): re-mounted. Opts: block_validity,barrier,user_xatr│#6  0xffffffff810577cc in invoke_softirq () at kernel/softirq.c:365
hello S98                                                                       │#7  irq_exit () at kernel/softirq.c:405
                                                                                │#8  0xffffffff818021ba in exiting_irq () at ./arch/x86/include/asm/apic.h:541
Apr 15 23:59:23 login[49]: root login on 'console'                              │#9  smp_apic_timer_interrupt (regs=<optimized out>)
hello /root/.profile                                                            │    at arch/x86/kernel/apic/apic.c:1052
# insmod /timer.ko                                                              │#10 0xffffffff8180190f in apic_timer_interrupt ()
[    6.791945] timer: loading out-of-tree module taints kernel.                 │    at arch/x86/entry/entry_64.S:857
# [    7.821621] 4294894248                                                     │#11 0xffffffff82003df8 in init_thread_union ()
[    8.851385] 4294894504                                                       │#12 0x0000000000000000 in ?? ()
                                                                                │(gdb)
....

==== How to hack stuff

Besides a seamless <<qemu-buildroot-setup-getting-started,initial build>>, this project also aims to make it effortless to modify and rebuild several major components of the system, to serve as an awesome development setup.

===== Your first Linux kernel hack

Let's hack up the <<linux-kernel-entry-point, Linux kernel entry point>>, which is an easy place to start.

Open the file:

....
vim submodules/linux/init/main.c
....

and find the `start_kernel` function, then add there a:

....
pr_info("I'VE HACKED THE LINUX KERNEL!!!");
....

Then rebuild the Linux kernel, quit QEMU and reboot the modified kernel:

....
./build-linux
./run
....

and, surely enough, your message has appeared at the beginning of the boot:

....
<6>[    0.000000] I'VE HACKED THE LINUX KERNEL!!!
....

So you are now officially a Linux kernel hacker, way to go!

We could have used just link:build[] to rebuild the kernel as in the <<qemu-buildroot-setup-getting-started,initial build>> instead of link:build-linux[], but building just the required individual components is preferred during development:

* saves a few seconds from parsing Make scripts and reading timestamps
* makes it easier to understand what is being done in more detail
* allows passing more specific options to customize the build

The link:build[] script is just a lightweight wrapper that calls the smaller build scripts, and you can see what `./build` does with:

....
./build --dry-run
....

When you reach difficulties, QEMU makes it possible to easily GDB step debug the Linux kernel source code, see: <<gdb>>.

===== Your first kernel module hack

Edit link:kernel_modules/hello.c[] to contain:

....
pr_info("hello init hacked\n");
....

and rebuild with:

....
./build-modules
....

Now there are two ways to test it out: the fast way, and the safe way.

The fast way is, without quitting or rebooting QEMU, just directly re-insert the module with:

....
insmod /mnt/9p/out_rootfs_overlay/lkmc/hello.ko
....

and the new `pr_info` message should now show on the terminal at the end of the boot.

This works because we have a <<9p>> mount there setup by default, which mounts the host directory that contains the build outputs on the guest:

....
ls "$(./getvar out_rootfs_overlay_dir)"
....

The fast method is slightly risky because your previously insmodded buggy kernel module attempt might have corrupted the kernel memory, which could affect future runs.

Such failures are however unlikely, and you should be fine if you don't see anything weird happening.

The safe way, is to fist <<rebuild-buildroot-while-running,quit QEMU>>, rebuild the modules, put them in the root filesystem, and then reboot:

....
./build-modules
./build-buildroot
./run --eval-after 'insmod hello.ko'
....

`./build-buildroot` is required after `./build-modules` because it re-generates the root filesystem with the modules that we compiled at `./build-modules`.

You can see that `./build` does that as well, by running:

....
./build --dry-run
....

`--eval-after` is optional: you could just type `insmod hello.ko` in the terminal, but this makes it run automatically at the end of boot, and then drops you into a shell.

If the guest and host are the same arch, typically x86_64, you can speed up boot further with <<kvm>>:

....
./run --kvm
....

All of this put together makes the safe procedure acceptably fast for regular development as well.

It is also easy to GDB step debug kernel modules with our setup, see: <<gdb-step-debug-kernel-module>>.

===== Your first QEMU hack

Not satisfied with mere software? OK then, let's hack up the QEMU x86 CPU identification:

....
vim submodules/qemu/target/i386/cpu.c
....

and modify:

....
.model_id = "QEMU Virtual CPU version " QEMU_HW_VERSION,
....

to contain:

....
.model_id = "QEMU Virtual CPU version HACKED " QEMU_HW_VERSION,
....

then as usual rebuild and re-run:

.....
./build-qemu
./run --eval-after 'grep "model name" /proc/cpuinfo'
.....

and once again, there is your message: QEMU communicated it to the Linux kernel, which printed it out.

You have now gone from newb to hardware hacker in a mere 15 minutes, your rate of progress is truly astounding!!!

Seriously though, if you want to be a real hardware hacker, it just can't be done with open source tools as of 2018. The root obstacle is that:

* link:https://en.wikipedia.org/wiki/Semiconductor_fabrication_plant[Silicon fabs] don't publish reveal their link:https://en.wikipedia.org/wiki/Design_rule_checking[design rules]
* which implies that there are no decent link:https://en.wikipedia.org/wiki/Standard_cell[standard cell libraries]. See also: https://www.quora.com/Are-there-good-open-source-standard-cell-libraries-to-learn-IC-synthesis-with-EDA-tools/answer/Ciro-Santilli
* which implies that people can't develop open source link:https://en.wikipedia.org/wiki/Electronic_design_automation[EDA tools]
* which implies that you can't get decent link:https://community.cadence.com/cadence_blogs_8/b/di/posts/hls-ppa-is-it-all-you-need-to-know[power, performance and area] estimates

The only thing you can do with open source is purely functional designs with link:https://en.wikipedia.org/wiki/Verilator[Verilator], but you will never know if it can be actually produced and how efficient it can be.

If you really want to develop semiconductors, your only choice is to join an university or a semiconductor company that has the EDA licenses.

See also: <<should-you-waste-your-life-with-systems-programming>>.

While hacking QEMU, you will likely want to GDB step its source. That is trivial since QEMU is just another userland program like any other, but our setup has a shortcut to make it even more convenient, see: <<debug-the-emulator>>.

===== Your first glibc hack

We use <<libc-choice,glibc as our default libc now>>, and it is tracked as an unmodified submodule at link:submodules/glibc[], at the exact same version that Buildroot has it, which can be found at: link:https://github.com/buildroot/buildroot/blob/2018.05/package/glibc/glibc.mk#L13[package/glibc/glibc.mk]. Buildroot 2018.05 applies no patches.

Let's hack up the `puts` function:

....
./build-buildroot -- glibc-reconfigure
....

with the patch:

....
diff --git a/libio/ioputs.c b/libio/ioputs.c
index 706b20b492..23185948f3 100644
--- a/libio/ioputs.c
+++ b/libio/ioputs.c
@@ -38,8 +38,9 @@ _IO_puts (const char *str)
   if ((_IO_vtable_offset (_IO_stdout) != 0
        || _IO_fwide (_IO_stdout, -1) == -1)
       && _IO_sputn (_IO_stdout, str, len) == len
+      && _IO_sputn (_IO_stdout, " hacked", 7) == 7
       && _IO_putc_unlocked ('\n', _IO_stdout) != EOF)
-    result = MIN (INT_MAX, len + 1);
+    result = MIN (INT_MAX, len + 1 + 7);

   _IO_release_lock (_IO_stdout);
   return result;
....

And then:

....
./run --eval-after './c/hello.out'
....

outputs:

....
hello hacked
....

Lol!

We can also test our hacked glibc on <<user-mode-simulation>> with:

....
./run --userland userland/c/hello.c
....

I just noticed that this is actually a good way to develop glibc for other archs.

In this example, we got away without recompiling the userland program because we made a change that did not affect the glibc ABI, see this answer for an introduction to ABI stability: https://stackoverflow.com/questions/2171177/what-is-an-application-binary-interface-abi/54967743#54967743

Note that for arch agnostic features that don't rely on bleeding kernel changes that you host doesn't yet have, you can develop glibc natively as explained at:

* https://stackoverflow.com/questions/10412684/how-to-compile-my-own-glibc-c-standard-library-from-source-and-use-it/52454710#52454710
* https://stackoverflow.com/questions/847179/multiple-glibc-libraries-on-a-single-host/52454603#52454603
* https://stackoverflow.com/questions/2856438/how-can-i-link-to-a-specific-glibc-version/52550158#52550158 more focus on symbol versioning, but no one knows how to do it, so I answered

Tested on a30ed0f047523ff2368d421ee2cce0800682c44e + 1.

===== Your first Binutils hack

Have you ever felt that a single `inc` instruction was not enough? Really? Me too!

So let's hack the <<gnu-gas-assembler>>, which is part of link:https://en.wikipedia.org/wiki/GNU_Binutils[GNU Binutils], to add a new shiny version of `inc` called... `myinc`!

GCC uses GNU GAS as its backend, so we will test out new mnemonic with an <<gcc-inline-assembly>> test program: link:userland/arch/x86_64/binutils_hack.c[], which is just a copy of link:userland/arch/x86_64/asm_hello.c[] but with `myinc` instead of `inc`.

The inline assembly is disabled with an `#ifdef`, so first modify the source to enable that.

Then, try to build userland:

....
./build-userland
....

and watch it fail with:

....
binutils_hack.c:8: Error: no such instruction: `myinc %rax'
....

Now, edit the file

....
vim submodules/binutils-gdb/opcodes/i386-tbl.h
....

and add a copy of the `"inc"` instruction just next to it, but with the new name `"myinc"`:

....
diff --git a/opcodes/i386-tbl.h b/opcodes/i386-tbl.h
index af583ce578..3cc341f303 100644
--- a/opcodes/i386-tbl.h
+++ b/opcodes/i386-tbl.h
@@ -1502,6 +1502,19 @@ const insn_template i386_optab[] =
     { { { 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
 	  0, 0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0,
 	  1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0 } } } },
+  { "myinc", 1, 0xfe, 0x0, 1,
+    { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } },
+    { 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+      0, 1, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0,
+      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+      0, 0, 0, 0, 0, 0 },
+    { { { 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	  0, 0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0,
+	  1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0 } } } },
   { "sub", 2, 0x28, None, 1,
     { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
         0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
....

Finally, rebuild Binutils, userland and test our program with <<user-mode-simulation>>:

....
./build-buildroot -- host-binutils-rebuild
./build-userland --static
./run --static --userland userland/arch/x86_64/binutils_hack.c
....

and we se that `myinc` worked since the assert did not fail!

Tested on b60784d59bee993bf0de5cde6c6380dd69420dda + 1.

===== Your first GCC hack

OK, now time to hack GCC.

For convenience, let's use the <<user-mode-simulation>>.

If we run the program link:userland/c/gcc_hack.c[]:

....
./build-userland --static
./run --static --userland userland/c/gcc_hack.c
....

it produces the normal boring output:

....
i = 2
j = 0
....

So how about we swap `++` and `--` to make things more fun?

Open the file:

....
vim submodules/gcc/gcc/c/c-parser.c
....

and find the function `c_parser_postfix_expression_after_primary`.

In that function, swap `case CPP_PLUS_PLUS` and `case CPP_MINUS_MINUS`:

....
diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index 101afb8e35f..89535d1759a 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -8529,7 +8529,7 @@ c_parser_postfix_expression_after_primary (c_parser *parser,
 		expr.original_type = DECL_BIT_FIELD_TYPE (field);
 	    }
 	  break;
-	case CPP_PLUS_PLUS:
+	case CPP_MINUS_MINUS:
 	  /* Postincrement.  */
 	  start = expr.get_start ();
 	  finish = c_parser_peek_token (parser)->get_finish ();
@@ -8548,7 +8548,7 @@ c_parser_postfix_expression_after_primary (c_parser *parser,
 	  expr.original_code = ERROR_MARK;
 	  expr.original_type = NULL;
 	  break;
-	case CPP_MINUS_MINUS:
+	case CPP_PLUS_PLUS:
 	  /* Postdecrement.  */
 	  start = expr.get_start ();
 	  finish = c_parser_peek_token (parser)->get_finish ();
....

Now rebuild GCC, the program and re-run it:

....
./build-buildroot -- host-gcc-final-rebuild
./build-userland --static
./run --static --userland userland/c/gcc_hack.c
....

and the new ouptut is now:

....
i = 2
j = 0
....

We need to use the ugly `-final` thing because GCC has to packages in Buildroot, `-initial` and `-final`: https://stackoverflow.com/questions/54992977/how-to-select-an-override-srcdir-source-for-gcc-when-building-buildroot No one is able to example precisely with a minimal example why this is required:

* https://stackoverflow.com/questions/39883865/why-multiple-passes-for-building-linux-from-scratch-lfs
* https://stackoverflow.com/questions/27457835/why-do-cross-compilers-have-a-two-stage-compilation

==== About the QEMU Buildroot setup

This is our reference setup, and the best supported one, use it unless you have good reason not to.

It was historically the first one we did, and all sections have been tested with this setup unless explicitly noted.

Read the following sections for further introductory material:

* <<introduction-to-qemu>>
* <<introduction-to-buildroot>>

=== gem5 Buildroot setup

==== About the gem5 Buildroot setup

This setup is like the <<qemu-buildroot-setup>>, but it uses link:http://gem5.org/[gem5] instead of QEMU as a system simulator.

QEMU tries to run as fast as possible and give correct results at the end, but it does not tell us how many CPU cycles it takes to do something, just the number of instructions it ran. This kind of simulation is known as functional simulation.

The number of instructions executed is a very poor estimator of performance because in modern computers, a lot of time is spent waiting for memory requests rather than the instructions themselves.

gem5 on the other hand, can simulate the system in more detail than QEMU, including:

* simplified CPU pipeline
* caches
* DRAM timing

and can therefore be used to estimate system performance, see: <<gem5-run-benchmark>> for an example.

The downside of gem5 much slower than QEMU because of the greater simulation detail.

See <<gem5-vs-qemu>> for a more thorough comparison.

==== gem5 Buildroot setup getting started

For the most part, if you just add the `--emulator gem5` option or `*-gem5` suffix to all commands and everything should magically work.

If you haven't built Buildroot yet for <<qemu-buildroot-setup>>, you can build from the beginning with:

....
./build --download-dependencies gem5-buildroot
./run --emulator gem5
....

If you have already built previously, don't be afraid: gem5 and QEMU use almost the same root filesystem and kernel, so `./build` will be fast.

Remember that the gem5 boot is <<benchmark-linux-kernel-boot,considerably slower>> than QEMU since the simulation is more detailed.

To get a terminal, either open a new shell and run:

....
./gem5-shell
....

You can quit the shell without killing gem5 by typing tilde followed by a period:

....
~.
....

If you are inside <<tmux>>, which I highly recommend, you can both run gem5 stdout and open the guest terminal on a split window with:

....
./run --emulator gem5 --tmux
....

See also: <<tmux-gem5>>.

At the end of boot, it might not be very clear that you have the shell since some <<printk>> messages may appear in front of the prompt like this:

....
# <6>[    1.215329] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x1cd486fa865, max_idle_ns: 440795259574 ns
<6>[    1.215351] clocksource: Switched to clocksource tsc
....

but if you look closely, the `PS1` prompt marker `#` is there already, just hit enter and a clear prompt line will appear.

If you forgot to open the shell and gem5 exit, you can inspect the terminal output post-mortem at:

....
less "$(./getvar --emulator gem5 m5out_dir)/system.pc.com_1.device"
....

More gem5 information is present at: <<gem5>>

Good next steps are:

* <<gem5-run-benchmark>>
* <<m5out-directory>>
* <<m5ops>>

[[docker]]
=== Docker host setup

This repository has been tested inside clean link:https://en.wikipedia.org/wiki/Docker_(software)[Docker] containers.

This is a good option if you are on a Linux host, but the native setup failed due to your weird host distribution, and you have better things to do with your life than to debug it. See also: <<supported-hosts>>.

For example, to do a <<qemu-buildroot-setup>> inside Docker, run:

....
sudo apt-get install docker
./run-docker create && \
./run-docker sh -- ./build --download-dependencies qemu-buildroot
./run-docker sh
....

You are now left inside a shell in the Docker! From there, just run as usual:

....
./run
....

The host git top level directory is mounted inside the guest with a link:https://stackoverflow.com/questions/23439126/how-to-mount-a-host-directory-in-a-docker-container[Docker volume], which means for example that you can use your host's GUI text editor directly on the files. Just don't forget that if you nuke that directory on the guest, then it gets nuked on the host as well!

Command breakdown:

* `./run-docker create`: create the image and container.
+
Needed only the very first time you use Docker, or if you run `./run-docker DESTROY` to restart for scratch, or save some disk space.
+
The image and container name is `lkmc`. The container shows under:
+
....
docker ps -a
....
+
and the image shows under:
+
....
docker images
....
* `./run-docker sh`: open a shell on the container.
+
If it has not been started previously, start it. This can also be done explicitly with:
+
....
./run-docker start
....
+
Quit the shell as usual with `Ctrl-D`
+
This can be called multiple times from different host terminals to open multiple shells.
* `./run-docker stop`: stop the container.
+
This might save a bit of CPU and RAM once you stop working on this project, but it should not be a lot.
* `./run-docker DESTROY`: delete the container and image.
+
This doesn't really clean the build, since we mount the guest's working directory on the host git top-level, so you basically just got rid of the `apt-get` installs.
+
To actually delete the Docker build, run on host:
+
....
# sudo rm -rf out.docker
....

To use <<gdb>> from inside Docker, you need a second shell inside the container. You can either do that from another shell with:

....
./run-docker sh
....

or even better, by starting a <<tmux>> session inside the container. We install `tmux` by default in the container.

You can also start a second shell and run a command in it at the same time with:

....
./run-docker sh -- ./run-gdb start_kernel
....

To use <<qemu-graphic-mode>> from Docker, run:

....
./run --graphic --vnc
....

and then on host:

....
sudo apt-get install vinagre
./vnc
....

TODO make files created inside Docker be owned by the current user in host instead of `root`:

* https://stackoverflow.com/questions/33681396/how-do-i-write-to-a-volume-container-as-non-root-in-docker
* https://stackoverflow.com/questions/23544282/what-is-the-best-way-to-manage-permissions-for-docker-shared-volumes
* https://stackoverflow.com/questions/31779802/shared-volume-file-permissions-ownership-docker

[[prebuilt]]
=== Prebuilt setup

==== About the prebuilt setup

This setup uses prebuilt binaries that we upload to GitHub from time to time.

We don't currently provide a full prebuilt because it would be too big to host freely, notably because of the cross toolchain.

Our prebuilts currently include:

* <<qemu-buildroot-setup>> binaries
** Linux kernel
** root filesystem
* <<baremetal-setup>> binaries for QEMU

For more details, see our our <<release,release procedure>>.

Advantage of this setup: saves time and disk space on the initial install, which is expensive in largely due to building the toolchain.

The limitations are severe however:

* can't <<gdb,GDB step debug the kernel>>, since the source and cross toolchain with GDB are not available. Buildroot cannot easily use a host toolchain: <<prebuilt-toolchain>>.
+
Maybe we could work around this by just downloading the kernel source somehow, and using a host prebuilt GDB, but we felt that it would be too messy and unreliable.
* you won't get the latest version of this repository. Our <<travis>> attempt to automate builds failed, and storing a release for every commit would likely make GitHub mad at us anyways.
* <<gem5>> is not currently supported. The major blocking point is how to avoid distributing the kernel images twice: once for gem5 which uses `vmlinux`, and once for QEMU which uses `arch/*` images, see also: <<vmlinux-vs-bzimage-vs-zimage-vs-image>>.

This setup might be good enough for those developing simulators, as that requires less image modification. But once again, if you are serious about this, why not just let your computer build the <<qemu-buildroot-setup,full featured setup>> while you take a coffee or a nap? :-)

==== Prebuilt setup getting started

Checkout to the latest tag and use the Ubuntu packaged QEMU to boot Linux:

....
sudo apt-get install qemu-system-x86
git clone https://github.com/cirosantilli/linux-kernel-module-cheat
cd linux-kernel-module-cheat
git checkout "$(git rev-list --tags --max-count=1)"
./release-download-latest
unzip lkmc-*.zip
./run --qemu-which host
....

You have to checkout to the latest tag to ensure that the scripts match the release format: https://stackoverflow.com/questions/1404796/how-to-get-the-latest-tag-name-in-current-branch-in-git

This is known not to work for aarch64 on an Ubuntu 16.04 host with QEMU 2.5.0, presumably because QEMU is too old, the terminal does not show any output. I haven't investigated why.

Or to run a baremetal example instead:

....
./run \
  --arch aarch64 \
  --baremetal baremetal/hello.c \
  --qemu-which host \
;
....

Be saner and use our custom built QEMU instead:

....
./build --download-dependencies qemu
./run
....

This also allows you to <<your-first-qemu-hack,modify QEMU>> if you're into that sort of thing.

To build the kernel modules as in <<your-first-kernel-module-hack>> do:

....
git submodule update --depth 1 --init --recursive "$(./getvar linux_source_dir)"
./build-linux --no-modules-install -- modules_prepare
./build-modules --gcc-which host
./run
....

TODO: for now the only way to test those modules out without <<qemu-buildroot-setup-getting-started,building Buildroot>> is with 9p, since we currently rely on Buildroot to manipulate the root filesystem.

Command explanation:

* `modules_prepare` does the minimal build procedure required on the kernel for us to be able to compile the kernel modules, and is way faster than doing a full kernel build. A full kernel build would also work however.
* `--gcc-which host` selects your host Ubuntu packaged GCC, since you don't have the Buildroot toolchain
* `--no-modules-install` is required otherwise the `make modules_install` target we run by default fails, since the kernel wasn't built

To modify the Linux kernel, build and use it as usual:

....
git submodule update --depth 1 --init --recursive "$(./getvar linux_source_dir)"
./build-linux
./run
....

////
For gem5, do:

....
git submodule update --init --depth 1 "$(./getvar linux_source_dir)"
sudo apt-get install qemu-utils
./build-gem5
./run --emulator gem5 --qemu-which host
....

`qemu-utils` is required because we currently distribute `.qcow2` files which <<gem5-qcow2,gem5 can't handle>>, so we need `qemu-img` to extract them first.

The Linux kernel is required for `extract-vmlinux` to convert the compressed kernel image which QEMU understands into the raw vmlinux that gem5 understands: https://superuser.com/questions/298826/how-do-i-uncompress-vmlinuz-to-vmlinux
////

////
[[ubuntu]]
=== Ubuntu guest setup

==== About the Ubuntu guest setup

This setup is similar to <<prebuilt>>, but instead of using Buildroot for the root filesystem, it downloads an Ubuntu image with Docker, and uses that as the root filesystem.

The rationale for choice of Ubuntu as a second distribution in addition to Buildroot can be found at: <<linux-distro-choice>>

Advantages over Buildroot:

* saves build time
* you get to play with a huge selection of Debian packages out of the box
* more representative of most non-embedded production systems than BusyBox

Disadvantages:

* less visibility: https://askubuntu.com/questions/82302/how-to-compile-ubuntu-from-source-code The fact that that question has no answer makes me cringe
* less compatibility, e.g. no one knows what the officially supported cross compilers are: https://askubuntu.com/questions/1046294/what-are-the-officially-supported-cross-compilers-for-ubuntu-server-alternative

Docker is used here just as an image download provider since it has a wide variety of images. Why we don't just download the regular Ubuntu disk image:

* that image is not ready to boot, but rather goes into an interactive installer: https://askubuntu.com/questions/884534/how-to-run-ubuntu-16-04-desktop-on-qemu/1046792#1046792
* the default Ubuntu image has a large collection of software, and is large. The docker version is much more minimal.

One alternative would be to use link:https://wiki.ubuntu.com/Base[Ubuntu base] which can be downloaded from: http://cdimage.ubuntu.com/ubuntu-base That provides a `.tgz` and comes very close to what we obtain with Docker, but without the need for `sudo`.

==== Ubuntu guest setup getting started

TODO

....
sudo ./build-docker
./run --docker
....

`sudo` is required for Docker operations: https://askubuntu.com/questions/477551/how-can-i-use-docker-without-sudo
////

[[host]]
=== Host kernel module setup

**THIS IS DANGEROUS (AND FUN), YOU HAVE BEEN WARNED**

This method runs the kernel modules directly on your host computer without a VM, and saves you the compilation time and disk usage of the virtual machine method.

It has however severe limitations:

* can't control which kernel version and build options to use. So some of the modules will likely not compile because of kernel API changes, since https://stackoverflow.com/questions/37098482/how-to-build-a-linux-kernel-module-so-that-it-is-compatible-with-all-kernel-rele/45429681#45429681[the Linux kernel does not have a stable kernel module API].
* bugs can easily break you system. E.g.:
** segfaults can trivially lead to a kernel crash, and require a reboot
** your disk could get erased. Yes, this can also happen with `sudo` from userland. But you should not use `sudo` when developing newbie programs. And for the kernel you don't have the choice not to use `sudo`.
** even more subtle system corruption such as https://unix.stackexchange.com/questions/78858/cannot-remove-or-reinsert-kernel-module-after-error-while-inserting-it-without-r[not being able to rmmod]
* can't control which hardware is used, notably the CPU architecture
* can't step debug it with <<gdb,GDB>> easily. The alternatives are link:https://en.wikipedia.org/wiki/JTAG[JTAG] or <<kgdb>>, but those are less reliable, and require extra hardware.

Still interested?

....
./build-modules --gcc-which host --host
....

Compilation will likely fail for some modules because of kernel or toolchain differences that we can't control on the host.

The best workaround is to compile just your modules with:

....
./build-modules --gcc-which host --host -- hello hello2
....

which is equivalent to:

....
./build-modules \
  --gcc-which host \
  --host \
  -- \
  kernel_modules/hello.c \
  kernel_modules/hello2.c \
;
....

Or just remove the `.c` extension from the failing files and try again:

....
cd "$(./getvar kernel_modules_source_dir)"
mv broken.c broken.c~
....

Once you manage to compile, and have come to terms with the fact that this may blow up your host, try it out with:

....
cd "$(./getvar kernel_modules_build_host_subdir)"
sudo insmod hello.ko

# Our module is there.
sudo lsmod | grep hello

# Last message should be: hello init
dmesg -T

sudo rmmod hello

# Last message should be: hello exit
dmesg -T

# Not present anymore
sudo lsmod | grep hello
....

==== Hello host

Minimal host build system example:

....
cd hello_host_kernel_module
make
sudo insmod hello.ko
dmesg
sudo rmmod hello.ko
dmesg
....

=== Userland setup

==== About the userland setup

In order to test the kernel and emulators, userland content in the form of executables and scripts is of course required, and we store it mostly under:

* link:userland/[]
* <<rootfs_overlay>>
* <<add-new-buildroot-packages>>

When we started this repository, it only contained content that interacted very closely with the kernel, or that had required performance analysis.

However, we soon started to notice that this had an increasing overlap with other userland test repositories: we were duplicating build and test infrastructure and even some examples.

Therefore, we decided to consolidate other userland tutorials that we had scattered around into this repository.

Notable userland content included / moving into this repository includes:

* <<userland-assembly>>
* <<c>>
* <<cpp>>
* <<posix>>
* https://github.com/cirosantilli/algorithm-cheat TODO will be good to move here for performance analysis <<gem5-run-benchmark,with gem5>>

==== Userland setup getting started

There are several ways to run our userland content, notably:

* natively on the host as shown at: <<userland-setup-getting-started-natively>>
+
Can only run examples compatible with your host CPU architecture and OS, but has the fastest setup and runtimes.
* from user mode simulation with:
+
--
** the host prebuilt toolchain: <<userland-setup-getting-started-with-prebuilt-toolchain-and-qemu-user-mode>>
** the Buildroot toolchain you built yourself: <<qemu-user-mode-getting-started>>
--
+
This setup:
+
--
** can run most examples, including those for other CPU architectures, with the notable exception of examples that rely on kernel modules
** can run reproducible approximate performance experiments with gem5, see e.g. <<bst-vs-heap>>
--
* from full system simulation as shown at: <<qemu-buildroot-setup-getting-started>>.
+
This is the most reproducible and controlled environment, and all examples work there. But also the slower one to setup.

===== Userland setup getting started natively

With this setup, we will use the host toolchain and execute executables directly on the host.

No installation or toolchain build is required, so you can just jump straight into it.

Build, run and example, and clean it in-tree with:

....
sudo apt-get install gcc
cd userland
./build c/hello
./c/hello.out
./build --clean
....

Source: link:userland/c/hello.c[].

Build an entire directory and test it:

....
cd userland
./build c
./test c
....

Build the current directory and test it:

....
cd userland/c
./build
./test
....

As mentioned at <<user-mode-tests>>, tests under link:userland/libs[] require certain optional libraries to be installed, and are not built or tested by default.

You can install those libraries with:

....
cd linux-kernel-module-cheat
./build --download-dependencies userland-host
....

and then build the examples and test with:

....
./build --package-all
./test --package-all
....

Pass custom compiler options:

....
./build --ccflags='-foptimize-sibling-calls -foptimize-strlen' --force-rebuild
....

Here we used `--force-rebuild` to force rebuild since the sources weren't modified since the last build.

Some CLI options have more specialized flags, e.g. `-O` optimization level:

....
./build --optimization-level 3 --force-rebuild
....

See also <<user-mode-static-executables>> for `--static`.

The `build` scripts inside link:userland/[] are just symlinks to link:build-userland-in-tree[] which you can also use from toplevel as:

....
./build-userland-in-tree
./build-userland-in-tree userland/c
./build-userland-in-tree userland/c/hello.c
....

`build-userland-in-tre` is in turn just a thin wrapper around link:build-userland[]:

....
./build-userland --gcc-which host --in-tree userland/c
....

So you can use any option supported by `build-userland` script freely with `build-userland-in-tree` and `build`.

The situation is analogous for link:userland/test[], link:test-user-mode-in-tree[] and link:test-user-mode[], which are further documented at: <<user-mode-tests>>.

Do a more clean out-of-tree build instead and run the program:

....
./build-userland --gcc-which host --userland-build-id host
./run --emulator native --userland userland/c/hello.c --userland-build-id host
....

Here we:

* put the host executables in a separate <<build-variants,build-variant>> to avoid conflict with Buildroot builds.
* ran with the `--emulator native` option to run the program natively

In this case you can debub the program with:

....
./run --debug-vm --emulator native --userland userland/c/hello.c --userland-build-id host
....

as shown at: <<debug-the-emulator>>, although direct GDB host usage works as well of course.

===== Userland setup getting started with prebuilt toolchain and QEMU user mode

If you are lazy to built the Buildroot toolchain and QEMU, but want to run e.g. ARM <<userland-assembly>> in <<user-mode-simulation>>, you can get away on Ubuntu 18.04 with just:

....
sudo apt-get install gcc-aarch64-linux-gnu qemu-system-aarch64
./build-userland \
  --arch aarch64 \
  --gcc-which host \
  --userland-build-id host \
;
./run \
  --arch aarch64 \
  --qemu-which host \
  --userland-build-id host \
  --userland userland/c/print_argv.c \
  --userland-args 'asdf "qw er"' \
;
....

where:

* `--gcc-which host`: use the host toolchain.
+
We must pass this to `./run` as well because QEMU must know which dynamic libraries to use. See also: <<user-mode-static-executables>>.
* `--userland-build-id host`: put the host built into a <<build-variants>>

This present the usual trade-offs of using prebuilts as mentioned at: <<prebuilt>>.

Other functionality are analogous, e.g. testing:

....
./test-user-mode \
  --arch aarch64 \
  --gcc-which host \
  --qemu-which host \
  --userland-build-id host \
;
....

and <<user-mode-gdb>>:

....
./run \
  --arch aarch64 \
  --gdb \
  --gcc-which host \
  --qemu-which host \
  --userland-build-id host \
  --userland userland/c/print_argv.c \
  --userland-args 'asdf "qw er"' \
;
....

===== Userland setup getting started full system

First ensure that <<qemu-buildroot-setup>> is working.

After doing that setup, you can already execute your userland programs from inside QEMU: the only missing step is how to rebuild executables and run them.

And the answer is exactly analogous to what is shown at: <<your-first-kernel-module-hack>>

For example, if we modify link:userland/c/hello.c[] to print out something different, we can just rebuild it with:

....
./build-userland
....

Source: link:build-userland[]. `./build` calls that script automatically for us when doing the initial full build.

Now, run the program either without rebooting use the <<9p>> mount:

....
/mnt/9p/out_rootfs_overlay/c/hello.out
....

or shutdown QEMU, add the executable to the root filesystem:

....
./build-buildroot
....

reboot and use the root filesystem as usual:

....
./hello.out
....

=== Baremetal setup

==== About the baremetal setup

This setup does not use the Linux kernel nor Buildroot at all: it just runs your very own minimal OS.

`x86_64` is not currently supported, only `arm` and `aarch64`: I had made some x86 bare metal examples at: https://github.com/cirosantilli/x86-bare-metal-examples but I'm lazy to port them here now. Pull requests are welcome.

The main reason this setup is included in this project, despite the word "Linux" being on the project name, is that a lot of the emulator boilerplate can be reused for both use cases.

This setup allows you to make a tiny OS and that runs just a few instructions, use it to fully control the CPU to better understand the simulators for example, or develop your own OS if you are into that.

You can also use C and a subset of the C standard library because we enable link:https://en.wikipedia.org/wiki/Newlib[Newlib] by default. See also: https://electronics.stackexchange.com/questions/223929/c-standard-libraries-on-bare-metal/400077#400077

Our C bare-metal compiler is built with link:https://github.com/crosstool-ng/crosstool-ng[crosstool-NG]. If you have already built <<qemu-buildroot-setup,Buildroot>> previously, you will end up with two GCCs installed. Unfortunately I don't see a solution for this, since we need separate toolchains for Newlib on baremetal and glibc on Linux: https://stackoverflow.com/questions/38956680/difference-between-arm-none-eabi-and-arm-linux-gnueabi/38989869#38989869

==== Baremetal setup getting started

Every `.c` file inside link:baremetal/[] and `.S` file inside `baremetal/arch/<arch>/` generates a separate baremetal image.

For example, to run link:baremetal/hello.c[] in QEMU do:

....
./build --arch aarch64 --download-dependencies qemu-baremetal
./run --arch aarch64 --baremetal baremetal/hello.c
....

The terminal prints:

....
hello
....

Now let's run link:baremetal/arch/aarch64/add.S[]:

....
./run --arch aarch64 --baremetal baremetal/arch/aarch64/add.S
....

This time, the terminal does not print anything, which indicates success.

If you look into the source, you will see that we just have an assertion there.

You can see a sample assertion fail in link:baremetal/interactive/assert_fail.c[]:

....
./run --arch aarch64 --baremetal baremetal/interactive/assert_fail.c
....

and the terminal contains:

....
lkmc_test_fail
error: simulation error detected by parsing logs
....

and the exit status of our script is 1:

....
echo $?
....

To modify a baremetal program, simply edit the file, e.g.

....
vim baremetal/hello.c
....

and rebuild:

....
./build-baremetal --arch aarch64
./run --arch aarch64 --baremetal baremetal/hello.c
....

`./build qemu-baremetal` that we run previously is only needed for the initial build. That script calls link:build-baremetal[] for us, in addition to building prerequisites such as QEMU and crosstool-NG.

`./build-baremetal` uses crosstool-NG, and so it must be preceded by link:build-crosstool-ng[], which `./build qemu-baremetal` also calls.

Alternatively, for the sake of tab completion, we also accept relative paths inside `baremetal/`, for example the following also work:

....
./run --arch aarch64 --baremetal baremetal/hello.c
./run --arch aarch64 --baremetal baremetal/arch/aarch64/add.S
....

Absolute paths however are used as is and must point to the actual executable:

....
./run --arch aarch64 --baremetal "$(./getvar --arch aarch64 baremetal_build_dir)/hello.elf"
....

To use gem5 instead of QEMU do:

....
./build --download-dependencies gem5-baremetal
./run --arch aarch64 --baremetal baremetal/hello.c --emulator gem5
....

and then <<qemu-buildroot-setup,as usual>> open a shell with:

....
./gem5-shell
....

Or as usual, <<tmux>> users can do both in one go with:

....
./run --arch aarch64 --baremetal baremetal/hello.c --emulator gem5 --tmux
....

TODO: the carriage returns are a bit different than in QEMU, see: <<gem5-baremetal-carriage-return>>.

Note that `./build-baremetal` requires the `--emulator gem5` option, and generates separate executable images for both, as can be seen from:

....
echo "$(./getvar --arch aarch64 --baremetal baremetal/hello.c --emulator qemu image)"
echo "$(./getvar --arch aarch64 --baremetal baremetal/hello.c --emulator gem5 image)"
....

This is unlike the Linux kernel that has a single image for both QEMU and gem5:

....
echo "$(./getvar --arch aarch64 --emulator qemu image)"
echo "$(./getvar --arch aarch64 --emulator gem5 image)"
....

The reason for that is that on baremetal we don't parse the <<device-tree,device tress>> from memory like the Linux kernel does, which tells the kernel for example the UART address, and many other system parameters.

`gem5` also supports the `RealViewPBX` machine, which represents an older hardware compared to the default `VExpress_GEM5_V1`:

....
./build-baremetal --arch aarch64 --emulator gem5 --machine RealViewPBX
./run --arch aarch64 --baremetal baremetal/hello.c --emulator gem5 --machine RealViewPBX
....

This generates yet new separate images with new magic constants:

....
echo "$(./getvar --arch aarch64 --baremetal baremetal/hello.c --emulator gem5 --machine VExpress_GEM5_V1 image)"
echo "$(./getvar --arch aarch64 --baremetal baremetal/hello.c --emulator gem5 --machine RealViewPBX      image)"
....

But just stick to newer and better `VExpress_GEM5_V1` unless you have a good reason to use `RealViewPBX`.

When doing baremetal programming, it is likely that you will want to learn userland assembly first, see: <<userland-assembly>>.

For more information on baremetal, see the section: <<baremetal>>.

The following subjects are particularly important:

* <<tracing>>
* <<baremetal-gdb-step-debug>>

[[gdb]]
== GDB step debug

=== GDB step debug kernel boot

`--gdb-wait` makes QEMU and gem5 wait for a GDB connection, otherwise we could accidentally go past the point we want to break at:

....
./run --gdb-wait
....

Say you want to break at `start_kernel`. So on another shell:

....
./run-gdb start_kernel
....

or at a given line:

....
./run-gdb init/main.c:1088
....

Now QEMU will stop there, and you can use the normal GDB commands:

....
list
next
continue
....

See also:

* http://stackoverflow.com/questions/11408041/how-to-debug-the-linux-kernel-with-gdb-and-qemu/33203642#33203642
* http://stackoverflow.com/questions/4943857/linux-kernel-live-debugging-how-its-done-and-what-tools-are-used/42316607#42316607

==== GDB step debug kernel boot other archs

Just don't forget to pass `--arch` to `./run-gdb`, e.g.:

....
./run --arch aarch64 --gdb-wait
....

and:

....
./run-gdb --arch aarch64 start_kernel
....

[[kernel-o0]]
==== Disable kernel compiler optimizations

https://stackoverflow.com/questions/29151235/how-to-de-optimize-the-linux-kernel-to-and-compile-it-with-o0

`O=0` is an impossible dream, `O=2` being the default.

So get ready for some weird jumps, and `<value optimized out>` fun. Why, Linux, why.

=== GDB step debug kernel post-boot

Let's observe the kernel `write` system call as it reacts to some userland actions.

Start QEMU with just:

....
./run
....

and after boot inside a shell run:

....
./count.sh
....

which counts to infinity to stdout. Source: link:rootfs_overlay/lkmc/count.sh[].

Then in another shell, run:

....
./run-gdb
....

and then hit:

....
Ctrl-C
break __x64_sys_write
continue
continue
continue
....

And you now control the counting on the first shell from GDB!

Before v4.17, the symbol name was just `sys_write`, the change happened at link:https://github.com/torvalds/linux/commit/d5a00528b58cdb2c71206e18bd021e34c4eab878[d5a00528b58cdb2c71206e18bd021e34c4eab878]. As of Linux v 4.19, the function is called `sys_write` in `arm`, and `__arm64_sys_write` in `aarch64`. One good way to find it if the name changes again is to try:

....
rbreak .*sys_write
....

or just have a quick look at the sources!

When you hit `Ctrl-C`, if we happen to be inside kernel code at that point, which is very likely if there are no heavy background tasks waiting, and we are just waiting on a `sleep` type system call of the command prompt, we can already see the source for the random place inside the kernel where we stopped.

=== tmux

tmux just makes things even more fun by allowing us to see both the terminal for:

* emulator stdout
* <<gdb>>

at once without dragging windows around!

First start `tmux` with:

....
tmux
....

Now that you are inside a shell inside tmux, you can start GDB simply with:

....
./run --gdb
....

which is just a convenient shortcut for:

....
./run --gdb-wait --tmux --tmux-args start_kernel
....

This splits the terminal into two panes:

* left: usual QEMU with terminal
* right: GDB

and focuses on the GDB pane.

Now you can navigate with the usual tmux shortcuts:

* switch between the two panes with: `Ctrl-B O`
* close either pane by killing its terminal with `Ctrl-D` as usual

See the tmux manual for further details:

....
man tmux
....

To start again, switch back to the QEMU pane with `Ctrl-O`, kill the emulator, and re-run:

....
./run --gdb
....

This automatically clears the GDB pane, and starts a new one.

The option `--tmux-args` determines which options will be passed to the program running on the second tmux pane, and is equivalent to:

This is equivalent to:

....
./run --gdb-wait
./run-gdb start_kernel
....

Due to Python's CLI parsing quicks, if the link:run-gdb[] arguments start with a dash `-`, you have to use the `=` sign, e.g. to <<gdb-step-debug-early-boot>>:

....
./run --gdb --tmux-args=--no-continue
....

Bibliography: https://unix.stackexchange.com/questions/152738/how-to-split-a-new-window-and-run-a-command-in-this-new-window-using-tmux/432111#432111

==== tmux gem5

If you are using gem5 instead of QEMU, `--tmux` has a different effect by default: it opens the gem5 terminal instead of the debugger:

....
./run --emulator gem5 --tmux
....

To open a new pane with GDB instead of the terminal, use:

....
./run --gdb
....

which is equivalent to:

....
./run --emulator gem5 --gdb-wait --tmux --tmux-args start_kernel --tmux-program gdb
....

`--tmux-program` implies `--tmux`, so we can just write:

....
./run --emulator gem5 --gdb-wait --tmux-program gdb
....

If you also want to see both GDB and the terminal with gem5, then you will need to open a separate shell manually as usual with `./gem5-shell`.

From inside tmux, you can create new terminals on a new window with `Ctrl-B C` split a pane yet again vertically with `Ctrl-B %` or horizontally with `Ctrl-B "`.

=== GDB step debug kernel module

http://stackoverflow.com/questions/28607538/how-to-debug-linux-kernel-modules-with-qemu/44095831#44095831

Loadable kernel modules are a bit trickier since the kernel can place them at different memory locations depending on load order.

So we cannot set the breakpoints before `insmod`.

However, the Linux kernel GDB scripts offer the `lx-symbols` command, which takes care of that beautifully for us.

Shell 1:

....
./run
....

Wait for the boot to end and run:

....
insmod timer.ko
....

Source: link:kernel_modules/timer.c[].

This prints a message to dmesg every second.

Shell 2:

....
./run-gdb
....

In GDB, hit `Ctrl-C`, and note how it says:

....
scanning for modules in /root/linux-kernel-module-cheat/out/kernel_modules/x86_64/kernel_modules
loading @0xffffffffc0000000: /root/linux-kernel-module-cheat/out/kernel_modules/x86_64/kernel_modules/timer.ko
....

That's `lx-symbols` working! Now simply:

....
break lkmc_timer_callback
continue
continue
continue
....

and we now control the callback from GDB!

Just don't forget to remove your breakpoints after `rmmod`, or they will point to stale memory locations.

TODO: why does `break work_func` for `insmod kthread.ko` not very well? Sometimes it breaks but not others.

[[gdb-step-debug-kernel-module-arm]]
==== GDB step debug kernel module insmodded by init on ARM

TODO on `arm` 51e31cdc2933a774c2a0dc62664ad8acec1d2dbe it does not always work, and `lx-symbols` fails with the message:

....
loading vmlinux
Traceback (most recent call last):
  File "/linux-kernel-module-cheat//out/arm/buildroot/build/linux-custom/scripts/gdb/linux/symbols.py", line 163, in invoke
    self.load_all_symbols()
  File "/linux-kernel-module-cheat//out/arm/buildroot/build/linux-custom/scripts/gdb/linux/symbols.py", line 150, in load_all_symbols
    [self.load_module_symbols(module) for module in module_list]
  File "/linux-kernel-module-cheat//out/arm/buildroot/build/linux-custom/scripts/gdb/linux/symbols.py", line 110, in load_module_symbols
    module_name = module['name'].string()
gdb.MemoryError: Cannot access memory at address 0xbf0000cc
Error occurred in Python command: Cannot access memory at address 0xbf0000cc
....

Can't reproduce on `x86_64` and `aarch64` are fine.

It is kind of random: if you just `insmod` manually and then immediately `./run-gdb --arch arm`, then it usually works.

But this fails most of the time: shell 1:

....
./run --arch arm --eval-after 'insmod hello.ko'
....

shell 2:

....
./run-gdb --arch arm
....

then hit `Ctrl-C` on shell 2, and voila.

Then:

....
cat /proc/modules
....

says that the load address is:

....
0xbf000000
....

so it is close to the failing `0xbf0000cc`.

`readelf`:

....
./run-toolchain readelf -- -s "$(./getvar kernel_modules_build_subdir)/hello.ko"
....

does not give any interesting hits at `cc`, no symbol was placed that far.

==== GDB module_init

TODO find a more convenient method. We have working methods, but they are not ideal.

This is not very easy, since by the time the module finishes loading, and `lx-symbols` can work properly, `module_init` has already finished running!

Possibly asked at:

* https://stackoverflow.com/questions/37059320/debug-a-kernel-module-being-loaded
* https://stackoverflow.com/questions/11888412/debug-the-init-module-call-of-a-linux-kernel-module

===== GDB module_init step into it

This is the best method we've found so far.

The kernel calls `module_init` synchronously, therefore it is not hard to step into that call.

As of 4.16, the call happens in `do_one_initcall`, so we can do in shell 1:

....
./run
....

shell 2 after boot finishes (because there are other calls to `do_init_module` at boot, presumably for the built-in modules):

....
./run-gdb do_one_initcall
....

then step until the line:

....
833         ret = fn();
....

which does the actual call, and then step into it.

For the next time, you can also put a breakpoint there directly:

....
./run-gdb init/main.c:833
....

How we found this out: first we got <<gdb-module_init-calculate-entry-address>> working, and then we did a `bt`. AKA cheating :-)

===== GDB module_init calculate entry address

This works, but is a bit annoying.

The key observation is that the load address of kernel modules is deterministic: there is a pre allocated memory region https://www.kernel.org/doc/Documentation/x86/x86_64/mm.txt "module mapping space" filled from bottom up.

So once we find the address the first time, we can just reuse it afterwards, as long as we don't modify the module.

Do a fresh boot and get the module:

....
./run --eval-after './pr_debug.sh;insmod fops.ko;./linux/poweroff.out'
....

The boot must be fresh, because the load address changes every time we insert, even after removing previous modules.

The base address shows on terminal:

....
0xffffffffc0000000 .text
....

Now let's find the offset of `myinit`:

....
./run-toolchain readelf -- \
  -s "$(./getvar kernel_modules_build_subdir)/fops.ko" | \
  grep myinit
....

which gives:

....
    30: 0000000000000240    43 FUNC    LOCAL  DEFAULT    2 myinit
....

so the offset address is `0x240` and we deduce that the function will be placed at:

....
0xffffffffc0000000 + 0x240 = 0xffffffffc0000240
....

Now we can just do a fresh boot on shell 1:

....
./run --eval 'insmod fops.ko;./linux/poweroff.out' --gdb-wait
....

and on shell 2:

....
./run-gdb '*0xffffffffc0000240'
....

GDB then breaks, and `lx-symbols` works.

===== GDB module_init break at the end of sys_init_module

TODO not working. This could be potentially very convenient.

The idea here is to break at a point late enough inside `sys_init_module`, at which point `lx-symbols` can be called and do its magic.

Beware that there are both `sys_init_module` and `sys_finit_module` syscalls, and `insmod` uses `fmodule_init` by default.

Both call `do_module_init` however, which is what `lx-symbols` hooks to.

If we try:

....
b sys_finit_module
....

then hitting:

....
n
....

does not break, and insertion happens, likely because of optimizations? <<kernel-o0>>

Then we try:

....
b do_init_module
....

A naive:

....
fin
....

also fails to break!

Finally, in despair we notice that <<pr_debug>> prints the kernel load address as explained at <<bypass-lx-symbols>>.

So, if we set a breakpoint just after that message is printed by searching where that happens on the Linux source code, we must be able to get the correct load address before `init_module` happens.

===== GDB module_init add trap instruction

This is another possibility: we could modify the module source by adding a trap instruction of some kind.

This appears to be described at: https://www.linuxjournal.com/article/4525

But it refers to a `gdbstart` script which is not in the tree anymore and beyond my `git log` capabilities.

And just adding:

....
asm( " int $3");
....

directly gives an <<oops,oops>> as I'd expect.

==== Bypass lx-symbols

Useless, but a good way to show how hardcore you are. Disable `lx-symbols` with:

....
./run-gdb --no-lxsymbols
....

From inside guest:

....
insmod timer.ko
cat /proc/modules
....

as mentioned at:

* https://stackoverflow.com/questions/6384605/how-to-get-address-of-a-kernel-module-loaded-using-insmod/6385818
* https://unix.stackexchange.com/questions/194405/get-base-address-and-size-of-a-loaded-kernel-module

This will give a line of form:

....
fops 2327 0 - Live 0xfffffffa00000000
....

And then tell GDB where the module was loaded with:

....
Ctrl-C
add-symbol-file ../../../rootfs_overlay/x86_64/timer.ko 0xffffffffc0000000
0xffffffffc0000000
....

Alternatively, if the module panics before you can read `/proc/modules`, there is a <<pr_debug>> which shows the load address:

....
echo 8 > /proc/sys/kernel/printk
echo 'file kernel/module.c +p' > /sys/kernel/debug/dynamic_debug/control
./linux/myinsmod.out hello.ko
....

And then search for a line of type:

....
[   84.877482]  0xfffffffa00000000 .text
....

Tested on 4f4749148273c282e80b58c59db1b47049e190bf + 1.

=== GDB step debug early boot

TODO successfully debug the very first instruction that the Linux kernel runs, before `start_kernel`!

Break at the very first instruction executed by QEMU:

....
./run-gdb --no-continue
....

TODO why can't we break at early startup stuff such as:

....
./run-gdb extract_kernel
./run-gdb main
....

Maybe it is because they are being copied around at specific locations instead of being run directly from inside the main image, which is where the debug information points to?

See also: https://stackoverflow.com/questions/2589845/what-are-the-first-operations-that-the-linux-kernel-executes-on-boot

<<gem5-tracing>> with `--debug-flags=Exec` does show the right symbols however! So in the worst case, we can just read their source. Amazing.

v4.19 also added a `CONFIG_HAVE_KERNEL_UNCOMPRESSED=y` option for having the kernel uncompressed which could make following the startup easier, but it is only available on s390. `aarch64` however is already uncompressed by default, so might be the easiest one. See also: <<vmlinux-vs-bzimage-vs-zimage-vs-image>>.

==== GDB step debug early boot by address

One possibility is to run:

....
./trace-boot --arch arm
....

and then find the second address (the first one does not work, already too late maybe):

....
less "$(./getvar --arch arm trace_txt_file)"
....

and break there:

....
./run --arch arm --gdb-wait
./run-gdb --arch arm '*0x1000'
....

but TODO: it does not show the source assembly under `arch/arm`: https://stackoverflow.com/questions/11423784/qemu-arm-linux-kernel-boot-debug-no-source-code

I also tried to hack `run-gdb` with:

....
@@ -81,7 +81,7 @@ else
 ${gdb} \
 -q \\
 -ex 'add-auto-load-safe-path $(pwd)' \\
--ex 'file vmlinux' \\
+-ex 'file arch/arm/boot/compressed/vmlinux' \\
 -ex 'target remote localhost:${port}' \\
 ${brk} \
 -ex 'continue' \\
....

and no I do have the symbols from `arch/arm/boot/compressed/vmlinux'`, but the breaks still don't work.

=== GDB step debug userland processes

QEMU's `-gdb` GDB breakpoints are set on virtual addresses, so you can in theory debug userland processes as well.

* https://stackoverflow.com/questions/26271901/is-it-possible-to-use-gdb-and-qemu-to-debug-linux-user-space-programs-and-kernel
* https://stackoverflow.com/questions/16273614/debug-init-on-qemu-using-gdb

You will generally want to use <<gdbserver>> for this as it is more reliable, but this method can overcome the following limitations of `gdbserver`:

* the emulator does not support host to guest networking. This seems to be the case for gem5: <<gem5-host-to-guest-networking>>
* cannot see the start of the `init` process easily
* `gdbserver` alters the working of the kernel, and makes your run less representative

Known limitations of direct userland debugging:

* the kernel might switch context to another process or to the kernel itself e.g. on a system call, and then TODO confirm the PIC would go to weird places and source code would be missing.
* TODO step into shared libraries. If I attempt to load them explicitly:
+
....
(gdb) sharedlibrary ../../staging/lib/libc.so.0
No loaded shared libraries match the pattern `../../staging/lib/libc.so.0'.
....
+
since GDB does not know that libc is loaded.

==== GDB step debug userland custom init

This is the userland debug setup most likely to work, since at init time there is only one userland executable running.

For executables from the link:userland/[] directory such as link:userland/posix/count.c[]:

* Shell 1:
+
....
./run --gdb-wait --kernel-cli 'init=/lkmc/posix/count.out'
....
* Shell 2:
+
....
./run-gdb-user userland/posix/count.c main
....
+
Alternatively, we could also pass the full path to the executable:
+
....
./run-gdb-user "$(./getvar userland_build_dir)/posix/count.out" main
....
+
Path resolution is analogous to <<baremetal-setup-getting-started,that of `./run --baremetal`>>.

Then, as soon as boot ends, we are left inside a debug session that looks just like what `gdbserver` would produce.

==== GDB step debug userland BusyBox init

BusyBox custom init process:

* Shell 1:
+
....
./run --gdb-wait --kernel-cli 'init=/bin/ls'
....
* Shell 2:
+
....
./run-gdb-user "$(./getvar buildroot_build_build_dir)"/busybox-*/busybox ls_main
....

This follows BusyBox' convention of calling the main for each executable as `<exec>_main` since the `busybox` executable has many "mains".

BusyBox default init process:

* Shell 1:
+
....
./run --gdb-wait
....
* Shell 2:
+
....
./run-gdb-user "$(./getvar buildroot_build_build_dir)"/busybox-*/busybox init_main
....

`init` cannot be debugged with <<gdbserver>> without modifying the source, or else `/sbin/init` exits early with:

....
"must be run as PID 1"
....

==== GDB step debug userland non-init

Non-init process:

* Shell 1:
+
....
./run --gdb-wait
....
* Shell 2:
+
....
./run-gdb-user userland/linux/myinsmod.c main
....
* Shell 1 after the boot finishes:
+
....
./linux/myinsmod.out hello.ko
....

This is the least reliable setup as there might be other processes that use the given virtual address.

===== GDB step debug userland non-init without --gdb-wait

TODO: without `--gdb-wait` and the `break main` that we do inside `./run-gdb-user` says:

....
Cannot access memory at address 0x10604
....

and then GDB never breaks. Tested at ac8663a44a450c3eadafe14031186813f90c21e4 + 1.

The exact behaviour seems to depend on the architecture:

* `arm`: happens always
* `x86_64`: appears to happen only if you try to connect GDB as fast as possible, before init has been reached.
* `aarch64`: could not observe the problem

We have also double checked the address with:

....
./run-toolchain --arch arm readelf -- \
  -s "$(./getvar --arch arm userland_build_dir)/linux/myinsmod.out" | \
  grep main
....

and from GDB:

....
info line main
....

and both give:

....
000105fc
....

which is just 8 bytes before `0x10604`.

`gdbserver` also says `0x10604`.

However, if do a `Ctrl-C` in GDB, and then a direct:

....
b *0x000105fc
....

it works. Why?!

On GEM5, x86 can also give the `Cannot access memory at address`, so maybe it is also unreliable on QEMU, and works just by coincidence.

=== GDB call

GDB can call functions as explained at: https://stackoverflow.com/questions/1354731/how-to-evaluate-functions-in-gdb

However this is failing for us:

* some symbols are not visible to `call` even though `b` sees them
* for those that are, `call` fails with an E14 error

E.g.: if we break on `__x64_sys_write` on `count.sh`:

....
>>> call printk(0, "asdf")
Could not fetch register "orig_rax"; remote failure reply 'E14'
>>> b printk
Breakpoint 2 at 0xffffffff81091bca: file kernel/printk/printk.c, line 1824.
>>> call fdget_pos(fd)
No symbol "fdget_pos" in current context.
>>> b fdget_pos
Breakpoint 3 at 0xffffffff811615e3: fdget_pos. (9 locations)
>>>
....

even though `fdget_pos` is the first thing `__x64_sys_write` does:

....
581 SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
582         size_t, count)
583 {
584     struct fd f = fdget_pos(fd);
....

I also noticed that I get the same error:

....
Could not fetch register "orig_rax"; remote failure reply 'E14'
....

when trying to use:

....
fin
....

on many (all?) functions.

See also: https://github.com/cirosantilli/linux-kernel-module-cheat/issues/19

=== GDB view ARM system registers

`info all-registers` shows some of them.

The implementation is described at: https://stackoverflow.com/questions/46415059/how-to-observe-aarch64-system-registers-in-qemu/53043044#53043044

=== GDB step debug multicore userland

For a more minimal baremetal multicore setup, see: <<arm-multicore>>.

We can set and get which cores the Linux kernel allows a program to run on with `sched_getaffinity` and `sched_setaffinity`:

....
./run --cpus 2 --eval-after './linux/sched_getaffinity.out'
....

Source: link:userland/linux/sched_getaffinity.c[]

Sample output:

....
sched_getaffinity = 1 1
sched_getcpu = 1
sched_getaffinity = 1 0
sched_getcpu = 0
....

Which shows us that:

* initially:
** all 2 cores were enabled as shown by `sched_getaffinity = 1 1`
** the process was randomly assigned to run on core 1 (the second one) as shown by `sched_getcpu = 1`. If we run this several times, it will also run on core 0 sometimes.
* then we restrict the affinity to just core 0, and we see that the program was actually moved to core 0

The number of cores is modified as explained at: <<number-of-cores>>

`taskset` from the util-linux package sets the initial core affinity of a program:

....
./build-buildroot \
  --config 'BR2_PACKAGE_UTIL_LINUX=y' \
  --config 'BR2_PACKAGE_UTIL_LINUX_SCHEDUTILS=y' \
;
./run --eval-after 'taskset -c 1,1 ./linux/sched_getaffinity.out'
....

output:

....
sched_getaffinity = 0 1
sched_getcpu = 1
sched_getaffinity = 1 0
sched_getcpu = 0
....

so we see that the affinity was restricted to the second core from the start.

Let's do a QEMU observation to justify this example being in the repository with <<gdb-step-debug-userland-non-init,userland breakpoints>>.

We will run our `./linux/sched_getaffinity.out` infinitely many time, on core 0 and core 1 alternatively:

....
./run \
  --cpus 2 \
  --eval-after 'i=0; while true; do taskset -c $i,$i ./linux/sched_getaffinity.out; i=$((! $i)); done' \
  --gdb-wait \
;
....

on another shell:

....
./run-gdb-user "$(./getvar userland_build_dir)/linux/sched_getaffinity.out" main
....

Then, inside GDB:

....
(gdb) info threads
  Id   Target Id         Frame
* 1    Thread 1 (CPU#0 [running]) main () at sched_getaffinity.c:30
  2    Thread 2 (CPU#1 [halted ]) native_safe_halt () at ./arch/x86/include/asm/irqflags.h:55
(gdb) c
(gdb) info threads
  Id   Target Id         Frame
  1    Thread 1 (CPU#0 [halted ]) native_safe_halt () at ./arch/x86/include/asm/irqflags.h:55
* 2    Thread 2 (CPU#1 [running]) main () at sched_getaffinity.c:30
(gdb) c
....

and we observe that `info threads` shows the actual correct core on which the process was restricted to run by `taskset`!

We should also try it out with kernel modules: https://stackoverflow.com/questions/28347876/set-cpu-affinity-on-a-loadable-linux-kernel-module

TODO we then tried:

....
./run --cpus 2 --eval-after './linux/sched_getaffinity_threads.out'
....

and:

....
./run-gdb-user "$(./getvar userland_build_dir)/linux/sched_getaffinity_threads.out"
....

to switch between two simultaneous live threads with different affinities, it just didn't break on our threads:

....
b main_thread_0
....

Bibliography:

* https://stackoverflow.com/questions/10490756/how-to-use-sched-getaffinity-and-sched-setaffinity-in-linux-from-c/50117787#50117787
* https://stackoverflow.com/questions/42800801/how-to-use-gdb-to-debug-qemu-with-smp-symmetric-multiple-processors

=== Linux kernel GDB scripts

We source the Linux kernel GDB scripts by default for `lx-symbols`, but they also contains some other goodies worth looking into.

Those scripts basically parse some in-kernel data structures to offer greater visibility with GDB.

All defined commands are prefixed by `lx-`, so to get a full list just try to tab complete that.

There aren't as many as I'd like, and the ones that do exist are pretty self explanatory, but let's give a few examples.

Show dmesg:

....
lx-dmesg
....

Show the <<kernel-command-line-parameters>>:

....
lx-cmdline
....

Dump the device tree to a `fdtdump.dtb` file in the current directory:

....
lx-fdtdump
pwd
....

List inserted kernel modules:

....
lx-lsmod
....

Sample output:

....
Address            Module                  Size  Used by
0xffffff80006d0000 hello                  16384  0
....

Bibliography:

* https://events.static.linuxfound.org/sites/events/files/slides/Debugging%20the%20Linux%20Kernel%20with%20GDB.pdf
* https://wiki.linaro.org/LandingTeams/ST/GDB

==== lx-ps

List all processes:

....
lx-ps
....

Sample output:

....
0xffff88000ed08000 1 init
0xffff88000ed08ac0 2 kthreadd
....

The second and third fields are obviously PID and process name.

The first one is more interesting, and contains the address of the `task_struct` in memory.

This can be confirmed with:

....
p ((struct task_struct)*0xffff88000ed08000
....

which contains the correct PID for all threads I've tried:

....
pid = 1,
....

TODO get the PC of the kthreads: https://stackoverflow.com/questions/26030910/find-program-counter-of-process-in-kernel Then we would be able to see where the threads are stopped in the code!

On ARM, I tried:

....
task_pt_regs((struct thread_info *)((struct task_struct)*0xffffffc00e8f8000))->uregs[ARM_pc]
....

but `task_pt_regs` is a `#define` and GDB cannot see defines without `-ggdb3`: https://stackoverflow.com/questions/2934006/how-do-i-print-a-defined-constant-in-gdb which are apparently not set?

Bibliography:

* https://stackoverflow.com/questions/9561546/thread-aware-gdb-for-kernel
* https://wiki.linaro.org/LandingTeams/ST/GDB
* https://events.static.linuxfound.org/sites/events/files/slides/Debugging%20the%20Linux%20Kernel%20with%20GDB.pdf presentation: https://www.youtube.com/watch?v=pqn5hIrz3A8

=== Debug the GDB remote protocol

For when it breaks again, or you want to add a new feature!

....
./run --debug
./run-gdb --before '-ex "set remotetimeout 99999" -ex "set debug remote 1"' start_kernel
....

See also: https://stackoverflow.com/questions/13496389/gdb-remote-protocol-how-to-analyse-packets

[[remote-g-packet]]
==== Remote 'g' packet reply is too long

This error means that the GDB server, e.g. in QEMU, sent more registers than the GDB client expected.

This can happen for the following reasons:

* you set the architecture of the client wrong, often 32 vs 64 bit as mentioned at: https://stackoverflow.com/questions/4896316/gdb-remote-cross-debugging-fails-with-remote-g-packet-reply-is-too-long
* there is a bug in the GDB server and the XML description does not match the number of registers actually sent
* the GDB server does not send XML target descriptions and your GDB expects a different number of registers by default. E.g., gem5 d4b3e064adeeace3c3e7d106801f95c14637c12f does not send the XML files

The XML target description format is described a bit further at: https://stackoverflow.com/questions/46415059/how-to-observe-aarch64-system-registers-in-qemu/53043044#53043044

== KGDB

KGDB is kernel dark magic that allows you to GDB the kernel on real hardware without any extra hardware support.

It is useless with QEMU since we already have full system visibility with `-gdb`. So the goal of this setup is just to prepare you for what to expect when you will be in the treches of real hardware.

KGDB is cheaper than JTAG (free) and easier to setup (all you need is serial), but with less visibility as it depends on the kernel working, so e.g.: dies on panic, does not see boot sequence.

First run the kernel with:

....
./run --kgdb
....

this passes the following options on the kernel CLI:

....
kgdbwait kgdboc=ttyS1,115200
....

`kgdbwait` tells the kernel to wait for KGDB to connect.

So the kernel sets things up enough for KGDB to start working, and then boot pauses waiting for connection:

....
<6>[    4.866050] Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled
<6>[    4.893205] 00:05: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
<6>[    4.916271] 00:06: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A
<6>[    4.987771] KGDB: Registered I/O driver kgdboc
<2>[    4.996053] KGDB: Waiting for connection from remote gdb...

Entering kdb (current=0x(____ptrval____), pid 1) on processor 0 due to Keyboard Entry
[0]kdb>
....

KGDB expects the connection at `ttyS1`, our second serial port after `ttyS0` which contains the terminal.

The last line is the KDB prompt, and is covered at: <<kdb>>. Typing now shows nothing because that prompt is expecting input from `ttyS1`.

Instead, we connect to the serial port `ttyS1` with GDB:

....
./run-gdb --kgdb --no-continue
....

Once GDB connects, it is left inside the function `kgdb_breakpoint`.

So now we can set breakpoints and continue as usual.

For example, in GDB:

....
continue
....

Then in QEMU:

....
./count.sh &
./kgdb.sh
....

link:rootfs_overlay/lkmc/kgdb.sh[] pauses the kernel for KGDB, and gives control back to GDB.

And now in GDB we do the usual:

....
break __x64_sys_write
continue
continue
continue
continue
....

And now you can count from KGDB!

If you do: `break __x64_sys_write` immediately after `./run-gdb --kgdb`, it fails with `KGDB: BP remove failed: <address>`. I think this is because it would break too early on the boot sequence, and KGDB is not yet ready.

See also:

* https://github.com/torvalds/linux/blob/v4.9/Documentation/DocBook/kgdb.tmpl
* https://stackoverflow.com/questions/22004616/qemu-kernel-debugging-with-kgdb/44197715#44197715

=== KGDB ARM

TODO: we would need a second serial for KGDB to work, but it is not currently supported on `arm` and `aarch64` with `-M virt` that we use: https://unix.stackexchange.com/questions/479085/can-qemu-m-virt-on-arm-aarch64-have-multiple-serial-ttys-like-such-as-pl011-t/479340#479340

One possible workaround for this would be to use <<kdb-arm>>.

Main more generic question: https://stackoverflow.com/questions/14155577/how-to-use-kgdb-on-arm

=== KGDB kernel modules

Just works as you would expect:

....
insmod timer.ko
./kgdb.sh
....

In GDB:

....
break lkmc_timer_callback
continue
continue
continue
....

and you now control the count.

=== KDB

KDB is a way to use KDB directly in your main console, without GDB.

Advantage over KGDB: you can do everything in one serial. This can actually be important if you only have one serial for both shell and .

Disadvantage: not as much functionality as GDB, especially when you use Python scripts. Notably, TODO confirm you can't see the the kernel source code and line step as from GDB, since the kernel source is not available on guest (ah, if only debugging information supported full source, or if the kernel had a crazy mechanism to embed it).

Run QEMU as:

....
./run --kdb
....

This passes `kgdboc=ttyS0` to the Linux CLI, therefore using our main console. Then QEMU:

....
[0]kdb> go
....

And now the `kdb>` prompt is responsive because it is listening to the main console.

After boot finishes, run the usual:

....
./count.sh &
./kgdb.sh
....

And you are back in KDB. Now you can count with:

....
[0]kdb> bp __x64_sys_write
[0]kdb> go
[0]kdb> go
[0]kdb> go
[0]kdb> go
....

And you will break whenever `__x64_sys_write` is hit.

You can get see further commands with:

....
[0]kdb> help
....

The other KDB commands allow you to step instructions, view memory, registers and some higher level kernel runtime data similar to the superior GDB Python scripts.

==== KDB graphic

You can also use KDB directly from the <<graphics,graphic>> window with:

....
./run --graphic --kdb
....

This setup could be used to debug the kernel on machines without serial, such as modern desktops.

This works because `--graphics` adds `kbd` (which stands for `KeyBoarD`!) to `kgdboc`.

==== KDB ARM

TODO neither `arm` and `aarch64` are working as of 1cd1e58b023791606498ca509256cc48e95e4f5b + 1.

`arm` seems to place and hit the breakpoint correctly, but no matter how many `go` commands I do, the `count.sh` stdout simply does not show.

`aarch64` seems to place the breakpoint correctly, but after the first `go` the kernel oopses with warning:

....
WARNING: CPU: 0 PID: 46 at /root/linux-kernel-module-cheat/submodules/linux/kernel/smp.c:416 smp_call_function_many+0xdc/0x358
....

and stack trace:

....
smp_call_function_many+0xdc/0x358
kick_all_cpus_sync+0x30/0x38
kgdb_flush_swbreak_addr+0x3c/0x48
dbg_deactivate_sw_breakpoints+0x7c/0xb8
kgdb_cpu_enter+0x284/0x6a8
kgdb_handle_exception+0x138/0x240
kgdb_brk_fn+0x2c/0x40
brk_handler+0x7c/0xc8
do_debug_exception+0xa4/0x1c0
el1_dbg+0x18/0x78
__arm64_sys_write+0x0/0x30
el0_svc_handler+0x74/0x90
el0_svc+0x8/0xc
....

My theory is that every serious ARM developer has JTAG, and no one ever tests this, and the kernel code is just broken.

== gdbserver

Step debug userland processes to understand how they are talking to the kernel.

First build `gdbserver` into the root filesystem:

....
./build-buildroot --config 'BR2_PACKAGE_GDB=y'
....

Then on guest, to debug link:userland/linux/myinsmod.c[]:

....
./gdbserver.sh ./linux/myinsmod.out hello.ko
....

Source: link:rootfs_overlay/lkmc/gdbserver.sh[].

And on host:

....
./run-gdbserver userland/linux/myinsmod.c
....

or alternatively with the path to the executable itself:

....
./run-gdbserver "$(./getvar userland_build_dir)/linux/myinsmod.out"
....

Bibliography: https://reverseengineering.stackexchange.com/questions/8829/cross-debugging-for-arm-mips-elf-with-qemu-toolchain/16214#16214

=== gdbserver BusyBox

Analogous to <<gdb-step-debug-userland-processes>>:

....
./gdbserver.sh ls
....

on host you need:

....
./run-gdbserver "$(./getvar buildroot_build_build_dir)"/busybox-*/busybox ls_main
....

=== gdbserver libc

Our setup gives you the rare opportunity to step debug libc and other system libraries.

For example in the guest:

....
./gdbserver.sh ./count.out
....

Then on host:

....
./run-gdbserver count
....

and inside GDB:

....
break sleep
continue
....

And you are now left inside the `sleep` function of our default libc implementation uclibc link:https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/tree/libc/unistd/sleep.c?h=v1.0.30#n91[`libc/unistd/sleep.c`]!

You can also step into the `sleep` call:

....
step
....

This is made possible by the GDB command that we use by default:

....
set sysroot ${common_buildroot_build_dir}/staging
....

which automatically finds unstripped shared libraries on the host for us.

See also: https://stackoverflow.com/questions/8611194/debugging-shared-libraries-with-gdbserver/45252113#45252113

=== gdbserver dynamic loader

TODO: try to step debug the dynamic loader. Would be even easier if `starti` is available: https://stackoverflow.com/questions/10483544/stopping-at-the-first-machine-code-instruction-in-gdb

Bibliography: https://stackoverflow.com/questions/20114565/gdb-step-into-dynamic-linkerld-so-code

== CPU architecture

The portability of the kernel and toolchains is amazing: change an option and most things magically work on completely different hardware.

To use `arm` instead of x86 for example:

....
./build-buildroot --arch arm
./run --arch arm
....

Debug:

....
./run --arch arm --gdb-wait
# On another terminal.
./run-gdb --arch arm
....

We also have one letter shorthand names for the architectures and `--arch` option:

....
# aarch64
./run -a A
# arm
./run -a a
# x86_64
./run -a x
....

Known quirks of the supported architectures are documented in this section.

=== x86_64

==== ring0

This example illustrates how reading from the x86 control registers with `mov crX, rax` can only be done from kernel land on ring0.

From kernel land:

....
insmod ring0.ko
....

works and output the registers, for example:

....
cr0 = 0xFFFF880080050033
cr2 = 0xFFFFFFFF006A0008
cr3 = 0xFFFFF0DCDC000
....

However if we try to do it from userland:

....
./ring0.out
....

stdout gives:

....
Segmentation fault
....

and dmesg outputs:

....
traps: ring0.out[55] general protection ip:40054c sp:7fffffffec20 error:0 in ring0.out[400000+1000]
....

Sources:

* link:kernel_modules/ring0.c[]
* link:lkmc/ring0.h[]
* link:userland/ring0.c[]

In both cases, we attempt to run the exact same code which is shared on the `ring0.h` header file.

Bibliography:

* https://stackoverflow.com/questions/7415515/how-to-access-the-control-registers-cr0-cr2-cr3-from-a-program-getting-segmenta/7419306#7419306
* https://stackoverflow.com/questions/18717016/what-are-ring-0-and-ring-3-in-the-context-of-operating-systems/44483439#44483439

=== arm

==== Run arm executable in aarch64

TODO Can you run arm executables in the aarch64 guest? https://stackoverflow.com/questions/22460589/armv8-running-legacy-32-bit-applications-on-64-bit-os/51466709#51466709

I've tried:

....
./run-toolchain --arch aarch64 gcc -- -static ~/test/hello_world.c -o "$(./getvar p9_dir)/a.out"
./run --arch aarch64 --eval-after '/mnt/9p/data/a.out'
....

but it fails with:

....
a.out: line 1: syntax error: unexpected word (expecting ")")
....

=== MIPS

We used to "support" it until f8c0502bb2680f2dbe7c1f3d7958f60265347005 (it booted) but dropped since one was testing it often.

If you want to revive and maintain it, send a pull request.

=== Other architectures

It should not be too hard to port this repository to any architecture that Buildroot supports. Pull requests are welcome.

== init

When the Linux kernel finishes booting, it runs an executable as the first and only userland process. This executable is called the `init` program.

The init process is then responsible for setting up the entire userland (or destroying everything when you want to have fun).

This typically means reading some configuration files (e.g. `/etc/initrc`) and forking a bunch of userland executables based on those files, including the very interactive shell that we end up on.

systemd provides a "popular" init implementation for desktop distros as of 2017.

BusyBox provides its own minimalistic init implementation which Buildroot, and therefore this repo, uses by default.

The `init` program can be either an executable shell text file, or a compiled ELF file. It becomes easy to accept this once you see that the `exec` system call handles both cases equally: https://unix.stackexchange.com/questions/174062/can-the-init-process-be-a-shell-script-in-linux/395375#395375

The `init` executable is searched for in a list of paths in the root filesystem, including `/init`, `/sbin/init` and a few others. For more details see: <<path-to-init>>

=== Replace init

To have more control over the system, you can replace BusyBox's init with your own.

The most direct way to replace `init` with our own is to just use the `init=` <<kernel-command-line-parameters,command line parameter>> directly:

....
./run --kernel-cli 'init=/lkmc/count.sh'
....

This just counts every second forever and does not give you a shell.

This method is not very flexible however, as it is hard to reliably pass multiple commands and command line arguments to the init with it, as explained at: <<init-environment>>.

For this reason, we have created a more robust helper method with the `--eval` option:

....
./run --eval 'echo "asdf qwer";insmod hello.ko;./linux/poweroff.out'
....

It is basically a shortcut for:

....
./run --kernel-cli 'init=/lkmc/eval_base64.sh - lkmc_eval="insmod hello.ko;./linux/poweroff.out"'
....

Source: link:rootfs_overlay/lkmc/eval_base64.sh[].

This allows quoting and newlines by base64 encoding on host, and decoding on guest, see: <<kernel-command-line-parameters-escaping>>.

It also automatically chooses between `init=` and `rcinit=` for you, see: <<path-to-init>>

`--eval` replaces BusyBox' init completely, which makes things more minimal, but also has has the following consequences:

* `/etc/fstab` mounts are not done, notably `/proc` and `/sys`, test it out with:
+
....
./run --eval 'echo asdf;ls /proc;ls /sys;echo qwer'
....
* no shell is launched at the end of boot for you to interact with the system. You could explicitly add a `sh` at the end of your commands however:
+
....
./run --eval 'echo hello;sh'
....

The best way to overcome those limitations is to use: <<init-busybox>>

If the script is large, you can add it to a gitignored file and pass that to `--eval` as in:

....
echo '
cd /lkmc
insmod hello.ko
./linux/poweroff.out
' > data/gitignore.sh
./run --eval "$(cat data/gitignore.sh)"
....

or add it to a file to the root filesystem guest and rebuild:

....
echo '#!/bin/sh
cd /lkmc
insmod hello.ko
./linux/poweroff.out
' > rootfs_overlay/lkmc/gitignore.sh
chmod +x rootfs_overlay/lkmc/gitignore.sh
./build-buildroot
./run --kernel-cli 'init=/lkmc/gitignore.sh'
....

Remember that if your init returns, the kernel will panic, there are just two non-panic possibilities:

* run forever in a loop or long sleep
* `poweroff` the machine

==== poweroff.out

Just using BusyBox' `poweroff` at the end of the `init` does not work and the kernel panics:

....
./run --eval poweroff
....

because BusyBox' `poweroff` tries to do some fancy stuff like killing init, likely to allow userland to shutdown nicely.

But this fails when we are `init` itself!

BusyBox' `poweroff` works more brutally and effectively if you add `-f`:

....
./run --eval 'poweroff -f'
....

but why not just use our minimal `./linux/poweroff.out` and be done with it?

....
./run --eval './linux/poweroff.out'
....

Source: link:userland/linux/poweroff.c[]

This also illustrates how to shutdown the computer from C: https://stackoverflow.com/questions/28812514/how-to-shutdown-linux-using-c-or-qt-without-call-to-system

==== sleep_forever.out

I dare you to guess what this does:

....
./run --eval './posix/sleep_forever.out'
....

Source: link:userland/posix/sleep_forever.c[]

This executable is a convenient simple init that does not panic and sleeps instead.

==== time_boot.out

Get a reasonable answer to "how long does boot take in guest time?":

....
./run --eval-after './linux/time_boot.c'
....

Source: link:userland/linux/time_boot.out[]

That executable writes to `dmesg` directly through `/dev/kmsg` a message of type:

....
[    2.188242] /path/to/linux-kernel-module-cheat/userland/linux/time_boot.c
....

which tells us that boot took `2.188242` seconds based on the dmesg timestamp.

Bibliography: https://stackoverflow.com/questions/12683169/measure-time-taken-for-linux-kernel-from-bootup-to-userpace/46517014#46517014

[[init-busybox]]
=== Run command at the end of BusyBox init

Use the `--eval-after` option is for you rely on something that BusyBox' init set up for you like `/etc/fstab`:

....
./run --eval-after 'echo asdf;ls /proc;ls /sys;echo qwer'
....

After the commands run, you are left on an interactive shell.

The above command is basically equivalent to:

....
./run --kernel-cli-after-dash 'lkmc_eval="insmod hello.ko;./linux/poweroff.out;"'
....

where the `lkmc_eval` option gets evaled by our default link:rootfs_overlay/etc/init.d/S98[] startup script.

Except that `--eval-after` is smarter and uses `base64` encoding.

Alternatively, you can also add the comamdns to run to a new `init.d` entry to run at the end o the BusyBox init:

....
cp rootfs_overlay/etc/init.d/S98 rootfs_overlay/etc/init.d/S99.gitignore
vim rootfs_overlay/etc/init.d/S99.gitignore
./build-buildroot
./run
....

and they will be run automatically before the login prompt.

Scripts under `/etc/init.d` are run by `/etc/init.d/rcS`, which gets called by the line `::sysinit:/etc/init.d/rcS` in link:rootfs_overlay/etc/inittab[`/etc/inittab`].

=== Path to init

The init is selected at:

* initrd or initramfs system: `/init`, a custom one can be set with the `rdinit=` <<kernel-command-line-parameters,kernel command line parameter>>
* otherwise: default is `/sbin/init`, followed by some other paths, a custom one can be set with `init=`

More details: https://unix.stackexchange.com/questions/30414/what-can-make-passing-init-path-to-program-to-the-kernel-not-start-program-as-i/430614#430614

=== Init environment

Documented at link:https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html[]:

____
The kernel parses parameters from the kernel command line up to "-"; if it doesn't recognize a parameter and it doesn't contain a '.', the parameter gets passed to init: parameters with '=' go into init's environment, others are passed as command line arguments to init. Everything after "-" is passed as an argument to init.
____

And you can try it out with:

....
./run --kernel-cli 'init=/lkmc/linux/init_env_poweroff.out - asdf=qwer zxcv'
....

Output:

....
args:
/lkmc/linux/init_env_poweroff.out
-
zxcv

env:
HOME=/
TERM=linux
asdf=qwer
....

Source: link:userland/linux/init_env_poweroff.c[].

==== init arguments

The annoying dash `-` gets passed as a parameter to `init`, which makes it impossible to use this method for most non custom executables.

Arguments with dots that come after `-` are still treated specially (of the form `subsystem.somevalue`) and disappear, from args, e.g.:

....
./run --kernel-cli 'init=/lkmc/linux/init_env_poweroff.out - /lkmc/linux/poweroff.out'
....

outputs:

....
args
/lkmc/linux/init_env_poweroff.out
-
ab
....

so see how `a.b` is gone.

The simple workaround is to just create a shell script that does it, e.g. as we've done at: link:rootfs_overlay/lkmc/gem5_exit.sh[].

==== init environment env

Wait, where do `HOME` and `TERM` come from? (greps the kernel). Ah, OK, the kernel sets those by default: https://github.com/torvalds/linux/blob/94710cac0ef4ee177a63b5227664b38c95bbf703/init/main.c#L173

....
const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
....

==== BusyBox shell init environment

On top of the Linux kernel, the BusyBox `/bin/sh` shell will also define other variables.

We can explore the shenanigans that the shell adds on top of the Linux kernel with:

....
./run --kernel-cli 'init=/bin/sh'
....

From there we observe that:

....
env
....

gives:

....
SHLVL=1
HOME=/
TERM=linux
PWD=/
....

therefore adding `SHLVL` and `PWD` to the default kernel exported variables.

Furthermore, to increase confusion, if you list all non-exported shell variables https://askubuntu.com/questions/275965/how-to-list-all-variables-names-and-their-current-values with:

....
set
....

then it shows more variables, notably:

....
PATH='/sbin:/usr/sbin:/bin:/usr/bin'
....

===== BusyBox shell initrc files

Login shells source some default files, notably:

....
/etc/profile
$HOME/.profile
....

In our case, `HOME` is set to `/` presumably by `init` at: https://git.busybox.net/busybox/tree/init/init.c?id=5059653882dbd86e3bbf48389f9f81b0fac8cd0a#n1114

We provide `/.profile` from link:rootfs_overlay/.profile[], and use the default BusyBox `/etc/profile`.

The shell knows that it is a login shell if the first character of `argv[0]` is `-`, see also: https://stackoverflow.com/questions/2050961/is-argv0-name-of-executable-an-accepted-standard-or-just-a-common-conventi/42291142#42291142

When we use just `init=/bin/sh`, the Linux kernel sets `argv[0]` to `/bin/sh`, which does not start with `-`.

However, if you use `::respawn:-/bin/sh` on inttab described at <<tty>>, BusyBox' init sets `argv[0][0]` to `-`, and so does `getty`. This can be observed with:

....
cat /proc/$$/cmdline
....

where `$$` is the PID of the shell itself: https://stackoverflow.com/questions/21063765/get-pid-in-shell-bash

Bibliography: https://unix.stackexchange.com/questions/176027/ash-profile-configuration-file

== initrd

The kernel can boot from an CPIO file, which is a directory serialization format much like tar: https://superuser.com/questions/343915/tar-vs-cpio-what-is-the-difference

The bootloader, which for us is provided by QEMU itself, is then configured to put that CPIO into memory, and tell the kernel that it is there.

This is very similar to the kernel image itself, which already gets put into memory by the QEMU `-kernel` option.

With this setup, you don't even need to give a root filesystem to the kernel: it just does everything in memory in a ramfs.

To enable initrd instead of the default ext2 disk image, do:

....
./build-buildroot --initrd
./run --initrd
....

By looking at the QEMU run command generated, you can see that we didn't give the `-drive` option at all:

....
cat "$(./getvar run_dir)/run.sh"
....

Instead, we used the QEMU `-initrd` option to point to the `.cpio` filesystem that Buildroot generated for us.

Try removing that `-initrd` option to watch the kernel panic without rootfs at the end of boot.

When using `.cpio`, there can be no <<disk-persistency,filesystem persistency>> across boots, since all file operations happen in memory in a tmpfs:

....
date >f
poweroff
cat f
# can't open 'f': No such file or directory
....

which can be good for automated tests, as it ensures that you are using a pristine unmodified system image every time.

Not however that we already disable disk persistency by default on ext2 filesystems even without `--initrd`: <<disk-persistency>>.

One downside of this method is that it has to put the entire filesystem into memory, and could lead to a panic:

....
end Kernel panic - not syncing: Out of memory and no killable processes...
....

This can be solved by increasing the memory with:

....
./run --initrd --memory 256M
....

The main ingredients to get initrd working are:

* `BR2_TARGET_ROOTFS_CPIO=y`: make Buildroot generate `images/rootfs.cpio` in addition to the other images.
+
It is also possible to compress that image with other options.
* `qemu -initrd`: make QEMU put the image into memory and tell the kernel about it.
* `CONFIG_BLK_DEV_INITRD=y`: Compile the kernel with initrd support, see also: https://unix.stackexchange.com/questions/67462/linux-kernel-is-not-finding-the-initrd-correctly/424496#424496
+
Buildroot forces that option when `BR2_TARGET_ROOTFS_CPIO=y` is given

TODO: how does the bootloader inform the kernel where to find initrd? https://unix.stackexchange.com/questions/89923/how-does-linux-load-the-initrd-image

=== initrd in desktop distros

Most modern desktop distributions have an initrd in their root disk to do early setup.

The rationale for this is described at: https://en.wikipedia.org/wiki/Initial_ramdisk

One obvious use case is having an encrypted root filesystem: you keep the initrd in an unencrypted partition, and then setup decryption from there.

I think GRUB then knows read common disk formats, and then loads that initrd to memory with a `/boot/grub/grub.cfg` directive of type:

....
initrd /initrd.img-4.4.0-108-generic
....

Related: https://stackoverflow.com/questions/6405083/initrd-and-booting-the-linux-kernel

=== initramfs

initramfs is just like <<initrd>>, but you also glue the image directly to the kernel image itself using the kernel's build system.

Try it out with:

....
./build-buildroot --initramfs
./build-linux --initramfs
./run --initramfs
....

Notice how we had to rebuild the Linux kernel this time around as well after Buildroot, since in that build we will be gluing the CPIO to the kernel image.

Now, once again, if we look at the QEMU run command generated, we see all that QEMU needs is the `-kernel` option, no `-drive` not even `-initrd`! Pretty cool:

....
cat "$(./getvar run_dir)/run.sh"
....

It is also interesting to observe how this increases the size of the kernel image if you do a:

....
ls -lh "$(./getvar linux_image)"
....

before and after using initramfs, since the `.cpio` is now glued to the kernel image.

Don't forget that to stop using initramfs, you must rebuild the kernel without `--initramfs` to get rid of the attached CPIO image:

....
./build-linux
./run
....

Alternatively, consider using <<linux-kernel-build-variants>> if you need to switch between initramfs and non initramfs often:

....
./build-buildroot --initramfs
./build-linux --initramfs --linux-build-id initramfs
./run --initramfs --linux-build-id
....

Setting up initramfs is very easy: our scripts just set `CONFIG_INITRAMFS_SOURCE` to point to the CPIO path.

http://nairobi-embedded.org/initramfs_tutorial.html shows a full manual setup.

=== rootfs

This is how `/proc/mounts` shows the root filesystem:

* hard disk: `/dev/root on / type ext2 (rw,relatime,block_validity,barrier,user_xattr)`. That file does not exist however.
* initrd: `rootfs on / type rootfs (rw)`
* initramfs: `rootfs on / type rootfs (rw)`

TODO: understand `/dev/root` better:

* https://unix.stackexchange.com/questions/295060/why-on-some-linux-systems-does-the-root-filesystem-appear-as-dev-root-instead
* https://superuser.com/questions/1213770/how-do-you-determine-the-root-device-if-dev-root-is-missing

==== /dev/root

See: <<rootfs>>

=== gem5 initrd

TODO we were not able to get it working yet: https://stackoverflow.com/questions/49261801/how-to-boot-the-linux-kernel-with-initrd-or-initramfs-with-gem5

This would require gem5 to load the CPIO into memory, just like QEMU. Grepping `initrd` shows some ARM hits under:

....
src/arch/arm/linux/atag.hh
....

but they are commented out.

=== gem5 initramfs

This could in theory be easier to make work than initrd since the emulator does not have to do anything special.

However, it didn't: boot fails at the end because it does not see the initramfs, but rather tries to open our dummy root filesystem, which unsurprisingly does not have a format in a way that the kernel understands:

....
VFS: Cannot open root device "sda" or unknown-block(8,0): error -5
....

We think that this might be because gem5 boots directly `vmlinux`, and not from the final compressed images that contain the attached rootfs such as `bzImage`, which is what QEMU does, see also: <<vmlinux-vs-bzimage-vs-zimage-vs-image>>.

To do this failed test, we automatically pass a dummy disk image as of gem5 7fa4c946386e7207ad5859e8ade0bbfc14000d91 since the scripts don't handle a missing `--disk-image` well, much like is currently done for <<baremetal>>.

Interestingly, using initramfs significantly slows down the gem5 boot, even though it did not work. For example, we've observed a 4x slowdown of as 17062a2e8b6e7888a14c3506e9415989362c58bf for aarch64. This must be because expanding the large attached CPIO must be expensive. We can clearly see from the kernel logs that the kernel just hangs at a point after the message `PCI: CLS 0 bytes, default 64` for a long time before proceeding further.

== Device tree

The device tree is a Linux kernel defined data structure that serves to inform the kernel how the hardware is setup.

<<platform_device>> contains a minimal runnable example of device tree manipulation.

Device trees serve to reduce the need for hardware vendors to patch the kernel: they just provide a device tree file instead, which is much simpler.

x86 does not use it device trees, but many other archs to, notably ARM.

This is notably because ARM boards:

* typically don't have discoverable hardware extensions like PCI, but rather just put everything on an SoC with magic register addresses
* are made by a wide variety of vendors due to ARM's licensing business model, which increases variability

The Linux kernel itself has several device trees under `./arch/<arch>/boot/dts`, see also: https://stackoverflow.com/questions/21670967/how-to-compile-dts-linux-device-tree-source-files-to-dtb/42839737#42839737

=== DTB files

Files that contain device trees have the `.dtb` extension when compiled, and `.dts` when in text form.

You can convert between those formats with:

....
"$(./getvar buildroot_host_dir)"/bin/dtc -I dtb -O dts -o a.dts a.dtb
"$(./getvar buildroot_host_dir)"/bin/dtc -I dts -O dtb -o a.dtb a.dts
....

Buildroot builds the tool due to `BR2_PACKAGE_HOST_DTC=y`.

On Ubuntu 18.04, the package is named:

....
sudo apt-get install device-tree-compiler
....

See also: https://stackoverflow.com/questions/14000736/tool-to-visualize-the-device-tree-file-dtb-used-by-the-linux-kernel/39931834#39931834

Device tree files are provided to the emulator just like the root filesystem and the Linux kernel image.

In real hardware, those components are also often provided separately. For example, on the Raspberry Pi 2, the SD card must contain two partitions:

* the first contains all magic files, including the Linux kernel and the device tree
* the second contains the root filesystem

See also: https://stackoverflow.com/questions/29837892/how-to-run-a-c-program-with-no-os-on-the-raspberry-pi/40063032#40063032

=== Device tree syntax

Good format descriptions:

* https://www.raspberrypi.org/documentation/configuration/device-tree.md

Minimal example

....
/dts-v1/;

/ {
    a;
};
....

Check correctness with:

....
dtc a.dts
....

Separate nodes are simply merged by node path, e.g.:

....
/dts-v1/;

/ {
    a;
};

/ {
    b;
};
....

then `dtc a.dts` gives:

....
/dts-v1/;

/ {
        a;
        b;
};
....

=== Get device tree from a running kernel

https://unix.stackexchange.com/questions/265890/is-it-possible-to-get-the-information-for-a-device-tree-using-sys-of-a-running/330926#330926

This is specially interesting because QEMU and gem5 are capable of generating DTBs that match the selected machine depending on dynamic command line parameters for some types of machines.

So observing the device tree from the guest allows to easily see what the emulator has generated.

Compile the `dtc` tool into the root filesystem:

....
./build-buildroot \
  --arch aarch64 \
  --config 'BR2_PACKAGE_DTC=y' \
  --config 'BR2_PACKAGE_DTC_PROGRAMS=y' \
;
....

`-M virt` for example, which we use by default for `aarch64`, boots just fine without the `-dtb` option:

....
./run --arch aarch64
....

Then, from inside the guest:

....
dtc -I fs -O dts /sys/firmware/devicetree/base
....

contains:

....
        cpus {
                #address-cells = <0x1>;
                #size-cells = <0x0>;

                cpu@0 {
                        compatible = "arm,cortex-a57";
                        device_type = "cpu";
                        reg = <0x0>;
                };
        };
....

=== Device tree emulator generation

Since emulators know everything about the hardware, they can automatically generate device trees for us, which is very convenient.

This is the case for both QEMU and gem5.

For example, if we increase the <<number-of-cores,number of cores>> to 2:

....
./run --arch aarch64 --cpus 2
....

QEMU automatically adds a second CPU to the DTB!

....
                cpu@0 {
                cpu@1 {
....

The action seems to be happening at: `hw/arm/virt.c`.

You can dump the DTB QEMU generated with:

....
./run --arch aarch64 -- -machine dumpdtb=dtb.dtb
....

as mentioned at: https://lists.gnu.org/archive/html/qemu-discuss/2017-02/msg00051.html

<<gem5-fs_biglittle>> 2a9573f5942b5416fb0570cf5cb6cdecba733392 can also generate its own DTB.

gem5 can generate DTBs on ARM with `--generate-dtb`. The generated DTB is placed in the <<m5out-directory>> named as `system.dtb`.

== KVM

link:https://en.wikipedia.org/wiki/Kernel-based_Virtual_Machine[KVM] is Linux kernel interface that <<benchmark-linux-kernel-boot,greatly speeds up>> execution of virtual machines.

You can make QEMU or gem5  by passing enabling KVM with:

....
./run --kvm
....

but it was broken in gem5 with pending patches: https://www.mail-archive.com/gem5-users@gem5.org/msg15046.html It fails immediately on:

....
panic: KVM: Failed to enter virtualized mode (hw reason: 0x80000021)
....

KVM works by running userland instructions natively directly on the real hardware instead of running a software simulation of those instructions.

Therefore, KVM only works if you the host architecture is the same as the guest architecture. This means that this will likely only work for x86 guests since almost all development machines are x86 nowadays. Unless you are link:https://www.youtube.com/watch?v=8ItXpmLsINs[running an ARM desktop for some weird reason] :-)

We don't enable KVM by default because:

* it limits visibility, since more things are running natively:
** can't use <<gdb,GDB>>
** can't do <<tracing,instruction tracing>>
** on gem5, you lose <<gem5-run-benchmark,cycle counts>> and therefor any notion of performance
* QEMU kernel boots are already <<benchmark-linux-kernel-boot,fast enough>> for most purposes without it

One important use case for KVM is to fast forward gem5 execution, often to skip boot, take a <<gem5-checkpoint>>, and then move on to a more detailed and slow simulation

=== KVM arm

TODO: we haven't gotten it to work yet, but it should be doable, and this is an outline of how to do it. Just don't expect this to tested very often for now.

We can test KVM on arm by running this repository inside an Ubuntu arm QEMU VM.

This produces no speedup of course, since the VM is already slow since it cannot use KVM on the x86 host.

First, obtain an Ubuntu arm64 virtual machine as explained at: https://askubuntu.com/questions/281763/is-there-any-prebuilt-qemu-ubuntu-image32bit-online/1081171#1081171

Then, from inside that image:

....
sudo apt-get install git
git clone https://github.com/cirosantilli/linux-kernel-module-cheat
cd linux-kernel-module-cheat
sudo ./setup -y
....

and then proceed exactly as in <<prebuilt>>.

We don't want to build the full Buildroot image inside the VM as that would be way too slow, thus the recommendation for the prebuilt setup.

TODO: do the right thing and cross compile QEMU and gem5. gem5's Python parts might be a pain. QEMU should be easy: https://stackoverflow.com/questions/26514252/cross-compile-qemu-for-arm

== User mode simulation

Both QEMU and gem5 have an user mode simulation mode in addition to full system simulation that we consider elsewhere in this project.

In QEMU, it is called just <<qemu-user-mode-getting-started,"user mode">>, and in gem5 it is called <<gem5-syscall-emulation-mode,syscall emulation mode>>.

In both, the basic idea is the same.

User mode simulation takes regular userland executables of any arch as input and executes them directly, without booting a kernel.

Instead of simulating the full system, it translates normal instructions like in full system mode, but magically forwards system calls to the host OS.

Advantages over full system simulation:

* the simulation may <<user-mode-vs-full-system-benchmark,run faster>> since you don't have to simulate the Linux kernel and several device models
* you don't need to build your own kernel or root filesystem, which saves time. You still need a toolchain however, but the pre-packaged ones may work fine.

Disadvantages:

* lower guest to host portability:
** TODO confirm: host OS == guest OS?
** TODO confirm: the host Linux kernel should be newer than the kernel the executable was built for.
+
It may still work even if that is not the case, but could fail is a missing system call is reached.
+
The target Linux kernel of the executable is a GCC toolchain build-time configuration.
** emulator implementers have to keep up with libc changes, some of which break even a C hello world due setup code executed before main.
+
See also: <<user-mode-simulation-with-glibc>>
* cannot be used to test the Linux kernel or any devices, and results are less representative of a real system since we are faking more

=== QEMU user mode getting started

Let's run link:userland/c/print_argv.c[] built with the Buildroot toolchain on QEMU user mode:

....
./build user-mode-qemu
./run \
  --userland userland/c/print_argv.c \
  --userland-args='asdf "qw er"' \
;
....

Output:

....
/path/to/linux-kernel-module-cheat/out/userland/default/x86_64/c/print_argv.out
asdf
qw er
....

`./run --userland` path resolution is analogous to <<baremetal-setup-getting-started,that of `./run --baremetal`>>.

`./build user-mode-qemu` first builds Buildroot, and then runs `./build-userland`, which is further documented at: <<userland-setup>>. It also builds QEMU. If you ahve already done a <<qemu-buildroot-setup>> previously, this will be very fast.

If you modify the userland programs, rebuild simply with:

....
./build-userland
....

==== User mode GDB

It's nice when <<gdb,the obvious>> just works, right?

....
./run \
  --arch aarch64 \
  --gdb-wait \
  --userland userland/c/print_argv.c \
  --userland-args 'asdf "qw er"' \
;
....

and on another shell:

....
./run-gdb \
  --arch aarch64 \
  --userland userland/c/print_argv.c \
  main \
;
....

Or alternatively, if you are using <<tmux>>, do everything in one go with:

....
./run \
  --arch aarch64 \
  --gdb \
  --userland userland/c/print_argv.c \
  --userland-args 'asdf "qw er"' \
;
....

To stop at the very first instruction of a freestanding program, just use `--no-continue` TODO example.

=== User mode tests

Automatically run all userland tests that can be run in user mode simulation, and check that they exit with status 0:

....
./build --all-archs test-user-mode
./test-user-mode --all-archs --all-emulators
....

Or just for QEMU:

....
./build --all-archs test-user-mode-qemu
./test-user-mode --all-archs --emulator qemu
....

Source: link:test-user-mode[]

This script skips a manually configured list of tests, notably:

* tests that depend on a full running kernel and cannot be run in user mode simulation, e.g. those that rely on kernel modules
* tests that require user interaction
* tests that take perceptible ammounts of time
* known bugs we didn't have time to fix ;-)

Tests under link:userland/libs/[] depend on certain libraries being available on the target, e.g. <<blas>> for link:userland/libs/blas[]. They are not run by default, but can be enabled with `--package` and `--package-all`.

The gem5 tests require building statically with build id `static`, see also: <<gem5-syscall-emulation-mode>>. TODO automate this better.

See: <<test-this-repo>> for more useful testing tips.

=== User mode Buildroot executables

If you followed <<qemu-buildroot-setup>>, you can now run the executables created by Buildroot directly as:

....
./run \
  --userland "$(./getvar buildroot_target_dir)/bin/echo" \
  --userland-args='asdf' \
;
....

Here is an interesting examples of this: <<linux-test-project>>

=== User mode simulation with glibc

At 125d14805f769104f93c510bedaa685a52ec025d we <<libc-choice,moved Buildroot from uClibc to glibc>>, and caused some user mode pain, which we document here.

==== FATAL: kernel too old

Happens on all gem5 <<user-mode-simulation>> setups, but not on QEMU on Ubuntu 18.04 host.

glibc has a check for kernel version, likely obtained from the `uname` syscall, and if the kernel is not new enough, it quits.

Determining the right number to put there is of course highly non-trivial and would require an extensive userland test suite, which most emulator don't have.

We don't have this failure for QEMU on an 18.04 host, only gem5.

QEMU by default copies the host `uname` value. However, our scripts set it by default to our the latest Buildroot kernel version with QEMU's `-r` option, which is exposed as `--kernel-version`:

....
./run --arch aarch64 --kernel-version 4.18 --userland userland/posix/uname.c
....

Source: link:userland/posix/uname.c[].

gem5 does not have such runtime configuration, but the error can be worked around for now by patching the hardcoded Linux version as mentioned at: https://stackoverflow.com/questions/48959349/how-to-solve-fatal-kernel-too-old-when-running-gem5-in-syscall-emulation-se-m to be a recent Linux version such as `v4.17.0`.

We override the default QEMU uname because otherwise all executables fail with "kernel too old" on older Ubuntu hosts. The downside is that you might hit syscalls which your host does not have for QEMU to forward to, but we'll let you be the judge of that.

The QEMU source that does this is at: https://github.com/qemu/qemu/blob/v3.1.0/linux-user/syscall.c#L8931

In gem5, there are tons of missing syscalls, and that number currently just gets bumped up randomly from time to time when someone gets fed up:

* https://stackoverflow.com/questions/53085048/how-to-compile-and-run-an-executable-in-gem5-syscall-emulation-mode-with-se-py/53085049#53085049
* https://gem5-review.googlesource.com/c/public/gem5/+/15855

The ID is just hardcoded on the source:

==== stack smashing detected

For some reason QEMU / glibc x86_64 picks up the host libc, which breaks things.

Other archs work as they different host libc is skipped. <<user-mode-static-executables>> also work.

We have worked around this with with https://bugs.launchpad.net/qemu/+bug/1701798/comments/12 from the thread: https://bugs.launchpad.net/qemu/+bug/1701798 by creating the file: link:rootfs_overlay/etc/ld.so.cache[] which is a symlink to a file that cannot exist: `/dev/null/nonexistent`.

Reproduction:

....
rm -f "$(./getvar buildroot_target_dir)/etc/ld.so.cache"
./run --userland userland/c/hello.c
./run --userland userland/c/hello.c --qemu-which host
....

Outcome:

....
*** stack smashing detected ***: <unknown> terminated
qemu: uncaught target signal 6 (Aborted) - core dumped
....

To get things working again, restore `ld.so.cache` with:

....
./build-buildroot
....

I've also tested on an Ubuntu 16.04 guest and the failure is different one:

....
qemu: uncaught target signal 4 (Illegal instruction) - core dumped
....

A non-QEMU-specific example of stack smashing is shown at: https://stackoverflow.com/questions/1345670/stack-smashing-detected/51897264#51897264

Tested at: 2e32389ebf1bedd89c682aa7b8fe42c3c0cf96e5 + 1.

=== User mode static executables

Example:

....
./build-userland \
  --arch aarch64 \
  --static \
;
./run \
  --arch aarch64 \
  --static \
  --userland userland/c/print_argv.c \
  --userland-args 'asdf "qw er"' \
;
....

Running dynamically linked executables in QEMU requires pointing it to the root filesystem with the `-L` option so that it can find the dynamic linker and shared libraries.

We pass `-L` by default, so everything just works.

However, in case something goes wrong, you can also try statically linked executables, since this mechanism tends to be a bit more stable, for example:

* gem5 user mode currently only supports static executables: <<gem5-syscall-emulation-mode>>
* QEMU x86_64 guest on x86_64 host was failing with <<stack-smashing-detected>>, but we found a workaround

==== User mode static executables with dynamic libraries

One limitation of static executables is that Buildroot mostly only builds dynamic versions of libraries (the libc is an exception).

So programs that rely on those libraries might not compile as GCC can't find the `.a` version of the library.

For example, if we try to build <<blas>> statically:

....
./build-userland --package openblas --static -- userland/libs/openblas/hello.c
....

it fails with:

....
ld: cannot find -lopenblas
....

=== gem5 syscall emulation mode

Less robust than QEMU's, but still usable:

* https://stackoverflow.com/questions/48986597/when-should-you-use-full-system-fs-vs-syscall-emulation-se-with-userland-program
* https://stackoverflow.com/questions/48959349/how-to-solve-fatal-kernel-too-old-when-running-gem5-in-syscall-emulation-se-m

There are much more unimplemented syscalls in gem5 than in QEMU. Many of those are trivial to implement however.

As of 185c2730cc78d5adda683d76c0e3b35e7cb534f0, dynamically linked executables only work on x86, and they can only use the host libraries, which is ugly:

* https://stackoverflow.com/questions/50542222/how-to-run-a-dynamically-linked-executable-syscall-emulation-mode-se-py-in-gem5
* https://www.mail-archive.com/gem5-users@gem5.org/msg15585.html

If you try dynamically linked executables on ARM, they fail with:

....
fatal: Unable to open dynamic executable's interpreter.
....

So let's just play with some static ones:

....
./build-userland \
  --arch aarch64 \
  --static \
;
./run \
  --arch aarch64 \
  --emulator gem5 \
  --userland userland/c/print_argv.c \
  --userland-args 'asdf "qw er"' \
;
....

TODO: how to escape spaces on the command line arguments?

<<user-mode-gdb,GDB step debug>> also works normally on gem5:

....
./run \
  --arch aarch64 \
  --emulator gem5 \
  --gdb-wait \
  --static \
  --userland userland/c/print_argv.c \
  --userland-args 'asdf "qw er"' \
;
./run-gdb \
  --arch aarch64 \
  --emulator gem5 \
  --static \
  --userland userland/c/print_argv.c \
  main \
;
....

==== gem5 syscall emulation exit status

As of gem5 7fa4c946386e7207ad5859e8ade0bbfc14000d91, the crappy `se.py` script does not forward the exit status of syscall emulation mode, you can test it with:

....
./run --dry-run --emulator gem5 --static --userland userland/c/false.c
....

Source: link:userland/c/false.c[].

Then manually run the generated gem5 CLI, and do:

....
echo $?
....

and the output is always `0`.

Instead, it just outputs a message to stdout just like for <<m5-fail>>:

....
Simulated exit code not 0! Exit code is 1
....

which we parse in link:run[] and then exit with the correct result ourselves...

Related thread: https://stackoverflow.com/questions/56032347/is-there-a-way-to-identify-if-gem5-run-got-over-successfully

==== gem5 syscall emulation mode program stdin

gem5 shows its own stdout to terminal, and does not allow you to type stdin to programs.

Instead, you must pass stdin non-interactively with the through a file with the `--se.py --input` option, e.g.:

....
printf a > f
./run --emulator gem5 --userland userland/c/getchar.c --static -- --input f
....

leads to gem5 output:

....
enter a character: you entered: a
....

Source: link:userland/c/getchar.c[]

==== User mode vs full system benchmark

Let's see if user mode runs considerably faster than full system or not.

First we build Dhrystone manually statically since dynamic linking is broken in gem5: <<gem5-syscall-emulation-mode>>.

gem5 user mode:

....
./build-buildroot --arch arm --config 'BR2_PACKAGE_DHRYSTONE=y'
make \
  -B \
  -C "$(./getvar --arch arm buildroot_build_build_dir)/dhrystone-2" \
  CC="$(./run-toolchain --arch arm --print-tool gcc)" \
  CFLAGS=-static \
;
time \
  ./run \
  --arch arm \
  --emulator gem5 \
  --userland "$(./getvar --arch arm buildroot_build_build_dir)/dhrystone-2/dhrystone" \
  --userland-args 'asdf qwer' \
;
....

gem5 full system:

....
time \
  ./run \
  --arch arm \
  --eval-after './gem5.sh' \
  --emulator gem5
  --gem5-readfile 'dhrystone 100000' \
;
....

QEMU user mode:

....
time qemu-arm "$(./getvar --arch arm buildroot_build_build_dir)/dhrystone-2/dhrystone" 100000000
....

QEMU full system:

....
time \
  ./run \
  --arch arm \
  --eval-after 'time dhrystone 100000000;./linux/poweroff.out' \
;
....

Result on <<p51>> at bad30f513c46c1b0995d3a10c0d9bc2a33dc4fa0:

* gem5 user: 33 seconds
* gem5 full system: 51 seconds
* QEMU user: 45 seconds
* QEMU full system: 223 seconds

=== QEMU user mode quirks

==== QEMU user mode does not show stdout immediately

At 8d8307ac0710164701f6e14c99a69ee172ccbb70 + 1, I noticed that if you run link:userland/posix/count.c[]:

....
./run --userland userland/posix/count.c --userland-args 3
....

it first waits for 3 seconds, and then dumps all the output at once, instead of counting once every second as expected.

The same can be reproduced by copying the raw QEMU command and piping it through `tee`, so I don't think it is a bug in our setup:

....
/path/to/linux-kernel-module-cheat/out/qemu/default/x86_64-linux-user/qemu-x86_64 \
  -L /path/to/linux-kernel-module-cheat/out/buildroot/build/default/x86_64/target \
  /path/to/linux-kernel-module-cheat/out/userland/default/x86_64/posix/count.out \
  3 \
| tee
....

TODO: investigate further and then possibly post on QEMU mailing list.

===== QEMU user mode does not show errors

Similarly to <<qemu-user-mode-does-not-show-stdout-immediately>>, QEMU error messages do not show at all through pipes.

In particular, it does not say anything if you pass it a non-existing executable:

....
qemu-x86_64 asdf | cat
....

So we just check ourselves manually

== Kernel module utilities

=== insmod

link:https://git.busybox.net/busybox/tree/modutils/insmod.c?h=1_29_3[Provided by BusyBox]:

....
./run --eval-after 'insmod hello.ko'
....

=== myinsmod

If you are feeling raw, you can insert and remove modules with our own minimal module inserter and remover!

....
# init_module
./linux/myinsmod.out hello.ko
# finit_module
./linux/myinsmod.out hello.ko "" 1
./linux/myrmmod.out hello
....

which teaches you how it is done from C code.

Source:

* link:userland/linux/myinsmod.c[]
* link:userland/linux/myrmmod.c[]

The Linux kernel offers two system calls for module insertion:

* `init_module`
* `finit_module`

and:

....
man init_module
....

documents that:

____
The finit_module() system call is like init_module(), but reads the module to be loaded from the file descriptor fd. It is useful when the authenticity of a kernel module can be determined from its location in the filesystem; in cases where that is possible, the overhead of using cryptographically signed modules to determine the authenticity of a module can be avoided. The param_values argument is as for init_module().
____

`finit` is newer and was added only in v3.8. More rationale: https://lwn.net/Articles/519010/

Bibliography: https://stackoverflow.com/questions/5947286/how-to-load-linux-kernel-modules-from-c-code

=== modprobe

Implemented as a BusyBox applet by default: https://git.busybox.net/busybox/tree/modutils/modprobe.c?h=1_29_stable

`modprobe` searches for modules installed under:

....
ls /lib/modules/<kernel_version>
....

and specified in the `modules.order` file.

This is the default install path for `CONFIG_SOME_MOD=m` modules built with `make modules_install` in the Linux kernel tree, with root path given by `INSTALL_MOD_PATH`, and therefore canonical in that sense.

Currently, there are only two kinds of kernel modules that you can try out with `modprobe`:

* modules built with Buildroot, see: <<kernel_modules-package>>
* modules built from the kernel tree itself, see: <<dummy-irq>>

We are not installing out custom `./build-modules` modules there, because:

* we don't know the right way. Why is there no `install` or `install_modules` target for kernel modules?
+
This can of course be solved by running Buildroot in verbose mode, and copying whatever it is doing, initial exploration at: https://stackoverflow.com/questions/22783793/how-to-install-kernel-modules-from-source-code-error-while-make-process/53169078#53169078
* we would have to think how to not have to include the kernel modules twice in the root filesystem, but still have <<9p>> working for fast development as described at: <<your-first-kernel-module-hack>>

=== kmod

The more "reference" kernel.org implementation of `lsmod`, `insmod`, `rmmod`, etc.: https://git.kernel.org/pub/scm/utils/kernel/kmod/kmod.git

Default implementation on desktop distros such as Ubuntu 16.04, where e.g.:

....
ls -l /bin/lsmod
....

gives:

....
lrwxrwxrwx 1 root root 4 Jul 25 15:35 /bin/lsmod -> kmod
....

and:

....
dpkg -l | grep -Ei
....

contains:

....
ii  kmod                                        22-1ubuntu5                                         amd64        tools for managing Linux kernel modules
....

BusyBox also implements its own version of those executables, see e.g. <<modprobe>>. Here we will only describe features that differ from kmod to the BusyBox implementation.

==== module-init-tools

Name of a predecessor set of tools.

==== kmod modprobe

kmod's `modprobe` can also load modules under different names to avoid conflicts, e.g.:

....
sudo modprobe vmhgfs -o vm_hgfs
....

== Filesystems

=== OverlayFS

link:https://en.wikipedia.org/wiki/OverlayFS[OverlayFS] is a filesystem merged in the Linux kernel in 3.18.

As the name suggests, OverlayFS allows you to merge multiple directories into one. The following minimal runnable examples should give you an intuition on how it works:

* https://askubuntu.com/questions/109413/how-do-i-use-overlayfs/1075564#1075564
* https://stackoverflow.com/questions/31044982/how-to-use-multiple-lower-layers-in-overlayfs/52792397#52792397

We are very interested in this filesystem because we are looking for a way to make host cross compiled executables appear on the guest root `/` without reboot.

This would have several advantages:

* makes it faster to test modified guest programs
** not rebooting is fundamental for <<gem5>>, where the reboot is very costly.
** no need to regenerate the root filesystem at all and reboot
** overcomes the `check_bin_arch` problem: <<rpath>>
* we could keep the base root filesystem very small, which implies:
** less host disk usage, no need to copy the entire `./getvar out_rootfs_overlay_dir` to the image again
** no need to worry about <<br2_target_rootfs_ext2_size>>

We can already make host files appear on the guest with <<9p>>, but they appear on a subdirectory instead of the root.

If they would appear on the root instead, that would be even more awesome, because you would just use the exact same paths relative to the root transparently.

For example, we wouldn't have to mess around with variables such as `PATH` and `LD_LIBRARY_PATH`.

The idea is to:

* 9P mount our overlay directory `./getvar out_rootfs_overlay_dir` on the guest, which we already do at `/mnt/9p/out_rootfs_overlay`
* then create an overlay with that directory and the root, and `chroot` into it.
+
I was unable to mount directly to `/` avoid the `chroot`:
** https://stackoverflow.com/questions/41119656/how-can-i-overlayfs-the-root-filesystem-on-linux
** https://unix.stackexchange.com/questions/316018/how-to-use-overlayfs-to-protect-the-root-filesystem
** https://unix.stackexchange.com/questions/420646/mount-root-as-overlayfs

We already have a prototype of this running from `fstab` on guest at `/mnt/overlay`, but it has the following shortcomings:

* changes to underlying filesystems are not visible on the overlay unless you remount with `mount -r remount /mnt/overlay`, as mentioned link:https://github.com/torvalds/linux/blob/v4.18/Documentation/filesystems/overlayfs.txt#L332[on the kernel docs]:
+
....
Changes to the underlying filesystems while part of a mounted overlay
filesystem are not allowed.  If the underlying filesystem is changed,
the behavior of the overlay is undefined, though it will not result in
a crash or deadlock.
....
+
This makes everything very inconvenient if you are inside `chroot` action. You would have to leave `chroot`, remount, then come back.
* the overlay does not contain sub-filesystems, e.g. `/proc`. We would have to re-mount them. But should be doable with some automation.

Even more awesome than `chroot` would be to `pivot_root`, but I couldn't get that working either:

* https://stackoverflow.com/questions/28015688/pivot-root-device-or-resource-busy
* https://unix.stackexchange.com/questions/179788/pivot-root-device-or-resource-busy

=== Secondary disk

A simpler and possibly less overhead alternative to <<9P>> would be to generate a secondary disk image with the benchmark you want to rebuild.

Then you can `umount` and re-mount on guest without reboot.

We don't support this yet, but it should not be too hard to hack it up, maybe by hooking into link:rootfs-post-build-script[].

This was not possible from gem5 `fs.py` as of 60600f09c25255b3c8f72da7fb49100e2682093a: https://stackoverflow.com/questions/50862906/how-to-attach-multiple-disk-images-in-a-simulation-with-gem5-fs-py/51037661#51037661

== Graphics

Both QEMU and gem5 are capable of outputting graphics to the screen, and taking mouse and keyboard input.

https://unix.stackexchange.com/questions/307390/what-is-the-difference-between-ttys0-ttyusb0-and-ttyama0-in-linux

=== QEMU text mode

Text mode is the default mode for QEMU.

The opposite of text mode is <<qemu-graphic-mode>>

In text mode, we just show the serial console directly on the current terminal, without opening a QEMU GUI window.

You cannot see any graphics from text mode, but text operations in this mode, including:

* scrolling up: <<scroll-up-in-graphic-mode>>
* copy paste to and from the terminal

making this a good default, unless you really need to use with graphics.

Text mode works by sending the terminal character by character to a serial device.

This is different from a display screen, where each character is a bunch of pixels, and it would be much harder to convert that into actual terminal text.

For more details, see:

* https://unix.stackexchange.com/questions/307390/what-is-the-difference-between-ttys0-ttyusb0-and-ttyama0-in-linux
* <<tty>>

Note that you can still see an image even in text mode with the VNC:

....
./run --vnc
....

and on another terminal:

....
./vnc
....

but there is not terminal on the VNC window, just the <<config_logo>> penguin.

==== Quit QEMU from text mode

https://superuser.com/questions/1087859/how-to-quit-the-qemu-monitor-when-not-using-a-gui

However, our QEMU setup captures Ctrl + C and other common signals and sends them to the guest, which makes it hard to quit QEMU for the first time since there is no GUI either.

The simplest way to quit QEMU, is to do:

....
Ctrl-A X
....

Alternative methods include:

* `quit` command on the <<qemu-monitor>>
* `pkill qemu`

=== QEMU graphic mode

Enable graphic mode with:

....
./run --graphic
....

Outcome: you see a penguin due to <<config_logo>>.

For a more exciting GUI experience, see: <<x11>>

Text mode is the default due to the following considerable advantages:

* copy and paste commands and stdout output to / from host
* get full panic traces when you start making the kernel crash :-) See also: https://unix.stackexchange.com/questions/208260/how-to-scroll-up-after-a-kernel-panic
* have a large scroll buffer, and be able to search it, e.g. by using tmux on host
* one less window floating around to think about in addition to your shell :-)
* graphics mode has only been properly tested on `x86_64`.

Text mode has the following limitations over graphics mode:

* you can't see graphics such as those produced by <<x11>>
* very early kernel messages such as `early console in extract_kernel` only show on the GUI, since at such early stages, not even the serial has been setup.

`x86_64` has a VGA device enabled by default, as can be seen as:

....
./qemu-monitor info qtree
....

and the Linux kernel picks it up through the link:https://en.wikipedia.org/wiki/Linux_framebuffer[fbdev] graphics system as can be seen from:

....
cat /dev/urandom > /dev/fb0
....

flooding the screen with colors. See also: https://superuser.com/questions/223094/how-do-i-know-if-i-have-kms-enabled

==== Scroll up in graphic mode

Scroll up in <<qemu-graphic-mode>>:

....
Shift-PgUp
....

but I never managed to increase that buffer:

* https://askubuntu.com/questions/709697/how-to-increase-scrollback-lines-in-ubuntu14-04-2-server-edition
* https://unix.stackexchange.com/questions/346018/how-to-increase-the-scrollback-buffer-size-for-tty

The superior alternative is to use text mode and GNU screen or <<tmux>>.

==== QEMU Graphic mode arm

===== QEMU graphic mode arm terminal

TODO: on arm, we see the penguin and some boot messages, but don't get a shell at then end:

....
./run --arch aarch64 --graphic
....

I think it does not work because the graphic window is <<drm>> only, i.e.:

....
cat /dev/urandom > /dev/fb0
....

fails with:

....
cat: write error: No space left on device
....

and has no effect, and the Linux kernel does not appear to have a built-in DRM console as it does for fbdev with <<fbcon,fbcon>>.

There is however one out-of-tree implementation: <<kmscon>>.

===== QEMU graphic mode arm terminal implementation

`arm` and `aarch64` rely on the QEMU CLI option:

....
-device virtio-gpu-pci
....

and the kernel config options:

....
CONFIG_DRM=y
CONFIG_DRM_VIRTIO_GPU=y
....

Unlike x86, `arm` and `aarch64` don't have a display device attached by default, thus the need for `virtio-gpu-pci`.

See also https://wiki.qemu.org/Documentation/Platforms/ARM (recently edited and corrected by yours truly... :-)).

===== QEMU graphic mode arm VGA

TODO: how to use VGA on ARM? https://stackoverflow.com/questions/20811203/how-can-i-output-to-vga-through-qemu-arm Tried:

....
-device VGA
....

But https://github.com/qemu/qemu/blob/v2.12.0/docs/config/mach-virt-graphical.cfg#L264 says:

....
# We use virtio-gpu because the legacy VGA framebuffer is
# very troublesome on aarch64, and virtio-gpu is the only
# video device that doesn't implement it.
....

so maybe it is not possible?

=== gem5 graphic mode

gem5 does not have a "text mode", since it cannot redirect the Linux terminal to same host terminal where the executable is running: you are always forced to connect to the terminal with `gem-shell`.

TODO could not get it working on `x86_64`, only ARM.

Overview: https://stackoverflow.com/questions/50364863/how-to-get-graphical-gui-output-and-user-touch-keyboard-mouse-input-in-a-ful/50364864#50364864

More concretely, first build the kernel with the <<gem5-arm-linux-kernel-patches>>, and then run:

....
./build-linux \
  --arch arm \
  --custom-config-file-gem5 \
  --linux-build-id gem5-v4.15 \
;
./run --arch arm --emulator gem5 --linux-build-id gem5-v4.15
....

and then on another shell:

....
vinagre localhost:5900
....

The <<config_logo>> penguin only appears after several seconds, together with kernel messages of type:

....
[    0.152755] [drm] found ARM HDLCD version r0p0
[    0.152790] hdlcd 2b000000.hdlcd: bound virt-encoder (ops 0x80935f94)
[    0.152795] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[    0.152799] [drm] No driver support for vblank timestamp query.
[    0.215179] Console: switching to colour frame buffer device 240x67
[    0.230389] hdlcd 2b000000.hdlcd: fb0:  frame buffer device
[    0.230509] [drm] Initialized hdlcd 1.0.0 20151021 for 2b000000.hdlcd on minor 0
....

The port `5900` is incremented by one if you already have something running on that port, `gem5` stdout tells us the right port on stdout as:

....
system.vncserver: Listening for connections on port 5900
....

and when we connect it shows a message:

....
info: VNC client attached
....

Alternatively, you can also dump each new frame to an image file with `--frame-capture`:

....
./run \
  --arch arm \
  --emulator gem5 \
  --linux-build-id gem5-v4.15 \
  -- --frame-capture \
;
....

This creates on compressed PNG whenever the screen image changes inside the <<m5out-directory>> with filename of type:

....
frames_system.vncserver/fb.<frame-index>.<timestamp>.png.gz
....

It is fun to see how we get one new frame whenever the white underscore cursor appears and reappears under the penguin!

The last frame is always available uncompressed at: `system.framebuffer.png`.

TODO <<kmscube>> failed on `aarch64` with:

....
kmscube[706]: unhandled level 2 translation fault (11) at 0x00000000, esr 0x92000006, in libgbm.so.1.0.0[7fbf6a6000+e000]
....

Tested on: link:http://github.com/cirosantilli/linux-kernel-module-cheat/commit/38fd6153d965ba20145f53dc1bb3ba34b336bde9[38fd6153d965ba20145f53dc1bb3ba34b336bde9]

==== Graphic mode gem5 aarch64

For `aarch64` we also need to configure the kernel with link:linux_config/display[]:

....
git -C "$(./getvar linux_source_dir)" fetch https://gem5.googlesource.com/arm/linux gem5/v4.15:gem5/v4.15
git -C "$(./getvar linux_source_dir)" checkout gem5/v4.15
./build-linux \
  --arch aarch64 \
  --config-fragment linux_config/display \
  --custom-config-file-gem5 \
  --linux-build-id gem5-v4.15 \
;
git -C "$(./getvar linux_source_dir)" checkout -
./run --arch aarch64 --emulator gem5 --linux-build-id gem5-v4.15
....

This is because the gem5 `aarch64` defconfig does not enable HDLCD like the 32 bit one `arm` one for some reason.

==== gem5 graphic mode DP650

TODO get working. There is an unmerged patchset at: https://gem5-review.googlesource.com/c/public/gem5/+/11036/1

The DP650 is a newer display hardware than HDLCD. TODO is its interface publicly documented anywhere? Since it has a gem5 model and link:https://github.com/torvalds/linux/blob/v4.19/drivers/gpu/drm/arm/Kconfig#L39[in-tree Linux kernel support], that information cannot be secret?

The key option to enable support in Linux is `DRM_MALI_DISPLAY=y` which we enable at link:linux_config/display[].

Build the kernel exactly as for <<graphic-mode-gem5-aarch64>> and then run with:

....
./run --arch aarch64 --dp650 --emulator gem5 --linux-build-id gem5-v4.15
....

==== Graphic mode gem5 internals

We cannot use mainline Linux because the <<gem5-arm-linux-kernel-patches>> are required at least to provide the `CONFIG_DRM_VIRT_ENCODER` option.

gem5 emulates the link:http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0541c/CHDBAIDI.html[HDLCD] ARM Holdings hardware for `arm` and `aarch64`.

The kernel uses HDLCD to implement the <<drm>> interface, the required kernel config options are present at: link:linux_config/display[].

TODO: minimize out the `--custom-config-file`. If we just remove it on `arm`: it does not work with a failing dmesg:

....
[    0.066208] [drm] found ARM HDLCD version r0p0
[    0.066241] hdlcd 2b000000.hdlcd: bound virt-encoder (ops drm_vencoder_ops)
[    0.066247] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[    0.066252] [drm] No driver support for vblank timestamp query.
[    0.066276] hdlcd 2b000000.hdlcd: Cannot do DMA to address 0x0000000000000000
[    0.066281] swiotlb: coherent allocation failed for device 2b000000.hdlcd size=8294400
[    0.066288] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.15.0 #1
[    0.066293] Hardware name: V2P-AARCH64 (DT)
[    0.066296] Call trace:
[    0.066301]  dump_backtrace+0x0/0x1b0
[    0.066306]  show_stack+0x24/0x30
[    0.066311]  dump_stack+0xb8/0xf0
[    0.066316]  swiotlb_alloc_coherent+0x17c/0x190
[    0.066321]  __dma_alloc+0x68/0x160
[    0.066325]  drm_gem_cma_create+0x98/0x120
[    0.066330]  drm_fbdev_cma_create+0x74/0x2e0
[    0.066335]  __drm_fb_helper_initial_config_and_unlock+0x1d8/0x3a0
[    0.066341]  drm_fb_helper_initial_config+0x4c/0x58
[    0.066347]  drm_fbdev_cma_init_with_funcs+0x98/0x148
[    0.066352]  drm_fbdev_cma_init+0x40/0x50
[    0.066357]  hdlcd_drm_bind+0x220/0x428
[    0.066362]  try_to_bring_up_master+0x21c/0x2b8
[    0.066367]  component_master_add_with_match+0xa8/0xf0
[    0.066372]  hdlcd_probe+0x60/0x78
[    0.066377]  platform_drv_probe+0x60/0xc8
[    0.066382]  driver_probe_device+0x30c/0x478
[    0.066388]  __driver_attach+0x10c/0x128
[    0.066393]  bus_for_each_dev+0x70/0xb0
[    0.066398]  driver_attach+0x30/0x40
[    0.066402]  bus_add_driver+0x1d0/0x298
[    0.066408]  driver_register+0x68/0x100
[    0.066413]  __platform_driver_register+0x54/0x60
[    0.066418]  hdlcd_platform_driver_init+0x20/0x28
[    0.066424]  do_one_initcall+0x44/0x130
[    0.066428]  kernel_init_freeable+0x13c/0x1d8
[    0.066433]  kernel_init+0x18/0x108
[    0.066438]  ret_from_fork+0x10/0x1c
[    0.066444] hdlcd 2b000000.hdlcd: Failed to set initial hw configuration.
[    0.066470] hdlcd 2b000000.hdlcd: master bind failed: -12
[    0.066477] hdlcd: probe of 2b000000.hdlcd failed with error -12
[
....

So what other options are missing from `gem5_defconfig`? It would be cool to minimize it out to better understand the options.

[[x11]]
=== X11 Buildroot

Once you've seen the `CONFIG_LOGO` penguin as a sanity check, you can try to go for a cooler X11 Buildroot setup.

Build and run:

....
./build-buildroot --config-fragment buildroot_config/x11
./run --graphic
....

Inside QEMU:

....
startx
....

And then from the GUI you can start exciting graphical programs such as:

....
xcalc
xeyes
....

Outcome:

image:x11.png[image]

We don't build X11 by default because it takes a considerable amount of time (about 20%), and is not expected to be used by most users: you need to pass the `-x` flag to enable it.

More details: https://unix.stackexchange.com/questions/70931/how-to-install-x11-on-my-own-linux-buildroot-system/306116#306116

Not sure how well that graphics stack represents real systems, but if it does it would be a good way to understand how it works.

To x11 packages have an `xserver` prefix as in:

....
./build-buildroot --config-fragment buildroot_config/x11 -- xserver_xorg-server-reconfigure
....

the easiest way to find them out is to just list `"$(./getvar buildroot_build_build_dir)/x*`.

TODO as of: c2696c978d6ca88e8b8599c92b1beeda80eb62b2 I noticed that `startx` leads to a <<bug_on>>:

....
[    2.809104] WARNING: CPU: 0 PID: 51 at drivers/gpu/drm/ttm/ttm_bo_vm.c:304 ttm_bo_vm_open+0x37/0x40
....

==== X11 Buildroot mouse not moving

TODO 9076c1d9bcc13b6efdb8ef502274f846d8d4e6a1 I'm 100% sure that it was working before, but I didn't run it forever, and it stopped working at some point. Needs bisection, on whatever commit last touched x11 stuff.

* https://askubuntu.com/questions/730891/how-can-i-get-a-mouse-cursor-in-qemu
* https://stackoverflow.com/questions/19665412/mouse-and-keyboard-not-working-in-qemu-emulator

`-show-cursor` did not help, I just get to see the host cursor, but the guest cursor still does not move.

Doing:

....
watch -n 1 grep i8042 /proc/interrupts
....

shows that interrupts do happen when mouse and keyboard presses are done, so I expect that it is some wrong either with:

* QEMU. Same behaviour if I try the host's QEMU 2.10.1 however.
* X11 configuration. We do have `BR2_PACKAGE_XDRIVER_XF86_INPUT_MOUSE=y`.

`/var/log/Xorg.0.log` contains the following interesting lines:

....
[    27.549] (II) LoadModule: "mouse"
[    27.549] (II) Loading /usr/lib/xorg/modules/input/mouse_drv.so
[    27.590] (EE) <default pointer>: Cannot find which device to use.
[    27.590] (EE) <default pointer>: cannot open input device
[    27.590] (EE) PreInit returned 2 for "<default pointer>"
[    27.590] (II) UnloadModule: "mouse"
....

The file `/dev/inputs/mice` does not exist.

Note that our current link:kernel_confi_fragment sets:

....
# CONFIG_INPUT_MOUSE is not set
# CONFIG_INPUT_MOUSEDEV_PSAUX is not set
....

for gem5, so you might want to remove those lines to debug this.

==== X11 Buildroot ARM

On ARM, `startx` hangs at a message:

....
vgaarb: this pci device is not a vga device
....

and nothing shows on the screen, and:

....
grep EE /var/log/Xorg.0.log
....

says:

....
(EE) Failed to load module "modesetting" (module does not exist, 0)
....

A friend told me this but I haven't tried it yet:

* `xf86-video-modesetting` is likely the missing ingredient, but it does not seem possible to activate it from Buildroot currently without patching things.
* `xf86-video-fbdev` should work as well, but we need to make sure fbdev is enabled, and maybe add some line to the `Xorg.conf`

== Networking

=== Enable networking

We disable networking by default because it starts an userland process, and we want to keep the number of userland processes to a minimum to make the system more understandable: <<resource-tradeoff-guidelines>>

To enable networking on Buildroot, simply run:

....
ifup -a
....

That command goes over all (`-a`) the interfaces in `/etc/network/interfaces` and brings them up.

Then test it with:

....
wget google.com
cat index.html
....

Disable networking with:

....
ifdown -a
....

To enable networking by default after boot, use the methods documented at <<init-busybox>>.

=== ping

`ping` does not work within QEMU by default, e.g.:

....
ping google.com
....

hangs after printing the header:

....
PING google.com (216.58.204.46): 56 data bytes
....

https://unix.stackexchange.com/questions/473448/how-to-ping-from-the-qemu-guest-to-an-external-url

=== Guest host networking

In this section we discuss how to interact between the guest and the host through networking.

First ensure that you can access the external network since that is easier to get working: <<networking>>.

==== Host to guest networking

===== nc host to guest

With `nc` we can create the most minimal example possible as a sanity check.

On guest run:

....
nc -l -p 45455
....

Then on host run:

....
echo asdf | nc localhost 45455
....

`asdf` appears on the guest.

This uses:

* BusyBox' `nc` utility, which is enabled with `CONFIG_NC=y`
* `nc` from the `netcat-openbsd` package on an Ubuntu 18.04 host

Only this specific port works by default since we have forwarded it on the QEMU command line.

We us this exact procedure to connect to <<gdbserver>>.

===== ssh into guest

Not enabled by default due to the build / runtime overhead. To enable, build with:

....
./build-buildroot --config 'BR2_PACKAGE_OPENSSH=y'
....

Then inside the guest turn on sshd:

....
./sshd.sh
....

Source: link:rootfs_overlay/lkmc/sshd.sh[]

And finally on host:

....
ssh root@localhost -p 45456
....

Bibliography: https://unix.stackexchange.com/questions/124681/how-to-ssh-from-host-to-guest-using-qemu/307557#307557

===== gem5 host to guest networking

Could not do port forwarding from host to guest, and therefore could not use `gdbserver`: https://stackoverflow.com/questions/48941494/how-to-do-port-forwarding-from-guest-to-host-in-gem5

==== Guest to host networking

First <<enable-networking>>.

Then in the host, start a server:

....
python -m SimpleHTTPServer 8000
....

And then in the guest, find the IP we need to hit with:

....
ip rounte
....

which gives:

.....
default via 10.0.2.2 dev eth0
10.0.2.0/24 dev eth0 scope link  src 10.0.2.15
.....

so we use in the guest:

....
wget 10.0.2.2:8000
....

Bibliography: https://serverfault.com/questions/769874/how-to-forward-a-port-from-guest-to-host-in-qemu-kvm/951835#951835

=== 9P

The link:https://en.wikipedia.org/wiki/9P_(protocol)[9p protocol] allows the guest to mount a host directory.

Both QEMU and <<9p-gem5>> support 9P.

==== 9P vs NFS

All of 9P and NFS (and sshfs) allow sharing directories between guest and host.

Advantages of 9P

* requires `sudo` on the host to mount
* we could share a guest directory to the host, but this would require running a server on the guest, which adds <<resource-tradeoff-guidelines,simulation overhead>>
+
Furthermore, this would be inconvenient, since what we usually want to do is to share host cross built files with the guest, and to do that we would have to copy the files over after the guest starts the server.
* QEMU implements 9P natively, which makes it very stable and convenient, and must mean it is a simpler protocol than NFS as one would expect.
+
This is not the case for gem5 7bfb7f3a43f382eb49853f47b140bfd6caad0fb8 unfortunately, which relies on the link:https://github.com/chaos/diod[diod] host daemon, although it is not unfeasible that future versions could implement it natively as well.

Advantages of NFS:

* way more widely used and therefore stable and available, not to mention that it also works on real hardware.
* the name does not start with a digit, which is an invalid identifier in all programming languages known to man. Who in their right mind would call a software project as such? It does not even match the natural order of Plan 9; Plan then 9: P9!

==== 9P getting started

As usual, we have already set everything up for you. On host:

....
cd "$(./getvar p9_dir)"
uname -a > host
....

Guest:

....
cd /mnt/9p/data
cat host
uname -a > guest
....

Host:

....
cat guest
....

The main ingredients for this are:

* `9P` settings in our <<kernel-configs-about,kernel configs>>
* `9p` entry on our link:rootfs_overlay/etc/fstab[]
+
Alternatively, you could also mount your own with:
+
....
mkdir /mnt/my9p
mount -t 9p -o trans=virtio,version=9p2000.L host0 /mnt/my9p
....
* Launch QEMU with `-virtfs` as in your link:run[] script
+
When we tried:
+
....
security_model=mapped
....
+
writes from guest failed due to user mismatch problems: https://serverfault.com/questions/342801/read-write-access-for-passthrough-9p-filesystems-with-libvirt-qemu

Bibliography:

* https://superuser.com/questions/628169/how-to-share-a-directory-with-the-host-without-networking-in-qemu
* https://wiki.qemu.org/Documentation/9psetup

==== 9P gem5

TODO seems possible! Lets do it:

* http://gem5.org/wiki/images/b/b8/Summit2017_wa_devlib.pdf
* http://gem5.org/WA-gem5

==== NFS

TODO: get working.

<<9p>> is better with emulation, but let's just get this working for fun.

First make sure that this works: <<guest-to-host-networking>>.

Then, build the kernel with NFS support:

....
./build-linux --config-fragment linux_config/nfs
....

Now on host:

....
sudo apt-get install nfs-kernel-server
....

Now edit `/etc/exports` to contain:

....
/tmp *(rw,sync,no_root_squash,no_subtree_check)
....

and restart the server:

....
sudo systemctl restart nfs-kernel-server
....

Now on guest:

....
mkdir /mnt/nfs
mount -t nfs 10.0.2.2:/tmp /mnt/nfs
....

TODO: failing with:

....
mount: mounting 10.0.2.2:/tmp on /mnt/nfs failed: No such device
....

And now the `/tmp` directory from host is not mounted on guest!

If you don't want to start the NFS server after the next boot automatically so save resources, link:https://askubuntu.com/questions/19320/how-to-enable-or-disable-services[do]:

....
systemctl disable nfs-kernel-server
....

== Linux kernel

=== Linux kernel configuration

==== Modify kernel config

To modify a single option on top of our <<kernel-configs-about,default kernel configs>>, do:

....
./build-linux --config 'CONFIG_FORTIFY_SOURCE=y'
....

Kernel modules depend on certain kernel configs, and therefore in general you might have to clean and rebuild the kernel modules after changing the kernel config:

....
./build-modules --clean
./build-modules
....

and then proceed as in <<your-first-kernel-module-hack>>.

You might often get way without rebuilding the kernel modules however.

To use an extra kernel config fragment file on top of our defaults, do:

....
printf '
CONFIG_IKCONFIG=y
CONFIG_IKCONFIG_PROC=y
' > data/myconfig
./build-linux --config-fragment 'data/myconfig'
....

To use just your own exact `.config` instead of our defaults ones, use:

....
./build-linux --custom-config-file data/myconfig
....

There is also a shortcut `--custom-config-file` to use the <<gem5-arm-linux-kernel-patches>>.

The following options can all be used together, sorted by decreasing config setting power precedence:

* `--config`
* `--config-fragment`
* `--custom-config-file`

To do a clean menu config yourself and use that for the build, do:

....
./build-linux --clean
./build-linux --custom-config-target menuconfig
....

But remember that every new build re-configures the kernel by default, so to keep your configs you will need to use on further builds:

....
./build-linux --no-configure
....

So what you likely want to do instead is to save that as a new `defconfig` and use it later as:

....
./build-linux --no-configure --no-modules-install savedefconfig
cp "$(./getvar linux_build_dir)/defconfig" data/myconfig
./build-linux --custom-config-file data/myconfig
....

You can also use other config generating targets such as `defconfig` with the same method as shown at: <<linux-kernel-defconfig>>.

==== Find the kernel config

Get the build config in guest:

....
zcat /proc/config.gz
....

or with our shortcut:

....
./conf.sh
....

or to conveniently grep for a specific option case insensitively:

....
./conf.sh ikconfig
....

Source: link:rootfs_overlay/lkmc/conf.sh[].

This is enabled by:

....
CONFIG_IKCONFIG=y
CONFIG_IKCONFIG_PROC=y
....

From host:

....
cat "$(./getvar linux_config)"
....

Just for fun link:https://stackoverflow.com/questions/14958192/how-to-get-the-config-from-a-linux-kernel-image/14958263#14958263[]:

....
./linux/scripts/extract-ikconfig "$(./getvar vmlinux)"
....

although this can be useful when someone gives you a random image.

[[kernel-configs-about]]
==== About our Linux kernel configs

By default, link:build-linux[] generates a `.config` that is a mixture of:

* a base config extracted from Buildroot's minimal per machine `.config`, which has the minimal options needed to boot: <<buildroot-kernel-config>>.
* small overlays put top of that

To find out which kernel configs are being used exactly, simply run:

....
./build-linux --dry-run
....

and look for the `merge_config.sh` call. This script from the Linux kernel tree, as the name suggests, merges multiple configuration files into one as explained at: https://unix.stackexchange.com/questions/224887/how-to-script-make-menuconfig-to-automate-linux-kernel-build-configuration/450407#450407

For each arch, the base of our configs are named as:

....
linux_config/buildroot-<arch>
....

e.g.: link:linux_config/buildroot-x86_64[].

These configs are extracted directly from a Buildroot build with link:update-buildroot-kernel-config[].

Note that Buildroot can `sed` override some of the configurations, e.g. it forces `CONFIG_BLK_DEV_INITRD=y` when `BR2_TARGET_ROOTFS_CPIO` is on. For this reason, those configs are not simply copy pasted from Buildroot files, but rather from a Buildroot kernel build, and then minimized with `make savedefconfig`: https://stackoverflow.com/questions/27899104/how-to-create-a-defconfig-file-from-a-config

On top of those, we add the following by default:

* link:linux_config/min[]: see: <<linux-kernel-min-config>>
* link:linux_config/default[]: other optional configs that we enable by default because they increase visibility, or expose some cool feature, and don't significantly increase build time nor add significant runtime overhead
+
We have since observed that the kernel size itself is very bloated compared to `defconfig`: <<linux-kernel-defconfig>>.

[[buildroot-kernel-config]]
===== About Buildroot's kernel configs

To see Buildroot's base configs, start from link:https://github.com/buildroot/buildroot/blob/2018.05/configs/qemu_x86_64_defconfig[`buildroot/configs/qemu_x86_64_defconfig`].

That file contains `BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="board/qemu/x86_64/linux-4.15.config"`, which points to the base config file used: link:https://github.com/buildroot/buildroot/blob/2018.05/board/qemu/x86_64/linux-4.15.config[board/qemu/x86_64/linux-4.15.config].

`arm`, on the other hand, uses link:https://github.com/buildroot/buildroot/blob/2018.05/configs/qemu_arm_vexpress_defconfig[`buildroot/configs/qemu_arm_vexpress_defconfig`], which contains `BR2_LINUX_KERNEL_DEFCONFIG="vexpress"`, and therefore just does a `make vexpress_defconfig`, and gets its config from the Linux kernel tree itself.

====== Linux kernel defconfig

To boot link:https://stackoverflow.com/questions/41885015/what-exactly-does-linux-kernels-make-defconfig-do[defconfig] from disk on Linux and see a shell, all we need is these missing virtio options:

....
./build-linux \
  --linux-build-id defconfig \
  --custom-config-target defconfig \
  --config CONFIG_VIRTIO_PCI=y \
  --config CONFIG_VIRTIO_BLK=y \
;
./run --linux-build-id defconfig
....

Oh, and check this out:

....
du -h \
  "$(./getvar vmlinux)" \
  "$(./getvar --linux-build-id defconfig vmlinux)" \
;
....

Output:

....
360M    /path/to/linux-kernel-module-cheat/out/linux/default/x86_64/vmlinux
47M     /path/to/linux-kernel-module-cheat/out/linux/defconfig/x86_64/vmlinux
....

Brutal. Where did we go wrong?

The extra virtio options are not needed if we use <<initrd>>:

....
./build-linux \
  --linux-build-id defconfig \
  --custom-config-target defconfig \
;
./run --initrd --linux-build-id defconfig
....

On aarch64, we can boot from initrd with:

....
./build-linux \
  --arch aarch64 \
  --linux-build-id defconfig \
  --custom-config-target defconfig \
;
./run \
  --arch aarch64 \
  --initrd \
  --linux-build-id defconfig \
  --memory 2G \
;
....

We need the 2G of memory because the CPIO is 600MiB due to a humongous amount of loadable kernel modules!

In aarch64, the size situation is inverted from x86_64, and this can be seen on the vmlinux size as well:

....
118M    /path/to/linux-kernel-module-cheat/out/linux/default/aarch64/vmlinux
240M    /path/to/linux-kernel-module-cheat/out/linux/defconfig/aarch64/vmlinux
....

So it seems that the ARM devs decided rather than creating a minimal config that boots QEMU, to try and make a single config that boots every board in existence. Terrible!

Bibliography: https://unix.stackexchange.com/questions/29439/compiling-the-kernel-with-default-configurations/204512#204512

Tested on 1e2b7f1e5e9e3073863dc17e25b2455c8ebdeadd + 1.

====== Linux kernel min config

link:linux_config/min[] contains minimal tweaks required to boot gem5 or for using our slightly different QEMU command line options than Buildroot on all archs.

It is one of the default config fragments we use, as explained at: <<kernel-configs-about>>>.

Having the same config working for both QEMU and gem5 (oh, the hours of bisection) means that you can deal with functional matters in QEMU, which runs much faster, and switch to gem5 only for performance issues.

We can build just with `min` on top of the base config with:

....
./build-linux \
  --arch aarch64 \
  --config-fragment linux_config/min \
  --custom-config-file linux_config/buildroot-aarch64 \
  --linux-build-id min \
;
....

vmlinux had a very similar size to the default. It seems that link:linux_config/buildroot-aarch64[] contains or implies most link:linux_config/default[] options already? TODO: that seems odd, really?

Tested on 649d06d6758cefd080d04dc47fd6a5a26a620874 + 1.

===== Notable alternate gem5 kernel configs

Other configs which we had previously tested at 4e0d9af81fcce2ce4e777cb82a1990d7c2ca7c1e are:

* `arm` and `aarch64` configs present in the official ARM gem5 Linux kernel fork: <<gem5-arm-linux-kernel-patches>>. Some of the configs present there are added by the patches.
* Jason's magic `x86_64` config: http://web.archive.org/web/20171229121642/http://www.lowepower.com/jason/files/config which is referenced at: link:http://web.archive.org/web/20171229121525/http://www.lowepower.com/jason/setting-up-gem5-full-system.html[]. QEMU boots with that by removing `# CONFIG_VIRTIO_PCI is not set`.

=== Kernel version

==== Find the kernel version

We try to use the latest possible kernel major release version.

In QEMU:

....
cat /proc/version
....

or in the source:

....
cd "$(./getvar linux_source_dir)"
git log | grep -E '    Linux [0-9]+\.' | head
....

==== Update the Linux kernel

During update all you kernel modules may break since the kernel API is not stable.

They are usually trivial breaks of things moving around headers or to sub-structs.

The userland, however, should simply not break, as Linus enforces strict backwards compatibility of userland interfaces.

This backwards compatibility is just awesome, it makes getting and running the latest master painless.

This also makes this repo the perfect setup to develop the Linux kernel.

In case something breaks while updating the Linux kernel, you can try to bisect it to understand the root cause: <<bisection>>.

==== Downgrade the Linux kernel

The kernel is not forward compatible, however, so downgrading the Linux kernel requires downgrading the userland too to the latest Buildroot branch that supports it.

The default Linux kernel version is bumped in Buildroot with commit messages of type:

....
linux: bump default to version 4.9.6
....

So you can try:

....
git log --grep 'linux: bump default to version'
....

Those commits change `BR2_LINUX_KERNEL_LATEST_VERSION` in `/linux/Config.in`.

You should then look up if there is a branch that supports that kernel. Staying on branches is a good idea as they will get backports, in particular ones that fix the build as newer host versions come out.

Finally, after downgrading Buildroot, if something does not work, you might also have to make some changes to how this repo uses Buildroot, as the Buildroot configuration options might have changed.

We don't expect those changes to be very difficult. A good way to approach the task is to:

* do a dry run build to get the equivalent Bash commands used:
+
....
./build-buildroot --dry-run
....
* build the Buildroot documentation for the version you are going to use, and check if all Buildroot build commands make sense there

Then, if you spot an option that is wrong, some grepping in this repo should quickly point you to the code you need to modify.

It also possible that you will need to apply some patches from newer Buildroot versions for it to build, due to incompatibilities with the host Ubuntu packages and that Buildroot version. Just read the error message, and try:

* `git log master -- packages/<pkg>`
* Google the error message for mailing list hits

Successful port reports:

* v3.18: https://github.com/cirosantilli/linux-kernel-module-cheat/issues/39#issuecomment-438525481

=== Kernel command line parameters

Bootloaders can pass a string as input to the Linux kernel when it is booting to control its behaviour, much like the `execve` system call does to userland processes.

This allows us to control the behaviour of the kernel without rebuilding anything.

With QEMU, QEMU itself acts as the bootloader, and provides the `-append` option and we expose it through `./run --kernel-cli`, e.g.:

....
./run --kernel-cli 'foo bar'
....

Then inside the host, you can check which options were given with:

....
cat /proc/cmdline
....

They are also printed at the beginning of the boot message:

....
dmesg | grep "Command line"
....

See also:

* https://unix.stackexchange.com/questions/48601/how-to-display-the-linux-kernel-command-line-parameters-given-for-the-current-bo
* https://askubuntu.com/questions/32654/how-do-i-find-the-boot-parameters-used-by-the-running-kernel

The arguments are documented in the kernel documentation: https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html

When dealing with real boards, extra command line options are provided on some magic bootloader configuration file, e.g.:

* GRUB configuration files: https://askubuntu.com/questions/19486/how-do-i-add-a-kernel-boot-parameter
* Raspberry pi `/boot/cmdline.txt` on a magic partition: https://raspberrypi.stackexchange.com/questions/14839/how-to-change-the-kernel-commandline-for-archlinuxarm-on-raspberry-pi-effectly

==== Kernel command line parameters escaping

Double quotes can be used to escape spaces as in `opt="a b"`, but double quotes themselves cannot be escaped, e.g. `opt"a\"b"`

This even lead us to use base64 encoding with `--eval`!

==== Kernel command line parameters definition points

There are two methods:

* `__setup` as in:
+
....
__setup("console=", console_setup);
....
* `core_param` as in:
+
....
core_param(panic, panic_timeout, int, 0644);
....

`core_param` suggests how they are different:

....
/**
 * core_param - define a historical core kernel parameter.

...

 * core_param is just like module_param(), but cannot be modular and
 * doesn't add a prefix (such as "printk.").  This is for compatibility
 * with __setup(), and it makes sense as truly core parameters aren't
 * tied to the particular file they're in.
 */
....

==== rw

By default, the Linux kernel mounts the root filesystem as readonly. TODO rationale?

This cannot be observed in the default BusyBox init, because by default our link:rootfs_overlay/etc/inittab[] does:

....
/bin/mount -o remount,rw /
....

Analogously, Ubuntu 18.04 does in its fstab something like:

....
UUID=/dev/sda1 / ext4 errors=remount-ro 0 1
....

which uses default mount `rw` flags.

We have however removed those setups init setups to keep things more minimal, and replaced them with the `rw` kernel boot parameter makes the root mounted as writable.

To observe the default readonly behaviour, hack the link:run[] script to remove <<replace-init,replace init>>, and then run on a raw shell:

....
./run --kernel-cli 'init=/bin/sh'
....

Now try to do:

....
touch a
....

which fails with:

....
touch: a: Read-only file system
....

We can also observe the read-onlyness with:

....
mount -t proc /proc
mount
....

which contains:

....
/dev/root on / type ext2 (ro,relatime,block_validity,barrier,user_xattr)
....

and so it is Read Only as shown by `ro`.

==== norandmaps

Disable userland address space randomization. Test it out by running <<rand_check-out>> twice:

....
./run --eval-after './linux/rand_check.out;./linux/poweroff.out'
./run --eval-after './linux/rand_check.out;./linux/poweroff.out'
....

If we remove it from our link:run[] script by hacking it up, the addresses shown by `linux/rand_check.out` vary across boots.

Equivalent to:

....
echo 0 > /proc/sys/kernel/randomize_va_space
....

=== printk

`printk` is the most simple and widely used way of getting information from the kernel, so you should familiarize yourself with its basic configuration.

We use `printk` a lot in our kernel modules, and it shows on the terminal by default, along with stdout and what you type.

Hide all `printk` messages:

....
dmesg -n 1
....

or equivalently:

....
echo 1 > /proc/sys/kernel/printk
....

See also: https://superuser.com/questions/351387/how-to-stop-kernel-messages-from-flooding-my-console

Do it with a <<kernel-command-line-parameters>> to affect the boot itself:

....
./run --kernel-cli 'loglevel=5'
....

and now only boot warning messages or worse show, which is useful to identify problems.

Our default `printk` format is:

....
<LEVEL>[TIMESTAMP] MESSAGE
....

e.g.:

....
<6>[    2.979121] Freeing unused kernel memory: 2024K
....

where:

* `LEVEL`: higher means less serious
* `TIMESTAMP`: seconds since boot

This format is selected by the following boot options:

* `console_msg_format=syslog`: add the `<LEVEL>` part. Added in v4.16.
* `printk.time=y`: add the `[TIMESTAMP]` part

The debug highest level is a bit more magic, see: <<pr_debug>> for more info.

==== ignore_loglevel

....
./run --kernel-cli 'ignore_loglevel'
....

enables all log levels, and is basically the same as:

....
./run --kernel-cli 'loglevel=8'
....

except that you don't need to know what is the maximum level.

==== pr_debug

https://stackoverflow.com/questions/28936199/why-is-pr-debug-of-the-linux-kernel-not-giving-any-output/49835405#49835405

Debug messages are not printable by default without recompiling.

But the awesome `CONFIG_DYNAMIC_DEBUG=y` option which we enable by default allows us to do:

....
echo 8 > /proc/sys/kernel/printk
echo 'file kernel/module.c +p' > /sys/kernel/debug/dynamic_debug/control
./linux/myinsmod.out hello.ko
....

and we have a shortcut at:

....
./pr_debug.sh
....

Source: link:rootfs_overlay/lkmc/pr_debug.sh[].

Syntax: https://www.kernel.org/doc/html/v4.11/admin-guide/dynamic-debug-howto.html

Wildcards are also accepted, e.g. enable all messages from all files:

....
echo 'file * +p' > /sys/kernel/debug/dynamic_debug/control
....

TODO: why is this not working:

....
echo 'func sys_init_module +p' > /sys/kernel/debug/dynamic_debug/control
....

Enable messages in specific modules:

....
echo 8 > /proc/sys/kernel/printk
echo 'module myprintk +p' > /sys/kernel/debug/dynamic_debug/control
insmod myprintk.ko
....

Source: link:kernel_modules/myprintk.c[]

This outputs the `pr_debug` message:

....
printk debug
....

but TODO: it also shows debug messages even without enabling them explicitly:

....
echo 8 > /proc/sys/kernel/printk
insmod myprintk.ko
....

and it shows as enabled:

....
# grep myprintk /sys/kernel/debug/dynamic_debug/control
/root/linux-kernel-module-cheat/out/kernel_modules/x86_64/kernel_modules/panic.c:12 [myprintk]myinit =p "pr_debug\012"
....

Enable `pr_debug` for boot messages as well, before we can reach userland and write to `/proc`:

....
./run --kernel-cli 'dyndbg="file * +p" loglevel=8'
....

Get ready for the noisiest boot ever, I think it overflows the `printk` buffer and funny things happen.

===== pr_debug != printk(KERN_DEBUG

When `CONFIG_DYNAMIC_DEBUG` is set,  `printk(KERN_DEBUG` is not the exact same as `pr_debug(` since `printk(KERN_DEBUG` messages are visible with:

....
./run --kernel-cli 'initcall_debug logleve=8'
....

which outputs lines of type:

....
<7>[    1.756680] calling  clk_disable_unused+0x0/0x130 @ 1
<7>[    1.757003] initcall clk_disable_unused+0x0/0x130 returned 0 after 111 usecs
....

which are `printk(KERN_DEBUG` inside `init/main.c` in v4.16.

Mentioned at: https://stackoverflow.com/questions/37272109/how-to-get-details-of-all-modules-drivers-got-initialized-probed-during-kernel-b

This likely comes from the ifdef split at `init/main.c`:

....
/* If you are writing a driver, please use dev_dbg instead */
#if defined(CONFIG_DYNAMIC_DEBUG)
#include <linux/dynamic_debug.h>

/* dynamic_pr_debug() uses pr_fmt() internally so we don't need it here */
#define pr_debug(fmt, ...) \
    dynamic_pr_debug(fmt, ##__VA_ARGS__)
#elif defined(DEBUG)
#define pr_debug(fmt, ...) \
    printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
#else
#define pr_debug(fmt, ...) \
    no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
#endif
....

=== Linux kernel entry point

`start_kernel` is a good definition of it: https://stackoverflow.com/questions/18266063/does-kernel-have-main-function/33422401#33422401

=== Kernel module APIs

==== Kernel module parameters

The Linux kernel allows passing module parameters at insertion time <<myinsmod,through the `init_module` and `finit_module` system calls>>:

....
./params.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/params.c[]
* link:rootfs_overlay/lkmc/params.sh[]

As shown in the example, module parameters can also be read and modified at runtime from <<sysfs>>.

We can obtain the help text of the parameters with:

....
modinfo params.ko
....

The output contains:

....
parm:           j:my second favorite int
parm:           i:my favorite int
....

===== modprobe.conf

<<modprobe>> insertion can also set default parameters via the link:rootfs_overlay/etc/modprobe.conf[`/etc/modprobe.conf`] file:

....
modprobe params
cat /sys/kernel/debug/lkmc_params
....

Output:

....
12 34
....

This is specially important when loading modules with <<kernel-module-dependencies>> or else we would have no opportunity of passing those.

`modprobe.conf` doesn't actually insmod anything for us: https://superuser.com/questions/397842/automatically-load-kernel-module-at-boot-angstrom/1267464#1267464

==== Kernel module dependencies

One module can depend on symbols of another module that are exported with `EXPORT_SYMBOL`:

....
./dep.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/dep.c[]
* link:kernel_modules/dep2.c[]
* link:rootfs_overlay/lkmc/dep.sh[]

The kernel deduces dependencies based on the `EXPORT_SYMBOL` that each module uses.

Symbols exported by `EXPORT_SYMBOL` can be seen with:

....
insmod dep.ko
grep lkmc_dep /proc/kallsyms
....

sample output:

....
ffffffffc0001030 r __ksymtab_lkmc_dep   [dep]
ffffffffc000104d r __kstrtab_lkmc_dep   [dep]
ffffffffc0002300 B lkmc_dep     [dep]
....

This requires `CONFIG_KALLSYMS_ALL=y`.

Dependency information is stored by the kernel module build system in the `.ko` files' <<module_info>>, e.g.:

....
modinfo dep2.ko
....

contains:

....
depends:        dep
....

We can double check with:

....
strings 3 dep2.ko  | grep -E 'depends'
....

The output contains:

....
depends=dep
....

Module dependencies are also stored at:

....
cd /lib/module/*
grep dep modules.dep
....

Output:

....
extra/dep2.ko: extra/dep.ko
extra/dep.ko:
....

TODO: what for, and at which point point does Buildroot / BusyBox generate that file?

===== Kernel module dependencies with modprobe

Unlike `insmod`, <<modprobe>> deals with kernel module dependencies for us.

First get <<kernel_modules-package>> working.

Then, for example:

....
modprobe buildroot_dep2
....

outputs to dmesg:

....
42
....

and then:

....
lsmod
....

outputs:

....
Module                  Size  Used by    Tainted: G
buildroot_dep2         16384  0
buildroot_dep          16384  1 buildroot_dep2
....

Sources:

* link:buildroot_packages/kernel_modules/buildroot_dep.c[]
* link:buildroot_packages/kernel_modules/buildroot_dep2.c[]

Removal also removes required modules that have zero usage count:

....
modprobe -r buildroot_dep2
....

`modprobe` uses information from the `modules.dep` file to decide the required dependencies. That file contains:

....
extra/buildroot_dep2.ko: extra/buildroot_dep.ko
....

Bibliography:

* https://askubuntu.com/questions/20070/whats-the-difference-between-insmod-and-modprobe
* https://stackoverflow.com/questions/22891705/whats-the-difference-between-insmod-and-modprobe

==== MODULE_INFO

Module metadata is stored on module files at compile time. Some of the fields can be retrieved through the `THIS_MODULE` `struct module`:

....
insmod module_info.ko
....

Dmesg output:

....
name = module_info
version = 1.0
....

Source: link:kernel_modules/module_info.c[]

Some of those are also present on sysfs:

....
cat /sys/module/module_info/version
....

Output:

....
1.0
....

And we can also observe them with the `modinfo` command line utility:

....
modinfo module_info.ko
....

sample output:

....
filename:       module_info.ko
license:        GPL
version:        1.0
srcversion:     AF3DE8A8CFCDEB6B00E35B6
depends:
vermagic:       4.17.0 SMP mod_unload modversions
....

Module information is stored in a special `.modinfo` section of the ELF file:

....
./run-toolchain readelf -- -SW "$(./getvar kernel_modules_build_subdir)/module_info.ko"
....

contains:

....
  [ 5] .modinfo          PROGBITS        0000000000000000 0000d8 000096 00   A  0   0  8
....

and:

....
./run-toolchain readelf -- -x .modinfo "$(./getvar kernel_modules_build_subdir)/module_info.ko"
....

gives:

....
  0x00000000 6c696365 6e73653d 47504c00 76657273 license=GPL.vers
  0x00000010 696f6e3d 312e3000 61736466 3d717765 ion=1.0.asdf=qwe
  0x00000020 72000000 00000000 73726376 65727369 r.......srcversi
  0x00000030 6f6e3d41 46334445 38413843 46434445 on=AF3DE8A8CFCDE
  0x00000040 42364230 30453335 42360000 00000000 B6B00E35B6......
  0x00000050 64657065 6e64733d 006e616d 653d6d6f depends=.name=mo
  0x00000060 64756c65 5f696e66 6f007665 726d6167 dule_info.vermag
  0x00000070 69633d34 2e31372e 3020534d 50206d6f ic=4.17.0 SMP mo
  0x00000080 645f756e 6c6f6164 206d6f64 76657273 d_unload modvers
  0x00000090 696f6e73 2000                       ions .
....

I think a dedicated section is used to allow the Linux kernel and command line tools to easily parse that information from the ELF file as we've done with `readelf`.

Bibliography:

* https://stackoverflow.com/questions/19467150/significance-of-this-module-in-linux-driver/49812248#49812248
* https://stackoverflow.com/questions/4839024/how-to-find-the-version-of-a-compiled-kernel-module/42556565#42556565
* https://unix.stackexchange.com/questions/238167/how-to-understand-the-modinfo-output

==== vermagic

Vermagic is a magic string present in the kernel and on <<module_info>> of kernel modules. It is used to verify that the kernel module was compiled against a compatible kernel version and relevant configuration:

....
insmod vermagic.ko
....

Possible dmesg output:

....
VERMAGIC_STRING = 4.17.0 SMP mod_unload modversions
....

Source: link:kernel_modules/vermagic.c[]

If we artificially create a mismatch with `MODULE_INFO(vermagic`, the insmod fails with:

....
insmod: can't insert 'vermagic_fail.ko': invalid module format
....

and `dmesg` says the expected and found vermagic found:

....
vermagic_fail: version magic 'asdfqwer' should be '4.17.0 SMP mod_unload modversions '
....

Source: link:kernel_modules/vermagic_fail.c[]

The kernel's vermagic is defined based on compile time configurations at link:https://github.com/torvalds/linux/blob/v4.17/include/linux/vermagic.h#L35[include/linux/vermagic.h]:

....
#define VERMAGIC_STRING                                                 \
        UTS_RELEASE " "                                                 \
        MODULE_VERMAGIC_SMP MODULE_VERMAGIC_PREEMPT                     \
        MODULE_VERMAGIC_MODULE_UNLOAD MODULE_VERMAGIC_MODVERSIONS       \
        MODULE_ARCH_VERMAGIC                                            \
        MODULE_RANDSTRUCT_PLUGIN
....

The `SMP` part of the string for example is defined on the same file based on the value of `CONFIG_SMP`:

....
#ifdef CONFIG_SMP
#define MODULE_VERMAGIC_SMP "SMP "
#else
#define MODULE_VERMAGIC_SMP ""
....

TODO how to get the vermagic from running kernel from userland? https://lists.kernelnewbies.org/pipermail/kernelnewbies/2012-October/006306.html

<<kmod-modprobe>> has a flag to skip the vermagic check:

....
--force-modversion
....

This option just strips `modversion` information from the module before loading, so it is not a kernel feature.

==== module_init

`init_module` and `cleantup_module` are an older alternative to the `module_init` and `module_exit` macros:

....
insmod init_module.ko
rmmod init_module
....

Dmesg output:

....
init_module
cleanup_module
....

Source: link:kernel_modules/module_init.c[]

TODO why were `module_init` and `module_exit` created? https://stackoverflow.com/questions/3218320/what-is-the-difference-between-module-init-and-init-module-in-a-linux-kernel-mod

=== Kernel panic and oops

To test out kernel panics and oops in controlled circumstances, try out the modules:

....
insmod panic.ko
insmod oops.ko
....

Source:

* link:kernel_modules/panic.c[]
* link:kernel_modules/oops.c[]

A panic can also be generated with:

....
echo c > /proc/sysrq-trigger
....

Panic vs oops: https://unix.stackexchange.com/questions/91854/whats-the-difference-between-a-kernel-oops-and-a-kernel-panic

How to generate them:

* https://unix.stackexchange.com/questions/66197/how-to-cause-kernel-panic-with-a-single-command
* https://stackoverflow.com/questions/23484147/generate-kernel-oops-or-crash-in-the-code

When a panic happens, <<linux-kernel-magic-keys,`Shift-PgUp`>> does not work as it normally does, and it is hard to get the logs if on are on <<qemu-graphic-mode>>:

* https://superuser.com/questions/848412/scrolling-up-the-failed-screen-with-kernel-panic
* https://superuser.com/questions/269228/write-qemu-booting-virtual-machine-output-to-a-file
* http://www.reactos.org/wiki/QEMU#Redirect_to_a_file

==== Kernel panic

On panic, the kernel dies, and so does our terminal.

The panic trace looks like:

....
panic: loading out-of-tree module taints kernel.
panic myinit
Kernel panic - not syncing: hello panic
CPU: 0 PID: 53 Comm: insmod Tainted: G           O     4.16.0 #6
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
Call Trace:
 dump_stack+0x7d/0xba
 ? 0xffffffffc0000000
 panic+0xda/0x213
 ? printk+0x43/0x4b
 ? 0xffffffffc0000000
 myinit+0x1d/0x20 [panic]
 do_one_initcall+0x3e/0x170
 do_init_module+0x5b/0x210
 load_module+0x2035/0x29d0
 ? kernel_read_file+0x7d/0x140
 ? SyS_finit_module+0xa8/0xb0
 SyS_finit_module+0xa8/0xb0
 do_syscall_64+0x6f/0x310
 ? trace_hardirqs_off_thunk+0x1a/0x32
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x7ffff7b36206
RSP: 002b:00007fffffffeb78 EFLAGS: 00000206 ORIG_RAX: 0000000000000139
RAX: ffffffffffffffda RBX: 000000000000005c RCX: 00007ffff7b36206
RDX: 0000000000000000 RSI: 000000000069e010 RDI: 0000000000000003
RBP: 000000000069e010 R08: 00007ffff7ddd320 R09: 0000000000000000
R10: 00007ffff7ddd320 R11: 0000000000000206 R12: 0000000000000003
R13: 00007fffffffef4a R14: 0000000000000000 R15: 0000000000000000
Kernel Offset: disabled
---[ end Kernel panic - not syncing: hello panic
....

Notice how our panic message `hello panic` is visible at:

....
Kernel panic - not syncing: hello panic
....

===== Kernel module stack trace to source line

The log shows which module each symbol belongs to if any, e.g.:

....
myinit+0x1d/0x20 [panic]
....

says that the function `myinit` is in the module `panic`.

To find the line that panicked, do:

....
./run-gdb
....

and then:

....
info line *(myinit+0x1d)
....

which gives us the correct line:

....
Line 7 of "/root/linux-kernel-module-cheat/out/kernel_modules/x86_64/kernel_modules/panic.c" starts at address 0xbf00001c <myinit+28> and ends at 0xbf00002c <myexit>.
....

as explained at: https://stackoverflow.com/questions/8545931/using-gdb-to-convert-addresses-to-lines/27576029#27576029

The exact same thing can be done post mortem with:

....
./run-toolchain gdb -- \
  -batch \
  -ex 'info line *(myinit+0x1d)' \
  "$(./getvar kernel_modules_build_subdir)/panic.ko" \
;
....

Related:

* https://stackoverflow.com/questions/6151538/addr2line-on-kernel-module
* https://stackoverflow.com/questions/13468286/how-to-read-understand-analyze-and-debug-a-linux-kernel-panic

===== BUG_ON

Basically just calls `panic("BUG!")` for most archs.

===== Exit emulator on panic

For testing purposes, it is very useful to quit the emulator automatically with exit status non zero in case of kernel panic, instead of just hanging forever.

====== Exit QEMU on panic

Enabled by default with:

* `panic=-1` command line option which reboots the kernel immediately on panic, see: <<reboot-on-panic>>
* QEMU `-no-reboot`, which makes QEMU exit when the guest tries to reboot

Also asked at https://unix.stackexchange.com/questions/443017/can-i-make-qemu-exit-with-failure-on-kernel-panic which also mentions the x86_64 `-device pvpanic`, but I don't see much advantage to it.

TODO neither method exits with exit status different from 0, so for now we are just grepping the logs for panic messages, which sucks.

One possibility that gets close would be to use <<gdb>> to break at the `panic` function, and then send a <<qemu-monitor-from-gdb>> `quit` command if that happens, but I don't see a way to exit with non-zero status to indicate error.

====== Exit gem5 on panic

gem5 9048ef0ffbf21bedb803b785fb68f83e95c04db8 (January 2019) can detect panics automatically if the option `system.panic_on_panic` is on.

It parses kernel symbols and detecting when the PC reaches the address of the `panic` function. gem5 then prints to stdout:

....
Kernel panic in simulated kernel
....

and exits with status -6.

At gem5 ff52563a214c71fcd1e21e9f00ad839612032e3b (July 2018) behaviour was different, and just exited 0: https://www.mail-archive.com/gem5-users@gem5.org/msg15870.html TODO find fixing commit.

We enable the `system.panic_on_panic` option by default on `arm` and `aarch64`, which makes gem5 exit immediately in case of panic, which is awesome!

If we don't set `system.panic_on_panic`, then gem5 just hangs on an infinite guest loop.

TODO: why doesn't gem5 x86 ff52563a214c71fcd1e21e9f00ad839612032e3b support `system.panic_on_panic` as well? Trying to set `system.panic_on_panic` there fails with:

....
tried to set or access non-existentobject parameter: panic_on_panic
....

However, at that commit panic on x86 makes gem5 crash with:

....
panic: i8042 "System reset" command not implemented.
....

which is a good side effect of an unimplemented hardware feature, since the simulation actually stops.

The implementation of panic detection happens at: https://github.com/gem5/gem5/blob/1da285dfcc31b904afc27e440544d006aae25b38/src/arch/arm/linux/system.cc#L73

....
        kernelPanicEvent = addKernelFuncEventOrPanic<Linux::KernelPanicEvent>(
            "panic", "Kernel panic in simulated kernel", dmesg_output);
....

Here we see that the symbol `"panic"` for the `panic()` function is the one being tracked.

Related thread: https://stackoverflow.com/questions/56032347/is-there-a-way-to-identify-if-gem5-run-got-over-successfully

===== Reboot on panic

Make the kernel reboot after n seconds after panic:

....
echo 1 > /proc/sys/kernel/panic
....

Can also be controlled with the `panic=` kernel boot parameter.

`0` to disable, `-1` to reboot immediately.

Bibliography:

* https://github.com/torvalds/linux/blob/v4.17/Documentation/admin-guide/kernel-parameters.txt#L2931
* https://unix.stackexchange.com/questions/29567/how-to-configure-the-linux-kernel-to-reboot-on-panic/29569#29569

===== Panic trace show addresses instead of symbols

If `CONFIG_KALLSYMS=n`, then addresses are shown on traces instead of symbol plus offset.

In v4.16 it does not seem possible to configure that at runtime. GDB step debugging with:

....
./run --eval-after 'insmod dump_stack.ko' --gdb-wait --tmux-args dump_stack
....

shows that traces are printed at `arch/x86/kernel/dumpstack.c`:

....
static void printk_stack_address(unsigned long address, int reliable,
                 char *log_lvl)
{
    touch_nmi_watchdog();
    printk("%s %s%pB\n", log_lvl, reliable ? "" : "? ", (void *)address);
}
....

and `%pB` is documented at `Documentation/core-api/printk-formats.rst`:

....
If KALLSYMS are disabled then the symbol address is printed instead.
....

I wasn't able do disable `CONFIG_KALLSYMS` to test this this out however, it is being selected by some other option? But I then used `make menuconfig` to see which options select it, and they were all off...

[[oops]]
==== Kernel oops

On oops, the shell still lives after.

However we:

* leave the normal control flow, and `oops after` never gets printed: an interrupt is serviced
* cannot `rmmod oops` afterwards

It is possible to make `oops` lead to panics always with:

....
echo 1 > /proc/sys/kernel/panic_on_oops
insmod oops.ko
....

An oops stack trace looks like:

....
BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
IP: myinit+0x18/0x30 [oops]
PGD dccf067 P4D dccf067 PUD dcc1067 PMD 0
Oops: 0002 [#1] SMP NOPTI
Modules linked in: oops(O+)
CPU: 0 PID: 53 Comm: insmod Tainted: G           O     4.16.0 #6
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
RIP: 0010:myinit+0x18/0x30 [oops]
RSP: 0018:ffffc900000d3cb0 EFLAGS: 00000282
RAX: 000000000000000b RBX: ffffffffc0000000 RCX: ffffffff81e3e3a8
RDX: 0000000000000001 RSI: 0000000000000086 RDI: ffffffffc0001033
RBP: ffffc900000d3e30 R08: 69796d2073706f6f R09: 000000000000013b
R10: ffffea0000373280 R11: ffffffff822d8b2d R12: 0000000000000000
R13: ffffffffc0002050 R14: ffffffffc0002000 R15: ffff88000dc934c8
FS:  00007ffff7ff66a0(0000) GS:ffff88000fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000000dcd2000 CR4: 00000000000006f0
Call Trace:
 do_one_initcall+0x3e/0x170
 do_init_module+0x5b/0x210
 load_module+0x2035/0x29d0
 ? SyS_finit_module+0xa8/0xb0
 SyS_finit_module+0xa8/0xb0
 do_syscall_64+0x6f/0x310
 ? trace_hardirqs_off_thunk+0x1a/0x32
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x7ffff7b36206
RSP: 002b:00007fffffffeb78 EFLAGS: 00000206 ORIG_RAX: 0000000000000139
RAX: ffffffffffffffda RBX: 000000000000005c RCX: 00007ffff7b36206
RDX: 0000000000000000 RSI: 000000000069e010 RDI: 0000000000000003
RBP: 000000000069e010 R08: 00007ffff7ddd320 R09: 0000000000000000
R10: 00007ffff7ddd320 R11: 0000000000000206 R12: 0000000000000003
R13: 00007fffffffef4b R14: 0000000000000000 R15: 0000000000000000
Code: <c7> 04 25 00 00 00 00 00 00 00 00 e8 b2 33 09 c1 31 c0 c3 0f 1f 44
RIP: myinit+0x18/0x30 [oops] RSP: ffffc900000d3cb0
CR2: 0000000000000000
---[ end trace 3cdb4e9d9842b503 ]---
....

To find the line that oopsed, look at the `RIP` register:

....
RIP: 0010:myinit+0x18/0x30 [oops]
....

and then on GDB:

....
./run-gdb
....

run

....
info line *(myinit+0x18)
....

which gives us the correct line:

....
Line 7 of "/root/linux-kernel-module-cheat/out/kernel_modules/x86_64/kernel_modules/panic.c" starts at address 0xbf00001c <myinit+28> and ends at 0xbf00002c <myexit>.
....

This-did not work on `arm` due to <<gdb-step-debug-kernel-module-arm>> so we need to either:

* <<gdb-module_init>>
* <<kernel-module-stack-trace-to-source-line>> post-mortem method

==== dump_stack

The `dump_stack` function produces a stack trace much like panic and oops, but causes no problems and we return to the normal control flow, and can cleanly remove the module afterwards:

....
insmod dump_stack.ko
....

Source: link:kernel_modules/dump_stack.c[]

==== WARN_ON

The `WARN_ON` macro basically just calls <<dump_stack,dump_stack>>.

One extra side effect is that we can make it also panic with:

....
echo 1 > /proc/sys/kernel/panic_on_warn
insmod warn_on.ko
....

Source: link:kernel_modules/warn_on.c[]

Can also be activated with the `panic_on_warn` boot parameter.

=== Pseudo filesystems

Pseudo filesystems are filesystems that don't represent actual files in a hard disk, but rather allow us to do special operations on filesystem-related system calls.

What each pseudo-file does for each related system call does is defined by its <<file-operations>>.

Bibliography:

* https://superuser.com/questions/1198292/what-is-a-pseudo-file-system-in-linux
* https://en.wikipedia.org/wiki/Synthetic_file_system

==== debugfs

Debugfs is the simplest pseudo filesystem to play around with:

....
./debugfs.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/debugfs.c[]
* link:rootfs_overlay/lkmc/debugfs.sh[]

Debugfs is made specifically to help test kernel stuff. Just mount, set <<file-operations>>, and we are done.

For this reason, it is the filesystem that we use whenever possible in our tests.

`debugfs.sh` explicitly mounts a debugfs at a custom location, but the most common mount point is `/sys/kernel/debug`.

This mount not done automatically by the kernel however: we, like most distros, do it from userland with our link:rootfs_overlay/etc/fstab[fstab].

Debugfs support requires the kernel to be compiled with `CONFIG_DEBUG_FS=y`.

Only the more basic file operations can be implemented in debugfs, e.g. `mmap` never gets called:

* https://patchwork.kernel.org/patch/9252557/
* https://github.com/torvalds/linux/blob/v4.9/fs/debugfs/file.c#L212

Bibliography: https://github.com/chadversary/debugfs-tutorial

==== procfs

Procfs is just another fops entry point:

....
./procfs.sh
echo $?
....

Outcome: the test passes:

....
0
....

Procfs is a little less convenient than <<debugfs>>, but is more used in serious applications.

Procfs can run all system calls, including ones that debugfs can't, e.g. <<mmap>>.

Sources:

* link:kernel_modules/procfs.c[]
* link:rootfs_overlay/lkmc/procfs.sh[]

Bibliography: https://stackoverflow.com/questions/8516021/proc-create-example-for-kernel-module/18924359#18924359

===== /proc/version

Its data is shared with `uname()`, which is a <<posix,POSIX C>> function and has a Linux syscall to back it up.

Where the data comes from and how to modify it:

* https://unix.stackexchange.com/questions/136959/where-does-uname-get-its-information-from/485962#485962
* https://stackoverflow.com/questions/23424174/how-to-customize-or-remove-extra-linux-kernel-version-details-shown-at-boot

In this repo, leaking host information, and to make builds more reproducible, we are setting:

- user and date to dummy values with `KBUILD_BUILD_USER` and `KBUILD_BUILD_TIMESTAMP`
- hostname to the kernel git commit with `KBUILD_BUILD_HOST` and  `KBUILD_BUILD_VERSION`

A sample result is:

....
Linux version 4.19.0-dirty (lkmc@84df9525b0c27f3ebc2ebb1864fa62a97fdedb7d) (gcc version 6.4.0 (Buildroot 2018.05-00002-gbc60382b8f)) #1 SMP Thu Jan 1 00:00:00 UTC 1970
....

==== sysfs

Sysfs is more restricted than <<procfs>>, as it does not take an arbitrary `file_operations`:

....
./sysfs.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/sysfs.c[]
* link:rootfs_overlay/lkmc/sysfs.sh[]

Vs procfs:

* https://unix.stackexchange.com/questions/4884/what-is-the-difference-between-procfs-and-sysfs
* https://stackoverflow.com/questions/37237835/how-to-attach-file-operations-to-sysfs-attribute-in-platform-driver

You basically can only do `open`, `close`, `read`, `write`, and `lseek` on sysfs files.

It is similar to a <<seq_file>> file operation, except that write is also implemented.

TODO: what are those `kobject` structs? Make a more complex example that shows what they can do.

Bibliography:

* https://github.com/t3rm1n4l/kern-dev-tutorial/blob/1f036ef40fc4378f5c8d2842e55bcea7c6f8894a/05-sysfs/sysfs.c
* https://www.kernel.org/doc/Documentation/kobject.txt
* https://www.quora.com/What-are-kernel-objects-Kobj
* http://www.makelinux.net/ldd3/chp-14-sect-1
* https://www.win.tue.nl/~aeb/linux/lk/lk-13.html

==== Character devices

Character devices can have arbitrary <<file-operations>> associated to them:

....
./character_device.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:rootfs_overlay/lkmc/character_device.sh[]
* link:rootfs_overlay/lkmc/mknoddev.sh[]
* link:kernel_modules/character_device.c[]

Unlike <<procfs>> entires, character device files are created with userland `mknod` or `mknodat` syscalls:

....
mknod </dev/path_to_dev> c <major> <minor>
....

Intuitively, for physical devices like keyboards, the major number maps to which driver, and the minor number maps to which device it is.

A single driver can drive multiple compatible devices.

The major and minor numbers can be observed with:

....
ls -l /dev/urandom
....

Output:

....
crw-rw-rw-    1 root     root        1,   9 Jun 29 05:45 /dev/urandom
....

which means:

* `c` (first letter): this is a character device. Would be `b` for a block device.
* `1,   9`: the major number is `1`, and the minor `9`

To avoid device number conflicts when registering the driver we:

* ask the kernel to allocate a free major number for us with: `register_chrdev(0`
* find ouf which number was assigned by grepping `/proc/devices` for the kernel module name

Bibliography: https://unix.stackexchange.com/questions/37829/understanding-character-device-or-character-special-files/371758#371758

===== Automatically create character device file on insmod

And also destroy it on `rmmod`:

....
./character_device_create.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/character_device_create.c[]
* link:rootfs_overlay/lkmc/character_device_create.sh[]

Bibliography: https://stackoverflow.com/questions/5970595/how-to-create-a-device-node-from-the-init-module-code-of-a-linux-kernel-module/45531867#45531867

=== Pseudo files

==== File operations

File operations are the main method of userland driver communication. `struct file_operations` determines what the kernel will do on filesystem system calls of <<pseudo-filesystems>>.

This example illustrates the most basic system calls: `open`, `read`, `write`, `close` and `lseek`:

....
./fops.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/fops.c[]
* link:rootfs_overlay/lkmc/fops.sh[]

Then give this a try:

....
sh -x ./fops.sh
....

We have put printks on each fop, so this allows you to see which system calls are being made for each command.

No, there no official documentation: http://stackoverflow.com/questions/15213932/what-are-the-struct-file-operations-arguments

==== seq_file

Writing trivial read <<file-operations>> is repetitive and error prone. The `seq_file` API makes the process much easier for those trivial cases:

....
./seq_file.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/seq_file.c[]
* link:rootfs_overlay/lkmc/seq_file.sh[]

In this example we create a debugfs file that behaves just like a file that contains:

....
0
1
2
....

However, we only store a single integer in memory and calculate the file on the fly in an iterator fashion.

`seq_file` does not provide `write`: https://stackoverflow.com/questions/30710517/how-to-implement-a-writable-proc-file-by-using-seq-file-in-a-driver-module

Bibliography:

* link:https://github.com/torvalds/linux/blob/v4.17/Documentation/filesystems/seq_file.txt[Documentation/filesystems/seq_file.txt]
* https://stackoverflow.com/questions/25399112/how-to-use-a-seq-file-in-linux-modules

===== seq_file single_open

If you have the entire read output upfront, `single_open` is an even more convenient version of <<seq_file>>:

....
./seq_file.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/seq_file_single_open.c[]
* link:rootfs_overlay/lkmc/seq_file_single_open.sh[]

This example produces a debugfs file that behaves like a file that contains:

....
ab
cd
....

==== poll

The poll system call allows an user process to do a non-busy wait on a kernel event:

....
./poll.sh
....

Outcome: `jiffies` gets printed to stdout every second from userland.

Sources:

* link:kernel_modules/poll.c[]
* link:include/poll.h[]
* link:rootfs_overlay/lkmc/poll.sh[]

Typically, we are waiting for some hardware to make some piece of data available available to the kernel.

The hardware notifies the kernel that the data is ready with an interrupt.

To simplify this example, we just fake the hardware interrupts with a <<kthread>> that sleeps for a second in an infinite loop.

Bibliography: https://stackoverflow.com/questions/30035776/how-to-add-poll-function-to-the-kernel-module-code/44645336#44645336

==== ioctl

The `ioctl` system call is the best way to pass an arbitrary number of parameters to the kernel in a single go:

....
./ioctl.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/ioctl.c[]
* link:lkmc/ioctl.h[]
* link:userland/kernel_modules/ioctl.c[]
* link:rootfs_overlay/lkmc/ioctl.sh[]

`ioctl` is one of the most important methods of communication with real device drivers, which often take several fields as input.

`ioctl` takes as input:

* an integer `request` : it usually identifies what type of operation we want to do on this call
* an untyped pointer to memory: can be anything, but is typically a pointer to a `struct`
+
The type of the `struct` often depends on the `request` input
+
This `struct` is defined on a uapi-style C header that is used both to compile the kernel module and the userland executable.
+
The fields of this `struct` can be thought of as arbitrary input parameters.

And the output is:

* an integer return value. `man ioctl` documents:
+
____
Usually, on success zero is returned. A few `ioctl()` requests use the return value as an output parameter and return a nonnegative value on success. On error, -1 is returned, and errno is set appropriately.
____
* the input pointer data may be overwritten to contain arbitrary output

Bibliography:

* https://stackoverflow.com/questions/2264384/how-do-i-use-ioctl-to-manipulate-my-kernel-module/44613896#44613896
* https://askubuntu.com/questions/54239/problem-with-ioctl-in-a-simple-kernel-module/926675#926675

==== mmap

The `mmap` system call allows us to share memory between user and kernel space without copying:

....
./mmap.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/mmap.c[]
* link:userland/kernel_modules/mmap.c[]
* link:rootfs_overlay/lkmc/mmap.sh[]

In this example, we make a tiny 4 byte kernel buffer available to user-space, and we then modify it on userspace, and check that the kernel can see the modification.

`mmap`, like most more complex <<file-operations>>, does not work with <<debugfs>> as of 4.9, so we use a <<procfs>> file for it.

Example adapted from: https://coherentmusings.wordpress.com/2014/06/10/implementing-mmap-for-transferring-data-from-user-space-to-kernel-space/

Bibliography:

* https://stackoverflow.com/questions/10760479/mmap-kernel-buffer-to-user-space/10770582#10770582
* https://stackoverflow.com/questions/1623008/allocating-memory-for-user-space-from-kernel-thread
* https://stackoverflow.com/questions/6967933/mmap-mapping-in-user-space-a-kernel-buffer-allocated-with-kmalloc
* https://github.com/jeremytrimble/ezdma
* https://github.com/simonjhall/dma
* https://github.com/ikwzm/udmabuf

==== Anonymous inode

Anonymous inodes allow getting multiple file descriptors from a single filesystem entry, which reduces namespace pollution compared to creating multiple device files:

....
./anonymous_inode.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/anonymous_inode.c[]
* link:lkmc/anonymous_inode.h[]
* link:userland/kernel_modules/anonymous_inode.c[]
* link:rootfs_overlay/lkmc/anonymous_inode.sh[]

This example gets an anonymous inode via <<ioctl>> from a debugfs entry by using `anon_inode_getfd`.

Reads to that inode return the sequence: `1`, `10`, `100`, ... `10000000`, `1`, `100`, ...

Bibliography: https://stackoverflow.com/questions/4508998/what-is-an-anonymous-inode-in-linux/44388030#44388030

==== netlink sockets

Netlink sockets offer a socket API for kernel / userland communication:

....
./netlink.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/netlink.c[]
* link:lkmc/netlink.h[]
* link:userland/kernel_modules/netlink.c[]
* link:rootfs_overlay/lkmc/netlink.sh[]

Launch multiple user requests in parallel to stress our socket:

....
insmod netlink.ko sleep=1
for i in `seq 16`; do ./netlink.out & done
....

TODO: what is the advantage over `read`, `write` and `poll`? https://stackoverflow.com/questions/16727212/how-netlink-socket-in-linux-kernel-is-different-from-normal-polling-done-by-appl

Bibliography:

* https://stackoverflow.com/questions/3299386/how-to-use-netlink-socket-to-communicate-with-a-kernel-module
* https://en.wikipedia.org/wiki/Netlink

=== kthread

Kernel threads are managed exactly like userland threads; they also have a backing `task_struct`, and are scheduled with the same mechanism:

....
insmod kthread.ko
....

Source: link:kernel_modules/kthread.c[]

Outcome: dmesg counts from `0` to `9` once every second infinitely many times:

....
0
1
2
...
8
9
0
1
2
...
....

The count stops when we `rmmod`:

....
rmmod kthread
....

The sleep is done with `usleep_range`, see: <<sleep>>.

Bibliography:

* http://stackoverflow.com/questions/10177641/proper-way-of-handling-threads-in-kernel
* http://stackoverflow.com/questions/4084708/how-to-wait-for-a-linux-kernel-thread-kthreadto-exit

==== kthreads

Let's launch two threads and see if they actually run in parallel:

....
insmod kthreads.ko
....

Source: link:kernel_modules/kthreads.c[]

Outcome: two threads count to dmesg from `0` to `9` in parallel.

Each line has output of form:

....
<thread_id> <count>
....

Possible very likely outcome:

....

1 0
2 0
1 1
2 1
1 2
2 2
1 3
2 3
....

The threads almost always interleaved nicely, thus confirming that they are actually running in parallel.

==== sleep

Count to dmesg every one second from `0` up to `n - 1`:

....
insmod sleep.ko n=5
....

Source: link:kernel_modules/sleep.c[]

The sleep is done with a call to link:https://github.com/torvalds/linux/blob/v4.17/kernel/time/timer.c#L1984[`usleep_range`] directly inside `module_init` for simplicity.

Bibliography:

* https://stackoverflow.com/questions/15994603/how-to-sleep-in-the-linux-kernel/44153288#44153288
* https://github.com/torvalds/linux/blob/v4.17/Documentation/timers/timers-howto.txt

==== Workqueues

A more convenient front-end for <<kthread>>:

....
insmod workqueue_cheat.ko
....

Outcome: count from `0` to `9` infinitely many times

Stop counting:

....
rmmod workqueue_cheat
....

Source: link:kernel_modules/workqueue_cheat.c[]

The workqueue thread is killed after the worker function returns.

We can't call the module just `workqueue.c` because there is already a built-in with that name: https://unix.stackexchange.com/questions/364956/how-can-insmod-fail-with-kernel-module-is-already-loaded-even-is-lsmod-does-not

Bibliography: https://github.com/torvalds/linux/blob/v4.17/Documentation/core-api/workqueue.rst

===== Workqueue from workqueue

Count from `0` to `9` every second infinitely many times by scheduling a new work item from a work item:

....
insmod work_from_work.ko
....

Stop:

....
rmmod work_from_work
....

The sleep is done indirectly through: link:https://github.com/torvalds/linux/blob/v4.17/include/linux/workqueue.h#L522[`queue_delayed_work`], which waits the specified time before scheduling the work.

Source: link:kernel_modules/work_from_work.c[]

==== schedule

Let's block the entire kernel! Yay:

.....
./run --eval-after 'dmesg -n 1;insmod schedule.ko schedule=0'
.....

Outcome: the system hangs, the only way out is to kill the VM.

Source: link:kernel_modules/schedule.c[]

kthreads only allow interrupting if you call `schedule()`, and the `schedule=0` <<kernel-module-parameters,kernel module parameter>> turns it off.

Sleep functions like `usleep_range` also end up calling schedule.

If we allow `schedule()` to be called, then the system becomes responsive:

.....
./run --eval-after 'dmesg -n 1;insmod schedule.ko schedule=1'
.....


and we can observe the counting with:

....
dmesg -w
....

The system also responds if we <<number-of-cores,add another core>>:

....
./run --cpus 2 --eval-after 'dmesg -n 1;insmod schedule.ko schedule=0'
....

==== Wait queues

Wait queues are a way to make a thread sleep until an event happens on the queue:

....
insmod wait_queue.c
....

Dmesg output:

....
0 0
1 0
2 0
# Wait one second.
0 1
1 1
2 1
# Wait one second.
0 2
1 2
2 2
...
....

Stop the count:

....
rmmod wait_queue
....

Source: link:kernel_modules/wait_queue.c[]

This example launches three threads:

* one thread generates events every with link:https://github.com/torvalds/linux/blob/v4.17/include/linux/wait.h#L195[`wake_up`]
* the other two threads wait for that with link:https://github.com/torvalds/linux/blob/v4.17/include/linux/wait.h#L286[`wait_event`], and print a dmesg when it happens.
+
The `wait_event` macro works a bit like:
+
....
while (!cond)
    sleep_until_event
....

=== Timers

Count from `0` to `9` infinitely many times in 1 second intervals using timers:

....
insmod timer.ko
....

Stop counting:

....
rmmod timer
....

Source: link:kernel_modules/timer.c[]

Timers are callbacks that run when an interrupt happens, from the interrupt context itself.

Therefore they produce more accurate timing than thread scheduling, which is more complex, but you can't do too much work inside of them.

Bibliography:

* http://stackoverflow.com/questions/10812858/timers-in-linux-device-drivers
* https://gist.github.com/yagihiro/310149

=== IRQ

==== irq.ko

Brute force monitor every shared interrupt that will accept us:

....
./run --eval-after 'insmod irq.ko' --graphic
....

Source: link:kernel_modules/irq.c[].

Now try the following:

* press a keyboard key and then release it after a few seconds
* press a mouse key, and release it after a few seconds
* move the mouse around

Outcome: dmesg shows which IRQ was fired for each action through messages of type:

....
handler irq = 1 dev = 250
....

`dev` is the character device for the module and never changes, as can be confirmed by:

....
grep lkmc_irq /proc/devices
....

The IRQs that we observe are:

* `1` for keyboard press and release.
+
If you hold the key down for a while, it starts firing at a constant rate. So this happens at the hardware level!
* `12` mouse actions

This only works if for IRQs for which the other handlers are registered as `IRQF_SHARED`.

We can see which ones are those, either via dmesg messages of type:

....
genirq: Flags mismatch irq 0. 00000080 (myirqhandler0) vs. 00015a00 (timer)
request_irq irq = 0 ret = -16
request_irq irq = 1 ret = 0
....

which indicate that `0` is not, but `1` is, or with:

....
cat /proc/interrupts
....

which shows:

....
  0:         31   IO-APIC   2-edge      timer
  1:          9   IO-APIC   1-edge      i8042, myirqhandler0
....

so only `1` has `myirqhandler0` attached but not `0`.

The <<qemu-monitor>> also has some interrupt statistics for x86_64:

....
./qemu-monitor info irq
....

TODO: properly understand how each IRQ maps to what number.

==== dummy-irq

The Linux kernel v4.16 mainline also has a `dummy-irq` module at `drivers/misc/dummy-irq.c` for monitoring a single IRQ.

We build it by default with:

....
CONFIG_DUMMY_IRQ=m
....

And then you can do

....
./run --graphic
....

and in guest:

....
modprobe dummy-irq irq=1
....

Outcome: when you click a key on the keyboard, dmesg shows:

....
dummy-irq: interrupt occurred on IRQ 1
....

However, this module is intended to fire only once as can be seen from its source:

....
    static int count = 0;

    if (count == 0) {
        printk(KERN_INFO "dummy-irq: interrupt occurred on IRQ %d\n",
            irq);
        count++;
    }
....

and furthermore interrupt `1` and `12` happen immediately TODO why, were they somehow pending?

So so see something interesting, you need to monitor an interrupt that is more rare than the keyboard, e.g. <<platform_device>>.

==== /proc/interrupts

In the guest with <<qemu-graphic-mode>>:

....
watch -n 1 cat /proc/interrupts
....

Then see how clicking the mouse and keyboard affect the interrupt counts.

This confirms that:

* 1: keyboard
* 12: mouse click and drags

The module also shows which handlers are registered for each IRQ, as we have observed at <<irq-ko>>

When in text mode, we can also observe interrupt line 4 with handler `ttyS0` increase continuously as IO goes through the UART.

=== Kernel utility functions

https://github.com/torvalds/linux/blob/v4.17/Documentation/core-api/kernel-api.rst

==== kstrto

Convert a string to an integer:

....
./kstrto.sh
echo $?
....

Outcome: the test passes:

....
0
....

Sources:

* link:kernel_modules/kstrto.c[]
* link:rootfs_overlay/lkmc/kstrto.sh[]

Bibliography: https://stackoverflow.com/questions/6139493/how-convert-char-to-int-in-linux-kernel/49811658#49811658

==== virt_to_phys

Convert a virtual address to physical:

....
insmod virt_to_phys.ko
cat /sys/kernel/debug/lkmc_virt_to_phys
....

Source: link:kernel_modules/virt_to_phys.c[]

Sample output:

....
*kmalloc_ptr = 0x12345678
kmalloc_ptr = ffff88000e169ae8
virt_to_phys(kmalloc_ptr) = 0xe169ae8
static_var = 0x12345678
&static_var = ffffffffc0002308
virt_to_phys(&static_var) = 0x40002308
....

We can confirm that the `kmalloc_ptr` translation worked with:

....
./qemu-monitor 'xp 0xe169ae8'
....

which reads four bytes from a given physical address, and gives the expected:

....
000000000e169ae8: 0x12345678
....

TODO it only works for kmalloc however, for the static variable:

....
./qemu-monitor 'xp 0x40002308'
....

it gave a wrong value of `00000000`.

Bibliography:

* https://stackoverflow.com/questions/5748492/is-there-any-api-for-determining-the-physical-address-from-virtual-address-in-li/45128487#45128487
* https://stackoverflow.com/questions/39134990/mmap-of-dev-mem-fails-with-invalid-argument-for-virt-to-phys-address-but-addre/45127582#45127582
* https://stackoverflow.com/questions/43325205/can-we-use-virt-to-phys-for-user-space-memory-in-kernel-module

===== Userland physical address experiments

Only tested in x86_64.

The Linux kernel exposes physical addresses to userland through:

* `/proc/<pid>/maps`
* `/proc/<pid>/pagemap`
* `/dev/mem`

In this section we will play with them.

First get a virtual address to play with:

....
./posix/virt_to_phys_test.out &
....

Source: link:userland/posix/virt_to_phys_test.c[]

Sample output:

....
vaddr 0x600800
pid 110
....

The program:

* allocates a `volatile` variable and sets is value to `0x12345678`
* prints the virtual address of the variable, and the program PID
* runs a while loop until until the value of the variable gets mysteriously changed somehow, e.g. by nasty tinkerers like us

Then, translate the virtual address to physical using `/proc/<pid>/maps` and `/proc/<pid>/pagemap`:

....
./linux/virt_to_phys_user.out 110 0x600800
....

Sample output physical address:

....
0x7c7b800
....

Source: link:userland/linux/virt_to_phys_user.c[]

Now we can verify that `linux/virt_to_phys_user.out` gave the correct physical address in the following ways:

* <<qemu-xp>>
* <<dev-mem>>

Bibliography:

* https://stackoverflow.com/questions/17021214/decode-proc-pid-pagemap-entry/45126141#45126141
* https://stackoverflow.com/questions/6284810/proc-pid-pagemaps-and-proc-pid-maps-linux/45500208#45500208

====== QEMU xp

The `xp` <<qemu-monitor>> command reads memory at a given physical address.

First launch `linux/virt_to_phys_user.out` as described at <<userland-physical-address-experiments>>.

On a second terminal, use QEMU to read the physical address:

....
./qemu-monitor 'xp 0x7c7b800'
....

Output:

....
0000000007c7b800: 0x12345678
....

Yes!!! We read the correct value from the physical address.

We could not find however to write to memory from the QEMU monitor, boring.

[[dev-mem]]
====== /dev/mem

`/dev/mem` exposes access to physical addresses, and we use it through the convenient `devmem` BusyBox utility.

First launch `linux/virt_to_phys_user.out` as described at <<userland-physical-address-experiments>>.

Next, read from the physical address:

....
devmem 0x7c7b800
....

Possible output:

....
Memory mapped at address 0x7ff7dbe01000.
Value at address 0X7C7B800 (0x7ff7dbe01800): 0x12345678
....

which shows that the physical memory contains the expected value `0x12345678`.

`0x7ff7dbe01000` is a new virtual address that `devmem` maps to the physical address to be able to read from it.

Modify the physical memory:

....
devmem 0x7c7b800 w 0x9abcdef0
....

After one second, we see on the screen:

....
i 9abcdef0
[1]+  Done                       ./posix/virt_to_phys_test.out
....

so the value changed, and the `while` loop exited!

This example requires:

* `CONFIG_STRICT_DEVMEM=n`, otherwise `devmem` fails with:
+
....
devmem: mmap: Operation not permitted
....
* `nopat` kernel parameter

which we set by default.

Bibliography: https://stackoverflow.com/questions/11891979/how-to-access-mmaped-dev-mem-without-crashing-the-linux-kernel

====== pagemap_dump.out

Dump the physical address of all pages mapped to a given process using `/proc/<pid>/maps` and `/proc/<pid>/pagemap`.

First launch `linux/virt_to_phys_user.out` as described at <<userland-physical-address-experiments>>. Suppose that the output was:

....
# ./posix/virt_to_phys_test.out &
vaddr 0x601048
pid 63
# ./linux/virt_to_phys_user.out 63 0x601048
0x1a61048
....

Now obtain the page map for the process:

....
./linux/pagemap_dump.out 63
....

Sample output excerpt:

....
vaddr pfn soft-dirty file/shared swapped present library
400000 1ede 0 1 0 1 ./posix/virt_to_phys_test.out
600000 1a6f 0 0 0 1 ./posix/virt_to_phys_test.out
601000 1a61 0 0 0 1 ./posix/virt_to_phys_test.out
602000 2208 0 0 0 1 [heap]
603000 220b 0 0 0 1 [heap]
7ffff78ec000 1fd4 0 1 0 1 /lib/libuClibc-1.0.30.so
....

Source: link:userland/linux/pagemap_dump.c[]

Adapted from: https://github.com/dwks/pagemap/blob/8a25747bc79d6080c8b94eac80807a4dceeda57a/pagemap2.c

Meaning of the flags:

* `vaddr`: first virtual address of a page the belongs to the process. Notably:
+
....
./run-toolchain readelf -- -l "$(./getvar userland_build_dir)/posix/virt_to_phys_test.out"
....
+
contains:
+
....
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
...
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x000000000000075c 0x000000000000075c  R E    0x200000
  LOAD           0x0000000000000e98 0x0000000000600e98 0x0000000000600e98
                 0x00000000000001b4 0x0000000000000218  RW     0x200000

 Section to Segment mapping:
  Segment Sections...
...
   02     .interp .hash .dynsym .dynstr .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame
   03     .ctors .dtors .jcr .dynamic .got.plt .data .bss
....
+
from which we deduce that:
+
** `400000` is the text segment
** `600000` is the data segment
* `pfn`: add three zeroes to it, and you have the physical address.
+
Three zeroes is 12 bits which is 4kB, which is the size of a page.
+
For example, the virtual address `0x601000` has `pfn` of `0x1a61`, which means that its physical address is `0x1a61000`
+
This is consistent with what `linux/virt_to_phys_user.out` told us: the virtual address `0x601048` has physical address `0x1a61048`.
+
`048` corresponds to the three last zeroes, and is the offset within the page.
+
Also, this value falls inside `0x601000`, which as previously analyzed is the data section, which is the normal location for global variables such as ours.
* `soft-dirty`: TODO
* `file/shared`: TODO. `1` seems to indicate that the page can be shared across processes, possibly for read-only pages? E.g. the text segment has `1`, but the data has `0`.
* `swapped`: TODO swapped to disk?
* `present`: TODO vs swapped?
* `library`: which executable owns that page

This program works in two steps:

* parse the human readable lines lines from `/proc/<pid>/maps`. This files contains lines of form:
+
....
7ffff7b6d000-7ffff7bdd000 r-xp 00000000 fe:00 658                        /lib/libuClibc-1.0.22.so
....
+
which tells us that:
+
** `7f8af99f8000-7f8af99ff000` is a virtual address range that belong to the process, possibly containing multiple pages.
** `/lib/libuClibc-1.0.22.so` is the name of the library that owns that memory
* loop over each page of each address range, and ask `/proc/<pid>/pagemap` for more information about that page, including the physical address

=== Linux kernel tracing

Good overviews:

* http://www.brendangregg.com/blog/2015-07-08/choosing-a-linux-tracer.html by Brendan Greg, AKA the master of tracing. Also: https://github.com/brendangregg/perf-tools
* https://jvns.ca/blog/2017/07/05/linux-tracing-systems/

I hope to have examples of all methods some day, since I'm obsessed with visibility.

==== CONFIG_PROC_EVENTS

Logs proc events such as process creation to a link:kernel_modules/netlink.c[netlink socket].

We then have a userland program that listens to the events and prints them out:

....
# ./linux/proc_events.out &
# set mcast listen ok
# sleep 2 & sleep 1
fork: parent tid=48 pid=48 -> child tid=79 pid=79
fork: parent tid=48 pid=48 -> child tid=80 pid=80
exec: tid=80 pid=80
exec: tid=79 pid=79
# exit: tid=80 pid=80 exit_code=0
exit: tid=79 pid=79 exit_code=0
echo a
a
#
....

Source: link:userland/linux/proc_events.c[]

TODO: why `exit: tid=79` shows after `exit: tid=80`?

Note how `echo a` is a Bash built-in, and therefore does not spawn a new process.

TODO: why does this produce no output?

....
./linux/proc_events.out >f &
....

* https://stackoverflow.com/questions/6075013/detect-launching-of-programs-on-linux-platform/8255487#8255487
* https://serverfault.com/questions/199654/does-anyone-know-a-simple-way-to-monitor-root-process-spawn
* https://unix.stackexchange.com/questions/260162/how-to-track-newly-created-processes

TODO can you get process data such as UID and process arguments? It seems not since `exec_proc_event` contains so little data: https://github.com/torvalds/linux/blob/v4.16/include/uapi/linux/cn_proc.h#L80 We could try to immediately read it from `/proc`, but there is a risk that the process finished and another one took its PID, so it wouldn't be reliable.

* https://unix.stackexchange.com/questions/163681/print-pids-and-names-of-processes-as-they-are-created/163689 requests process name
* https://serverfault.com/questions/199654/does-anyone-know-a-simple-way-to-monitor-root-process-spawn requests UID

===== CONFIG_PROC_EVENTS aarch64

0111ca406bdfa6fd65a2605d353583b4c4051781 was failing with:

....
>>> kernel_modules 1.0 Building
/usr/bin/make -j8 -C '/linux-kernel-module-cheat//out/aarch64/buildroot/build/kernel_modules-1.0/user' BR2_PACKAGE_OPENBLAS="" CC="/linux-kernel-module-cheat//out/aarch64/buildroot/host/bin/aarch64-buildroot-linux-uclibc-gcc" LD="/linux-kernel-module-cheat//out/aarch64/buildroot/host/bin/aarch64-buildroot-linux-uclibc-ld"
/linux-kernel-module-cheat//out/aarch64/buildroot/host/bin/aarch64-buildroot-linux-uclibc-gcc  -ggdb3 -fopenmp -O0 -std=c99 -Wall -Werror -Wextra -o 'proc_events.out' 'proc_events.c'
In file included from /linux-kernel-module-cheat//out/aarch64/buildroot/host/aarch64-buildroot-linux-uclibc/sysroot/usr/include/signal.h:329:0,
                 from proc_events.c:12:
/linux-kernel-module-cheat//out/aarch64/buildroot/host/aarch64-buildroot-linux-uclibc/sysroot/usr/include/sys/ucontext.h:50:16: error: field ‘uc_mcontext’ has incomplete type
     mcontext_t uc_mcontext;
                ^~~~~~~~~~~
....

so we commented it out.

Related threads:

* https://mailman.uclibc-ng.org/pipermail/devel/2018-January/001624.html
* https://github.com/DynamoRIO/dynamorio/issues/2356

If we try to naively update uclibc to 1.0.29 with `buildroot_override`, which contains the above mentioned patch, clean `aarch64` test build fails with:

....
../utils/ldd.c: In function 'elf_find_dynamic':
../utils/ldd.c:238:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     return (void *)byteswap_to_host(dynp->d_un.d_val);
            ^
/tmp/user/20321/cciGScKB.o: In function `process_line_callback':
msgmerge.c:(.text+0x22): undefined reference to `escape'
/tmp/user/20321/cciGScKB.o: In function `process':
msgmerge.c:(.text+0xf6): undefined reference to `poparser_init'
msgmerge.c:(.text+0x11e): undefined reference to `poparser_feed_line'
msgmerge.c:(.text+0x128): undefined reference to `poparser_finish'
collect2: error: ld returned 1 exit status
Makefile.in:120: recipe for target '../utils/msgmerge.host' failed
make[2]: *** [../utils/msgmerge.host] Error 1
make[2]: *** Waiting for unfinished jobs....
/tmp/user/20321/ccF8V8jF.o: In function `process':
msgfmt.c:(.text+0xbf3): undefined reference to `poparser_init'
msgfmt.c:(.text+0xc1f): undefined reference to `poparser_feed_line'
msgfmt.c:(.text+0xc2b): undefined reference to `poparser_finish'
collect2: error: ld returned 1 exit status
Makefile.in:120: recipe for target '../utils/msgfmt.host' failed
make[2]: *** [../utils/msgfmt.host] Error 1
package/pkg-generic.mk:227: recipe for target '/data/git/linux-kernel-module-cheat/out/aarch64/buildroot/build/uclibc-custom/.stamp_built' failed
make[1]: *** [/data/git/linux-kernel-module-cheat/out/aarch64/buildroot/build/uclibc-custom/.stamp_built] Error 2
Makefile:79: recipe for target '_all' failed
make: *** [_all] Error 2
....

Buildroot master has already moved to uclibc 1.0.29 at f8546e836784c17aa26970f6345db9d515411700, but it is not yet in any tag... so I'm not tempted to update it yet just for this.

==== ftrace

Trace a single function:

....
cd /sys/kernel/debug/tracing/

# Stop tracing.
echo 0 > tracing_on

# Clear previous trace.
echo > trace

# List the available tracers, and pick one.
cat available_tracers
echo function > current_tracer

# List all functions that can be traced
# cat available_filter_functions
# Choose one.
echo __kmalloc > set_ftrace_filter
# Confirm that only __kmalloc is enabled.
cat enabled_functions

echo 1 > tracing_on

# Latest events.
head trace

# Observe trace continuously, and drain seen events out.
cat trace_pipe &
....

Sample output:

....
# tracer: function
#
# entries-in-buffer/entries-written: 97/97   #P:1
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
            head-228   [000] ....   825.534637: __kmalloc <-load_elf_phdrs
            head-228   [000] ....   825.534692: __kmalloc <-load_elf_binary
            head-228   [000] ....   825.534815: __kmalloc <-load_elf_phdrs
            head-228   [000] ....   825.550917: __kmalloc <-__seq_open_private
            head-228   [000] ....   825.550953: __kmalloc <-tracing_open
            head-229   [000] ....   826.756585: __kmalloc <-load_elf_phdrs
            head-229   [000] ....   826.756627: __kmalloc <-load_elf_binary
            head-229   [000] ....   826.756719: __kmalloc <-load_elf_phdrs
            head-229   [000] ....   826.773796: __kmalloc <-__seq_open_private
            head-229   [000] ....   826.773835: __kmalloc <-tracing_open
            head-230   [000] ....   827.174988: __kmalloc <-load_elf_phdrs
            head-230   [000] ....   827.175046: __kmalloc <-load_elf_binary
            head-230   [000] ....   827.175171: __kmalloc <-load_elf_phdrs
....

Trace all possible functions, and draw a call graph:

....
echo 1 > max_graph_depth
echo 1 > events/enable
echo function_graph > current_tracer
....

Sample output:

....
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |
 0)   2.173 us    |                  } /* ntp_tick_length */
 0)               |                  timekeeping_update() {
 0)   4.176 us    |                    ntp_get_next_leap();
 0)   5.016 us    |                    update_vsyscall();
 0)               |                    raw_notifier_call_chain() {
 0)   2.241 us    |                      notifier_call_chain();
 0) + 19.879 us   |                    }
 0)   3.144 us    |                    update_fast_timekeeper();
 0)   2.738 us    |                    update_fast_timekeeper();
 0) ! 117.147 us  |                  }
 0)               |                  _raw_spin_unlock_irqrestore() {
 0)   4.045 us    |                    _raw_write_unlock_irqrestore();
 0) + 22.066 us   |                  }
 0) ! 265.278 us  |                } /* update_wall_time */
....

TODO: what do `+` and `!` mean?

Each `enable` under the `events/` tree enables a certain set of functions, the higher the `enable` more functions are enabled.

TODO: can you get function arguments? https://stackoverflow.com/questions/27608752/does-ftrace-allow-capture-of-system-call-arguments-to-the-linux-kernel-or-only

===== ftrace system calls

https://stackoverflow.com/questions/29840213/how-do-i-trace-a-system-call-in-linux/51856306#51856306

===== trace-cmd

TODO example:

....
./build-buildroot --config 'BR2_PACKAGE_TRACE_CMD=y'
....

==== Kprobes

kprobes is an instrumentation mechanism that injects arbitrary code at a given address in a trap instruction, much like GDB. Oh, the good old kernel. :-)

....
./build-linux --config 'CONFIG_KPROBES=y'
....

Then on guest:

....
insmod kprobe_example.ko
sleep 4 & sleep 4 &'
....

Outcome: dmesg outputs on every fork:

....
<_do_fork> pre_handler: p->addr = 0x00000000e1360063, ip = ffffffff810531d1, flags = 0x246
<_do_fork> post_handler: p->addr = 0x00000000e1360063, flags = 0x246
<_do_fork> pre_handler: p->addr = 0x00000000e1360063, ip = ffffffff810531d1, flags = 0x246
<_do_fork> post_handler: p->addr = 0x00000000e1360063, flags = 0x246
....

Source: link:kernel_modules/kprobe_example.c[]

TODO: it does not work if I try to immediately launch `sleep`, why?

....
insmod kprobe_example.ko
sleep 4 & sleep 4 &
....

I don't think your code can refer to the surrounding kernel code however: the only visible thing is the value of the registers.

You can then hack it up to read the stack and read argument values, but do you really want to?

There is also a kprobes + ftrace based mechanism with `CONFIG_KPROBE_EVENTS=y` which does read the memory for us based on format strings that indicate type... https://github.com/torvalds/linux/blob/v4.16/Documentation/trace/kprobetrace.txt Horrendous. Used by: https://github.com/brendangregg/perf-tools/blob/98d42a2a1493d2d1c651a5c396e015d4f082eb20/execsnoop

Bibliography:

* https://github.com/torvalds/linux/blob/v4.16/Documentation/kprobes.txt
* https://github.com/torvalds/linux/blob/v4.17/samples/kprobes/kprobe_example.c

==== Count boot instructions

TODO: didn't port during refactor after 3b0a343647bed577586989fb702b760bd280844a. Reimplementing should not be hard.

* https://www.quora.com/How-many-instructions-does-a-typical-Linux-kernel-boot-take
* https://github.com/cirosantilli/chat/issues/31
* https://rwmj.wordpress.com/2016/03/17/tracing-qemu-guest-execution/
* `qemu/docs/tracing.txt` and `qemu/docs/replay.txt`
* https://stackoverflow.com/questions/39149446/how-to-use-qemus-simple-trace-backend/46497873#46497873

Results (boot not excluded):

[options="header"]
|===
|Commit |Arch |Simulator |Instruction count

|7228f75ac74c896417fb8c5ba3d375a14ed4d36b
|arm
|QEMU
|680k

|7228f75ac74c896417fb8c5ba3d375a14ed4d36b
|arm
|gem5 AtomicSimpleCPU
|160M

|7228f75ac74c896417fb8c5ba3d375a14ed4d36b
|arm
|gem5 HPI
|155M

|7228f75ac74c896417fb8c5ba3d375a14ed4d36b
|x86_64
|QEMU
|3M

|7228f75ac74c896417fb8c5ba3d375a14ed4d36b
|x86_64
|gem5 AtomicSimpleCPU
|528M

|===

QEMU:

....
./trace-boot --arch x86_64
....

sample output:

....
instructions 1833863
entry_address 0x1000000
instructions_firmware 20708
....

gem5:

....
./run --arch aarch64 --emulator gem5 --eval 'm5 exit'
# Or:
# ./run --arch aarch64 --emulator gem5 --eval 'm5 exit' -- --cpu-type=HPI --caches
./gem5-stat --arch aarch64 sim_insts
....

Notes:

* `0x1000000` is the address where QEMU puts the Linux kernel at with `-kernel` in x86.
+
It can be found from:
+
....
./run-toolchain readelf -- -e "$(./getvar vmlinux)" | grep Entry
....
+
TODO confirm further. If I try to break there with:
+
....
./run-gdb *0x1000000
....
+
but I have no corresponding source line. Also note that this line is not actually the first line, since the kernel messages such as `early console in extract_kernel` have already shown on screen at that point. This does not break at all:
+
....
./run-gdb extract_kernel
....
+
It only appears once on every log I've seen so far, checked with `grep 0x1000000 trace.txt`
+
Then when we count the instructions that run before the kernel entry point, there is only about 100k instructions, which is insignificant compared to the kernel boot itself.
+
TODO `--arch arm` and `--arch aarch64` does not count firmware instructions properly because the entry point address of the ELF file (`ffffff8008080000` for `aarch64`) does not show up on the trace at all. Tested on link:http://github.com/cirosantilli/linux-kernel-module-cheat/commit/f8c0502bb2680f2dbe7c1f3d7958f60265347005[f8c0502bb2680f2dbe7c1f3d7958f60265347005].
* We can also discount the instructions after `init` runs by using `readelf` to get the initial address of `init`. One easy way to do that now is to just run:
+
....
./run-gdb-user "$(./getvar userland_build_dir)/linux/poweroff.out" main
....
+
And get that from the traces, e.g. if the address is `4003a0`, then we search:
+
....
grep -n 4003a0 trace.txt
....
+
I have observed a single match for that instruction, so it must be the init, and there were only 20k instructions after it, so the impact is negligible.
* to disable networking. Is replacing `init` enough?
+
--
** https://superuser.com/questions/181254/how-do-you-boot-linux-with-networking-disabled
** https://superuser.com/questions/684005/how-does-one-permanently-disable-gnu-linux-networking/1255015#1255015
--
+
`CONFIG_NET=n` did not significantly reduce instruction counts, so maybe replacing `init` is enough.
* gem5 simulates memory latencies. So I think that the CPU loops idle while waiting for memory, and counts will be higher.

=== Linux kernel hardening

Make it harder to get hacked and easier to notice that you were, at the cost of some (small?) runtime overhead.

==== CONFIG_FORTIFY_SOURCE

Detects buffer overflows for us:

....
./build-linux --config 'CONFIG_FORTIFY_SOURCE=y' --linux-build-id fortify
./build-modules --clean
./build-modules
./build-buildroot
./run --eval-after 'insmod strlen_overflow.ko' --linux-build-id fortify
....

Possible dmesg output:

....
strlen_overflow: loading out-of-tree module taints kernel.
detected buffer overflow in strlen
------------[ cut here ]------------
....

followed by a trace.

You may not get this error because this depends on `strlen` overflowing at least until the next page: if a random `\0` appears soon enough, it won't blow up as desired.

TODO not always reproducible. Find a more reproducible failure. I could not observe it on:

....
insmod memcpy_overflow.ko
....

Source: link:kernel_modules/strlen_overflow.c[]

Bibliography: https://www.reddit.com/r/hacking/comments/8h4qxk/what_a_buffer_overflow_in_the_linux_kernel_looks/

==== Linux security modules

https://en.wikipedia.org/wiki/Linux_Security_Modules

===== SELinux

TODO get a hello world permission control working:

....
./build-linux \
  --config-fragment linux_config/selinux \
  --linux-build-id selinux \
;
./build-buildroot --config 'BR2_PACKAGE_REFPOLICY=y'
./run --enable-kvm --linux-build-id selinux
....

Source: link:linux_config/selinux[]

This builds:

* `BR2_PACKAGE_REFPOLICY`, which includes a reference `/etc/selinux/config` policy: https://github.com/SELinuxProject/refpolicy
+
refpolicy in turn depends on:
* `BR2_PACKAGE_SETOOLS`, which contains tools such as `getenforced`: https://github.com/SELinuxProject/setools
+
setools depends on:
* `BR2_PACKAGE_LIBSELINUX`, which is the backing userland library

After boot finishes, we see:

....
Starting auditd: mkdir: invalid option -- 'Z'
....

which comes from `/etc/init.d/S01auditd`, because BusyBox' `mkdir` does not have the crazy `-Z` option like Ubuntu. That's amazing!

The kernel logs contain:

....
SELinux:  Initializing.
....

Inside the guest we now have:

....
getenforce
....

which initially says:

....
Disabled
....

TODO: if we try to enforce:

....
setenforce 1
....

it does not work and outputs:

....
setenforce: SELinux is disabled
....

SELinux requires glibc: <<libc-choice>>.

=== User mode Linux

I once got link:https://en.wikipedia.org/wiki/User-mode_Linux[UML] running on a minimal Buildroot setup at: https://unix.stackexchange.com/questions/73203/how-to-create-rootfs-for-user-mode-linux-on-fedora-18/372207#372207

But in part because it is dying, I didn't spend much effort to integrate it into this repo, although it would be a good fit in principle, since it is essentially a virtualization method.

Maybe some brave soul will send a pull request one day.

=== UIO

UIO is a kernel subsystem that allows to do certain types of driver operations from userland.

This would be awesome to improve debuggability and safety of kernel modules.

VFIO looks like a newer and better UIO replacement, but there do not exist any examples of how to use it: https://stackoverflow.com/questions/49309162/interfacing-with-qemu-edu-device-via-userspace-i-o-uio-linux-driver

TODO get something interesting working. I currently don't understand the behaviour very well.

TODO how to ACK interrupts? How to ensure that every interrupt gets handled separately?

TODO how to write to registers. Currently using `/dev/mem` and `lspci`.

This example should handle interrupts from userland and print a message to stdout:

....
./uio_read.sh
....

TODO: what is the expected behaviour? I should have documented this when I wrote this stuff, and I'm that lazy right now that I'm in the middle of a refactor :-)

UIO interface in a nutshell:

* blocking read / poll: waits until interrupts
* `write`: call `irqcontrol` callback. Default: 0 or 1 to enable / disable interrupts.
* `mmap`: access device memory

Sources:

* link:userland/kernel_modules/uio_read.c[]
* link:rootfs_overlay/lkmc/uio_read.sh[]

Bibliography:

* https://stackoverflow.com/questions/15286772/userspace-vs-kernel-space-driver
* https://01.org/linuxgraphics/gfx-docs/drm/driver-api/uio-howto.html
* https://stackoverflow.com/questions/7986260/linux-interrupt-handling-in-user-space
* https://yurovsky.github.io/2014/10/10/linux-uio-gpio-interrupt/
* https://github.com/bmartini/zynq-axis/blob/65a3a448fda1f0ea4977adfba899eb487201853d/dev/axis.c
* https://yurovsky.github.io/2014/10/10/linux-uio-gpio-interrupt/
* http://nairobi-embedded.org/uio_example.html that website has QEMU examples for everything as usual. The example has a kernel-side which creates the memory mappings and is used by the user.
* https://stackoverflow.com/questions/49309162/interfacing-with-qemu-edu-device-via-userspace-i-o-uio-linux-driver
* userland driver stability questions:
** https://stackoverflow.com/questions/8030758/getting-kernel-version-from-linux-kernel-module-at-runtime/45430233#45430233
** https://stackoverflow.com/questions/37098482/how-to-build-a-linux-kernel-module-so-that-it-is-compatible-with-all-kernel-rele/45429681#45429681
** https://liquidat.wordpress.com/2007/07/21/linux-kernel-2623-to-have-stable-userspace-driver-api/

=== Linux kernel interactive stuff

[[fbcon]]
==== Linux kernel console fun

Requires <<graphics>>.

You can also try those on the `Ctrl-Alt-F3` of your Ubuntu host, but it is much more fun inside a VM!

Stop the cursor from blinking:

....
echo 0 > /sys/class/graphics/fbcon/cursor_blink
....

Rotate the console 90 degrees! https://askubuntu.com/questions/237963/how-do-i-rotate-my-display-when-not-using-an-x-server

....
echo 1 > /sys/class/graphics/fbcon/rotate
....

Relies on: `CONFIG_FRAMEBUFFER_CONSOLE_ROTATION=y`.

Documented under: `Documentation/fb/`.

TODO: font and keymap. Mentioned at: https://cmcenroe.me/2017/05/05/linux-console.html and I think can be done with BusyBox `loadkmap` and `loadfont`, we just have to understand their formats, related:

* https://unix.stackexchange.com/questions/177024/remap-keyboard-on-the-linux-console
* https://superuser.com/questions/194202/remapping-keys-system-wide-in-linux-not-just-in-x

==== Linux kernel magic keys

Requires <<graphics>>.

Let's have some fun.

I think most are implemented under:

....
drivers/tty
....

TODO find all.

Scroll up / down the terminal:

....
Shift-PgDown
Shift-PgUp
....

Or inside `./qemu-monitor`:

....
sendkey shift-pgup
sendkey shift-pgdown
....

===== Ctrl Alt Del

Run `/sbin/reboot` on guest:

....
Ctrl-Alt-Del
....

Enabled from our link:rootfs_overlay/etc/inittab[]:

....
::ctrlaltdel:/sbin/reboot
....

Linux tries to reboot, and QEMU shutdowns due to the `-no-reboot` option which we set by default for: <<exit-emulator-on-panic>>.

Under the hood, behaviour is controlled by the `reboot` syscall:

....
man 2 reboot
....

`reboot` calls can set either of the these behaviours for `Ctrl-Alt-Del`:

* do a hard shutdown syscall. Set in ublibc C code with:
+
....
reboot(RB_ENABLE_CAD)
....
+
or from procfs with:
+
....
echo 1 > /proc/sys/kernel/ctrl-alt-del
....
* send a SIGINT to the init process. This is what BusyBox' init does, and it then execs the string set in `inittab`.
+
Set in uclibc C code with:
+
....
reboot(RB_DISABLE_CAD)
....
+
or from procfs with:
+
....
echo 0 > /proc/sys/kernel/ctrl-alt-del
....

Minimal example:

....
./run --kernel-cli 'init=/lkmc/linux/ctrl_alt_del.out' --graphic
....

Source: link:userland/linux/ctrl_alt_del.c[]

When you hit `Ctrl-Alt-Del` in the guest, our tiny init handles a `SIGINT` sent by the kernel and outputs to stdout:

....
cad
....

To map between `man 2 reboot` and the uClibc `RB_*` magic constants see:

....
less "$(./getvar buildroot_build_build_dir)"/uclibc-*/include/sys/reboot.h"
....

The procfs mechanism is documented at:

....
less linux/Documentation/sysctl/kernel.txt
....

which says:

....
When the value in this file is 0, ctrl-alt-del is trapped and
sent to the init(1) program to handle a graceful restart.
When, however, the value is > 0, Linux's reaction to a Vulcan
Nerve Pinch (tm) will be an immediate reboot, without even
syncing its dirty buffers.

Note: when a program (like dosemu) has the keyboard in 'raw'
mode, the ctrl-alt-del is intercepted by the program before it
ever reaches the kernel tty layer, and it's up to the program
to decide what to do with it.
....

Bibliography:

* https://superuser.com/questions/193652/does-linux-have-a-ctrlaltdel-equivalent/1324415#1324415
* https://unix.stackexchange.com/questions/42573/meaning-and-commands-for-ctrlaltdel/444969#444969

===== SysRq

We cannot test these actual shortcuts on QEMU since the host captures them at a lower level, but from:

....
./qemu-monitor
....

we can for example crash the system with:

....
sendkey alt-sysrq-c
....

Same but boring because no magic key:

....
echo c > /proc/sysrq-trigger
....

Implemented in:

....
drivers/tty/sysrq.c
....

On your host, on modern systems that don't have the `SysRq` key you can do:

....
Alt-PrtSc-space
....

which prints a message to `dmesg` of type:

....
sysrq: SysRq : HELP : loglevel(0-9) reboot(b) crash(c) terminate-all-tasks(e) memory-full-oom-kill(f) kill-all-tasks(i) thaw-filesystems(j) sak(k) show-backtrace-all-active-cpus(l) show-memory-usage(m) nice-all-RT-tasks(n) poweroff(o) show-registers(p) show-all-timers(q) unraw(r) sync(s) show-task-states(t) unmount(u) show-blocked-tasks(w) dump-ftrace-buffer(z)
....

Individual SysRq can be enabled or disabled with the bitmask:

....
/proc/sys/kernel/sysrq
....

The bitmask is documented at:

....
less linux/Documentation/admin-guide/sysrq.rst
....

Bibliography: https://en.wikipedia.org/wiki/Magic_SysRq_key

==== TTY

In order to play with TTYs, do this:

....
printf '
tty2::respawn:/sbin/getty -n -L -l /lkmc/loginroot.sh tty2 0 vt100
tty3::respawn:-/bin/sh
tty4::respawn:/sbin/getty 0 tty4
tty63::respawn:-/bin/sh
::respawn:/sbin/getty -L ttyS0 0 vt100
::respawn:/sbin/getty -L ttyS1 0 vt100
::respawn:/sbin/getty -L ttyS2 0 vt100
# Leave one serial empty.
#::respawn:/sbin/getty -L ttyS3 0 vt100
' >> rootfs_overlay/etc/inittab
./build-buildroot
./run --graphic -- \
  -serial telnet::1235,server,nowait \
  -serial vc:800x600 \
  -serial telnet::1236,server,nowait \
;
....

and on a second shell:

....
telnet localhost 1235
....

We don't add more TTYs by default because it would spawn more processes, even if we use `askfirst` instead of `respawn`.

On the GUI, switch TTYs with:

* `Alt-Left` or `Alt-Right:` go to previous / next populated `/dev/ttyN` TTY. Skips over empty TTYs.
* `Alt-Fn`: go to the nth TTY. If it is  not populated, don't go there.
* `chvt <n>`: go to the n-th virtual TTY, even if it is empty: https://superuser.com/questions/33065/console-commands-to-change-virtual-ttys-in-linux-and-openbsd

You can also test this on most hosts such as Ubuntu 18.04, except that when in the GUI, you must use `Ctrl-Alt-Fx` to switch to another terminal.

Next, we also have the following shells running on the serial ports, hit enter to activate them:

* `/dev/ttyS0`: first shell that was used to run QEMU, corresponds to QEMU's `-serial mon:stdio`.
+
It would also work if we used `-serial stdio`, but:
+
--
** `Ctrl-C` would kill QEMU instead of going to the guest
** `Ctrl-A C` wouldn't open the QEMU console there
--
+
see also: https://stackoverflow.com/questions/49716931/how-to-run-qemu-with-nographic-and-monitor-but-still-be-able-to-send-ctrlc-to
* `/dev/ttyS1`: second shell running `telnet`
* `/dev/ttyS2`: go on the GUI and enter `Ctrl-Alt-2`, corresponds to QEMU's `-serial vc`. Go back to the main console with `Ctrl-Alt-1`.

although we cannot change between terminals from there.

Each populated TTY contains a "shell":

* `-/bin/sh`: goes directly into an `sh` without a login prompt.
+
The trailing dash `-` can be used on any command. It makes the command that follows take over the TTY, which is what we typically want for interactive shells: https://askubuntu.com/questions/902998/how-to-check-which-tty-am-i-using
+
The `getty` executable however also does this operation and therefore dispenses the `-`.
* `/sbin/getty` asks for password, and then gives you an `sh`
+
We can overcome the password prompt with the `-l /lkmc/loginroot.sh` technique explained at: https://askubuntu.com/questions/902998/how-to-check-which-tty-am-i-using but I don't see any advantage over `-/bin/sh` currently.

Identify the current TTY with the command:

....
tty
....

Bibliography:

* https://unix.stackexchange.com/questions/270272/how-to-get-the-tty-in-which-bash-is-running/270372
* https://unix.stackexchange.com/questions/187319/how-to-get-the-real-name-of-the-controlling-terminal
* https://unix.stackexchange.com/questions/77796/how-to-get-the-current-terminal-name
* https://askubuntu.com/questions/902998/how-to-check-which-tty-am-i-using

This outputs:

* `/dev/console` for the initial GUI terminal. But I think it is the same as `/dev/tty1`, because if I try to do
+
....
tty1::respawn:-/bin/sh
....
+
it makes the terminal go crazy, as if multiple processes are randomly eating up the characters.
* `/dev/ttyN` for the other graphic TTYs. Note that there are only 63 available ones, from `/dev/tty1` to `/dev/tty63` (`/dev/tty0` is the current one): link:https://superuser.com/questions/449781/why-is-there-so-many-linux-dev-tty[]. I think this is determined by:
+
....
#define MAX_NR_CONSOLES 63
....
+
in `linux/include/uapi/linux/vt.h`.
* `/dev/ttySN` for the text shells.
+
These are Serial ports, see this to understand what those represent physically: https://unix.stackexchange.com/questions/307390/what-is-the-difference-between-ttys0-ttyusb0-and-ttyama0-in-linux/367882#367882
+
There are only 4 serial ports, I think this is determined by QEMU. TODO check.
+
See also: https://stackoverflow.com/questions/16706423/two-instances-of-busybox-on-separate-serial-lines-ttysn

Get the TTY in bulk for all processes:

....
./psa.sh
....

Source: link:rootfs_overlay/lkmc/psa.sh[].

The TTY appears under the `TT` section, which is enabled by `-o tty`. This shows the TTY device number, e.g.:

....
4,1
....

and we can then confirm it with:

....
ls -l /dev/tty1
....

Next try:

....
insmod kthread.ko
....

and switch between virtual terminals, to understand that the dmesg goes to whatever current virtual terminal you are on, but not the others, and not to the serial terminals.

Bibliography:

* https://serverfault.com/questions/119736/how-to-enable-multiple-virtual-consoles-on-linux
* https://github.com/mirror/busybox/blob/1_28_3/examples/inittab#L60
* http://web.archive.org/web/20180117124612/http://nairobi-embedded.org/qemu_serial_port_system_console.html

===== Start a getty from outside of init

TODO: https://unix.stackexchange.com/questions/196704/getty-start-from-command-line

TODO: how to place an `sh` directly on a TTY as well without `getty`?

If I try the exact same command that the `inittab` is doing from a regular shell after boot:

....
/sbin/getty 0 tty1
....

it fails with:

....
getty: setsid: Operation not permitted
....

The following however works:

....
./run --eval 'getty 0 tty1 & getty 0 tty2 & getty 0 tty3 & sleep 99999999' --graphic
....

presumably because it is being called from `init` directly?

Outcome: `Alt-Right` cycles between three TTYs, `tty1` being the default one that appears under the boot messages.

`man 2 setsid` says that there is only one failure possibility:

____
EPERM  The process group ID of any process equals the PID of the calling process.  Thus, in particular, setsid() fails if the calling process is already a process group leader.
____

We can get some visibility into it to try and solve the problem with:

....
./psa.sh
....

===== console kernel boot parameter

Take the command described at <<tty>> and try adding the following:

* `-e 'console=tty7'`: boot messages still show on `/dev/tty1` (TODO how to change that?), but we don't get a shell at the end of boot there.
+
Instead, the shell appears on `/dev/tty7`.
* `-e 'console=tty2'` like `/dev/tty7`, but `/dev/tty2` is broken, because we have two shells there:
** one due to the `::respawn:-/bin/sh` entry which uses whatever `console` points to
** another one due to the `tty2::respawn:/sbin/getty` entry we added
* `-e 'console=ttyS0'` much like `tty2`, but messages show only on serial, and the terminal is broken due to having multiple shells on it
* `-e 'console=tty1 console=ttyS0'`: boot messages show on both `tty1` and `ttyS0`, but only `S0` gets a shell because it came last

==== CONFIG_LOGO

If you run in <<graphics>>, then you get a Penguin image for <<number-of-cores,every core>> above the console! https://askubuntu.com/questions/80938/is-it-possible-to-get-the-tux-logo-on-the-text-based-boot

This is due to the link:https://github.com/torvalds/linux/blob/v4.17/drivers/video/logo/Kconfig#L5[`CONFIG_LOGO=y`] option which we enable by default.

`reset` on the terminal then kills the poor penguins.

When `CONFIG_LOGO=y` is set, the logo can be disabled at boot with:

....
./run --kernel-cli 'logo.nologo'
....

* https://stackoverflow.com/questions/39872463/how-can-i-disable-the-startup-penguins-and-boot-text-on-linaro-ubuntu
* https://unix.stackexchange.com/questions/332198/centos-remove-penguin-logo-at-startup

Looks like a recompile is needed to modify the image...

* https://superuser.com/questions/736423/changing-kernel-bootsplash-image
* https://unix.stackexchange.com/questions/153975/how-to-change-boot-logo-in-linux-mint

=== DRM

DRM / DRI is the new interface that supersedes `fbdev`:

....
./build-buildroot --config 'BR2_PACKAGE_LIBDRM=y'
./build-userland --package libdrm -- userland/libs/libdrm/modeset.c
./run --eval-after './libs/libdrm/modeset.out' --graphic
....

Source: link:userland/libs/libdrm_modeset.c[]

Outcome: for a few seconds, the screen that contains the terminal gets taken over by changing colors of the rainbow.

TODO not working for `aarch64`, it takes over the screen for a few seconds and the kernel messages disappear, but the screen stays black all the time.

....
./build-buildroot --config 'BR2_PACKAGE_LIBDRM=y'
./build-userland --package libdrm
./build-buildroot
./run --eval-after './libs/libdrm/modeset.out' --graphic
....

<<kmscube>> however worked, which means that it must be a bug with this demo?

We set `CONFIG_DRM=y` on our default kernel configuration, and it creates one device file for each display:

....
# ls -l /dev/dri
total 0
crw-------    1 root     root      226,   0 May 28 09:41 card0
# grep 226 /proc/devices
226 drm
# ls /sys/module/drm /sys/module/drm_kms_helper/
....

Try creating new displays:

....
./run --arch aarch64 --graphic -- -device virtio-gpu-pci
....

to see multiple `/dev/dri/cardN`, and then use a different display with:

....
./run --eval-after './libs/libdrm/modeset.out' --graphic
....

Bibliography:

* https://dri.freedesktop.org/wiki/DRM/
* https://en.wikipedia.org/wiki/Direct_Rendering_Infrastructure
* https://en.wikipedia.org/wiki/Direct_Rendering_Manager
* https://en.wikipedia.org/wiki/Mode_setting KMS

Tested on: link:http://github.com/cirosantilli/linux-kernel-module-cheat/commit/93e383902ebcc03d8a7ac0d65961c0e62af9612b[93e383902ebcc03d8a7ac0d65961c0e62af9612b]

==== kmscube

....
./build-buildroot --config-fragment buildroot_config/kmscube
....

Outcome: a colored spinning cube coded in OpenGL + EGL takes over your display and spins forever: https://www.youtube.com/watch?v=CqgJMgfxjsk

It is a bit amusing to see OpenGL running outside of a window manager window like that: https://stackoverflow.com/questions/3804065/using-opengl-without-a-window-manager-in-linux/50669152#50669152

TODO: it is very slow, about 1FPS. I tried Buildroot master ad684c20d146b220dd04a85dbf2533c69ec8ee52 with:

....
make qemu_x86_64_defconfig
printf "
BR2_CCACHE=y
BR2_PACKAGE_HOST_QEMU=y
BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE=n
BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE=y
BR2_PACKAGE_HOST_QEMU_VDE2=y
BR2_PACKAGE_KMSCUBE=y
BR2_PACKAGE_MESA3D=y
BR2_PACKAGE_MESA3D_DRI_DRIVER_SWRAST=y
BR2_PACKAGE_MESA3D_OPENGL_EGL=y
BR2_PACKAGE_MESA3D_OPENGL_ES=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y
" >> .config
....

and the FPS was much better, I estimate something like 15FPS.

On Ubuntu 18.04 with NVIDIA proprietary drivers:

....
sudo apt-get instll kmscube
kmscube
....

fails with:

....
drmModeGetResources failed: Invalid argument
failed to initialize legacy DRM
....

See also: https://github.com/robclark/kmscube/issues/12 and https://stackoverflow.com/questions/26920835/can-egl-application-run-in-console-mode/26921287#26921287

Tested on: link:http://github.com/cirosantilli/linux-kernel-module-cheat/commit/2903771275372ccfecc2b025edbb0d04c4016930[2903771275372ccfecc2b025edbb0d04c4016930]

==== kmscon

TODO get working.

Implements a console for <<drm>>.

The Linux kernel has a built-in fbdev console: <<fbcon,fbcon>> but not for <<drm>> it seems.

The upstream project seems dead with last commit in 2014: https://www.freedesktop.org/wiki/Software/kmscon/

Build failed in Ubuntu 18.04 with: https://github.com/dvdhrm/kmscon/issues/131 but this fork compiled but didn't run on host: https://github.com/Aetf/kmscon/issues/2#issuecomment-392484043

Haven't tested the fork on QEMU too much insanity.

==== libdri2

TODO get working.

Looks like a more raw alternative to libdrm:

....
./build-buildroot --config 'BR2_PACKABE_LIBDRI2=y'
wget \
  -O "$(./getvar userland_source_dir)/dri2test.c" \
  https://raw.githubusercontent.com/robclark/libdri2/master/test/dri2test.c \
;
./build-userland
....

but then I noticed that that example requires multiple files, and I don't feel like integrating it into our build.

When I build it on Ubuntu 18.04 host, it does not generate any executable, so I'm confused.

=== Linux kernel testing

Bibliography: https://stackoverflow.com/questions/3177338/how-is-the-linux-kernel-tested

==== Linux Test Project

https://github.com/linux-test-project/ltp

Tests a lot of Linux and POSIX userland visible interfaces.

Buildroot already has a package, so it is trivial to build it:

....
./build-buildroot --config 'BR2_PACKAGE_LTP_TESTSUITE=y'
....

So now let's try and see if the `exit` system call is working:

....
/usr/lib/ltp-testsuite/testcases/bin/exit01
....

which gives successful output:

....
exit01      1  TPASS  :  exit() test PASSED
....

and has source code at: https://github.com/linux-test-project/ltp/blob/20190115/testcases/kernel/syscalls/exit/exit01.c

Besides testing any kernel modifications you make, LTP can also be used to the system call implementation of <<user-mode-simulation>> as shown at <<user-mode-buildroot-executables>>:

....
./run --userland "$(./getvar buildroot_target_dir)/usr/lib/ltp-testsuite/testcases/bin/exit01"
....

Tested at: 287c83f3f99db8c1ff9bbc85a79576da6a78e986 + 1.

==== stress

<<posix>> userland stress. Two versions:

....
./build-buildroot \
  --config 'BR2_PACKAGE_STRESS=y' \
  --config 'BR2_PACKAGE_STRESS_NG=y' \
;
....

`STRESS_NG` is likely the best, but it requires glibc: <<libc-choice>>.

Websites:

* https://people.seas.harvard.edu/~apw/stress/
* https://github.com/ColinIanKing/stress-ng

`stress` usage:

....
stress --help
stress -c 16 &
ps
....

and notice how 16 threads were created in addition to a parent worker thread.

It just runs forever, so kill it when you get tired:

....
kill %1
....

`stress -c 1 -t 1` makes gem5 irresponsive for a very long time.

== Linux kernel build system

=== vmlinux vs bzImage vs zImage vs Image

Between all archs on QEMU and gem5 we touch all of those kernel built output files.

We are trying to maintain a description of each at: https://unix.stackexchange.com/questions/5518/what-is-the-difference-between-the-following-kernel-makefile-terms-vmlinux-vml/482978#482978

QEMU does not seem able to boot ELF files like `vmlinux`, only `objdump` code: https://superuser.com/questions/1376944/can-qemu-boot-linux-from-vmlinux-instead-of-bzimage

Converting `arch/*` images to `vmlinux` is possible in x86 with link:https://github.com/torvalds/linux/blob/master/scripts/extract-vmlinux[`extract-vmlinux`]. But for arm it fails with:

....
run-detectors: unable to find an interpreter for
....

as mentioned at:

* https://unix.stackexchange.com/questions/352215/how-do-i-extract-vmlinux-from-an-arm-image
* https://raspberrypi.stackexchange.com/questions/88621/why-doesnt-extract-vmlinux-work-with-raspbians-boot-kernel-img

== QEMU

=== Introduction to QEMU

link:https://en.wikipedia.org/wiki/QEMU[QEMU] is a system simulator: it simulates a CPU and devices such as interrupt handlers, timers, UART, screen, keyboard, etc.

If you are familiar with link:https://en.wikipedia.org/wiki/VirtualBox[VirtualBox], then QEMU then basically does the same thing: it opens a "window" inside your desktop that can run an operating system inside your operating system.

Also both can use very similar techniques: either link:lhttps://en.wikipedia.org/wiki/Binary_translation[binary translation] or <<KVM>>. VirtualBox' binary translator is / was based on QEMU's it seems: https://en.wikipedia.org/wiki/VirtualBox#Software-based_virtualization

The huge advantage of QEMU over VirtualBox is that is supports cross arch simulation, e.g. simulate an ARM guest on an x86 host.

QEMU is likely the leading cross arch system simulator as of 2018. It is even the default <<android>> simulator that developers get with Android Studio 3 to develop apps without real hardware.

Another advantage of QEMU over virtual box is that it doesn't have Oracle' hands all all over it, more like RedHat + ARM.

Another advantage of QEMU is that is has no nice configuration GUI. Because who needs GUIs when you have 50 million semi-documented CLI options? Android Studio adds a custom GUI configuration tool on top of it.

QEMU is also supported by Buildroot in-tree, see e.g.: https://github.com/buildroot/buildroot/blob/2018.05/configs/qemu_aarch64_virt_defconfig We however just build our own manually with link:build-qemu[], as it gives more flexibility, and building QEMU is very easy!

All of this makes QEMU the natural choice of reference system simulator for this repo.

=== Disk persistency

We disable disk persistency for both QEMU and gem5 by default, to prevent the emulator from putting the image in an unknown state.

For QEMU, this is done by passing the `snapshot` option to `-drive`, and for gem5 it is the default behaviour.

If you hack up our link:run[] script to remove that option, then:

....
./run --eval-after 'date >f;poweroff'

....

followed by:

....
./run --eval-after 'cat f'
....

gives the date, because `poweroff` without `-n` syncs before shutdown.

The `sync` command also saves the disk:

....
sync
....

When you do:

....
./build-buildroot
....

the disk image gets overwritten by a fresh filesystem and you lose all changes.

Remember that if you forcibly turn QEMU off without `sync` or `poweroff` from inside the VM, e.g. by closing the QEMU window, disk changes may not be saved.

Persistency is also turned off when booting from <<initrd>> with a CPIO instead of with a disk.

Disk persistency is useful to re-run shell commands from the history of a previous session with `Ctrl-R`, but we felt that the loss of determinism was not worth it.

==== gem5 disk persistency

TODO how to make gem5 disk writes persistent?

As of cadb92f2df916dbb47f428fd1ec4932a2e1f0f48 there are some `read_only` entries in the <<config-ini>> under cow sections, but hacking them to true did not work:

....
diff --git a/configs/common/FSConfig.py b/configs/common/FSConfig.py
index 17498c42b..76b8b351d 100644
--- a/configs/common/FSConfig.py
+++ b/configs/common/FSConfig.py
@@ -60,7 +60,7 @@ os_types = { 'alpha' : [ 'linux' ],
            }

 class CowIdeDisk(IdeDisk):
-    image = CowDiskImage(child=RawDiskImage(read_only=True),
+    image = CowDiskImage(child=RawDiskImage(read_only=False),
                          read_only=False)

     def childImage(self, ci):
....

The directory of interest is `src/dev/storage`.

=== gem5 qcow2

qcow2 does not appear supported, there are not hits in the source tree, and there is a mention on Nate's 2009 wishlist: http://gem5.org/Nate%27s_Wish_List

This would be good to allow storing smaller sparse ext2 images locally on disk.

=== Snapshot

QEMU allows us to take snapshots at any time through the monitor.

You can then restore CPU, memory and disk state back at any time.

qcow2 filesystems must be used for that to work.

To test it out, login into the VM with and run:

....
./run --eval-after 'umount /mnt/9p/*;./count.sh'
....

On another shell, take a snapshot:

....
./qemu-monitor savevm my_snap_id
....

The counting continues.

Restore the snapshot:

....
./qemu-monitor loadvm my_snap_id
....

and the counting goes back to where we saved. This shows that CPU and memory states were reverted.

The `umount` is needed because snapshotting conflicts with <<9p>>, which we felt is a more valuable default. If you forget to unmount, the following error appears on the QEMU monitor:

.....
Migration is disabled when VirtFS export path '/linux-kernel-module-cheat/out/x86_64/buildroot/build' is mounted in the guest using mount_tag 'host_out'
.....

We can also verify that the disk state is also reversed. Guest:

....
echo 0 >f
....

Monitor:

....
./qemu-monitor savevm my_snap_id
....

Guest:

....
echo 1 >f
....

Monitor:

....
./qemu-monitor loadvm my_snap_id
....

Guest:

....
cat f
....

And the output is `0`.

Our setup does not allow for snapshotting while using <<initrd>>.

Bibliography: https://stackoverflow.com/questions/40227651/does-qemu-emulator-have-checkpoint-function/48724371#48724371

==== Snapshot internals

Snapshots are stored inside the `.qcow2` images themselves.

They can be observed with:

....
"$(./getvar buildroot_host_dir)/bin/qemu-img" info "$(./getvar qcow2_file)"
....

which after `savevm my_snap_id` and `savevm asdf` contains an output of type:

....
image: out/x86_64/buildroot/images/rootfs.ext2.qcow2
file format: qcow2
virtual size: 512M (536870912 bytes)
disk size: 180M
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         my_snap_id              47M 2018-04-27 21:17:50   00:00:15.251
2         asdf                    47M 2018-04-27 21:20:39   00:00:18.583
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
....

As a consequence:

* it is possible to restore snapshots across boots, since they stay on the same image the entire time
* it is not possible to use snapshots with <<initrd>> in our setup, since we don't pass `-drive` at all when initrd is enabled

=== Device models

This section documents:

* how to interact with peripheral hardware device models through device drivers
* how to write your own hardware device models for our emulators, see also: https://stackoverflow.com/questions/28315265/how-to-add-a-new-device-in-qemu-source-code

For the more complex interfaces, we focus on simplified educational devices, either:

* present in the QEMU upstream:
** <<qemu-edu>>
* added in link:https://github.com/cirosantilli/qemu[our fork of QEMU]:
** <<pci_min>>
** <<platform_device>>

==== PCI

Only tested in x86.

===== pci_min

PCI driver for our minimal `pci_min.c` QEMU fork device:

....
./run -- -device lkmc_pci_min
....

then:

....
insmod pci_min.ko
....

Sources:

* Kernel module: link:kernel_modules/pci_min.c[].
* QEMU device: https://github.com/cirosantilli/qemu/blob/lkmc/hw/misc/lkmc_pci_min.c

Outcome:

....
<4>[   10.608241] pci_min: loading out-of-tree module taints kernel.
<6>[   10.609935] probe
<6>[   10.651881] dev->irq = 11
lkmc_pci_min mmio_write addr = 0 val = 12345678 size = 4
<6>[   10.668515] irq_handler irq = 11 dev = 251
lkmc_pci_min mmio_write addr = 4 val = 0 size = 4
....

What happened:

* right at probe time, we write to a register
* our hardware model is coded such that it generates an interrupt when written to
* the Linux kernel interrupt handler write to another register, which tells the hardware to stop sending interrupts

Kernel messages and printks from inside QEMU are shown all together, to see that more clearly, run in <<qemu-graphic-mode>> instead.

We don't enable the device by default because it does not work for vanilla QEMU, which we often want to test with this repository.

Probe already does a MMIO write, which generates an IRQ and tests everything.

[[qemu-edu]]
===== QEMU edu PCI device

Small upstream educational PCI device:

....
./qemu_edu.sh
....

This tests a lot of features of the edu device, to understand the results, compare the inputs with the documentation of the hardware: https://github.com/qemu/qemu/blob/v2.12.0/docs/specs/edu.txt

Sources:

* kernel module: link:kernel_modules/qemu_edu.c[]
* QEMU device: https://github.com/qemu/qemu/blob/v2.12.0/hw/misc/edu.c
* test script: link:rootfs_overlay/lkmc/qemu_edu.sh[]

Works because we add to our default QEMU CLI:

....
-device edu
....

This example uses:

* the QEMU `edu` educational device, which is a minimal educational in-tree PCI example
* the `pci.ko` kernel module, which exercises the `edu` hardware.
+
I've contacted the awesome original author author of `edu` link:https://github.com/jirislaby[Jiri Slaby], and he told there is no official kernel module example because this was created for a kernel module university course that he gives, and he didn't want to give away answers. link:https://github.com/cirosantilli/how-to-teach-efficiently[I don't agree with that philosophy], so students, cheat away with this repo and go make startups instead.

TODO exercise DMA on the kernel module. The `edu` hardware model has that feature:

* https://stackoverflow.com/questions/32592734/are-there-any-dma-driver-example-pcie-and-fpga/44716747#44716747
* https://stackoverflow.com/questions/17913679/how-to-instantiate-and-use-a-dma-driver-linux-module

===== Manipulate PCI registers directly

In this section we will try to interact with PCI devices directly from userland without kernel modules.

First identify the PCI device with:

....
lspci
....

In our case for example, we see:

....
00:06.0 Unclassified device [00ff]: Device 1234:11e8 (rev 10)
00:07.0 Unclassified device [00ff]: Device 1234:11e9
....

which we identify as being `edu` and `pci_min` respectively by the magic numbers: `1234:11e?`

Alternatively, we can also do use the QEMU monitor:

....
./qemu-monitor info qtree
....

which gives:

....
      dev: lkmc_pci_min, id ""
        addr = 07.0
        romfile = ""
        rombar = 1 (0x1)
        multifunction = false
        command_serr_enable = true
        x-pcie-lnksta-dllla = true
        x-pcie-extcap-init = true
        class Class 00ff, addr 00:07.0, pci id 1234:11e9 (sub 1af4:1100)
        bar 0: mem at 0xfeb54000 [0xfeb54007]
      dev: edu, id ""
        addr = 06.0
        romfile = ""
        rombar = 1 (0x1)
        multifunction = false
        command_serr_enable = true
        x-pcie-lnksta-dllla = true
        x-pcie-extcap-init = true
        class Class 00ff, addr 00:06.0, pci id 1234:11e8 (sub 1af4:1100)
        bar 0: mem at 0xfea00000 [0xfeafffff]
....

See also: https://serverfault.com/questions/587189/list-all-devices-emulated-for-a-vm/913622#913622

Read the configuration registers as binary:

....
hexdump /sys/bus/pci/devices/0000:00:06.0/config
....

Get nice human readable names and offsets of the registers and some enums:

....
setpci --dumpregs
....

Get the values of a given config register from its human readable name, either with either bus or device id:

....
setpci -s 0000:00:06.0 BASE_ADDRESS_0
setpci -d 1234:11e9 BASE_ADDRESS_0
....

Note however that `BASE_ADDRESS_0` also appears when you do:

....
lspci -v
....

as:

....
Memory at feb54000
....

Then you can try messing with that address with <<dev-mem>>:

....
devmem 0xfeb54000 w 0x12345678
....

which writes to the first register of our <<pci_min>> device.

The device then fires an interrupt at irq 11, which is unhandled, which leads the kernel to say you are a bad boy:

....
lkmc_pci_min mmio_write addr = 0 val = 12345678 size = 4
<5>[ 1064.042435] random: crng init done
<3>[ 1065.567742] irq 11: nobody cared (try booting with the "irqpoll" option)
....

followed by a trace.

Next, also try using our <<irq-ko>> IRQ monitoring module before triggering the interrupt:

....
insmod irq.ko
devmem 0xfeb54000 w 0x12345678
....

Our kernel module handles the interrupt, but does not acknowledge it like our proper <<pci_min>> kernel module, and so it keeps firing, which leads to infinitely many messages being printed:

....
handler irq = 11 dev = 251
....

===== pciutils

There are two versions of `setpci` and `lspci`:

* a simple one from BusyBox
* a more complete one from link:https://github.com/pciutils/pciutils[pciutils] which Buildroot has a package for, and is the default on Ubuntu 18.04 host. This is the one we enable by default.

===== Introduction to PCI

The PCI standard is non-free, obviously like everything in low level: https://pcisig.com/specifications but Google gives several illegal PDF hits :-)

And of course, the best documentation available is: http://wiki.osdev.org/PCI

Like every other hardware, we could interact with PCI on x86 using only IO instructions and memory operations.

But PCI is a complex communication protocol that the Linux kernel implements beautifully for us, so let's use the kernel API.

Bibliography:

* edu device source and spec in QEMU tree:
** https://github.com/qemu/qemu/blob/v2.7.0/hw/misc/edu.c
** https://github.com/qemu/qemu/blob/v2.7.0/docs/specs/edu.txt
* http://www.zarb.org/~trem/kernel/pci/pci-driver.c inb outb runnable example (no device)
* LDD3 PCI chapter
* another QEMU device + module, but using a custom QEMU device:
** https://github.com/levex/kernel-qemu-pci/blob/31fc9355161b87cea8946b49857447ddd34c7aa6/module/levpci.c
** https://github.com/levex/kernel-qemu-pci/blob/31fc9355161b87cea8946b49857447ddd34c7aa6/qemu/hw/char/lev-pci.c
* https://is.muni.cz/el/1433/podzim2016/PB173/um/65218991/ course given by the creator of the edu device. In Czech, and only describes API
* http://nairobi-embedded.org/linux_pci_device_driver.html

===== PCI BFD

`lspci -k` shows something like:

....
00:04.0 Class 00ff: 1234:11e8 lkmc_pci
....

Meaning of the first numbers:

....
<8:bus>:<5:device>.<3:function>
....

Often abbreviated to BDF.

* bus: groups PCI slots
* device: maps to one slot
* function: https://stackoverflow.com/questions/19223394/what-is-the-function-number-in-pci/44735372#44735372

Sometimes a fourth number is also added, e.g.:

....
0000:00:04.0
....

TODO is that the domain?

Class: pure magic: https://www-s.acm.illinois.edu/sigops/2007/roll_your_own/7.c.1.html TODO: does it have any side effects? Set in the edu device at:

....
k->class_id = PCI_CLASS_OTHERS
....

===== PCI BAR

https://stackoverflow.com/questions/30190050/what-is-base-address-register-bar-in-pcie/44716618#44716618

Each PCI device has 6 BAR IOs (base address register) as per the PCI spec.

Each BAR corresponds to an address range that can be used to communicate with the PCI.

Each BAR is of one of the two types:

* `IORESOURCE_IO`: must be accessed with `inX` and `outX`
* `IORESOURCE_MEM`: must be accessed with `ioreadX` and `iowriteX`. This is the saner method apparently, and what the edu device uses.

The length of each region is defined by the hardware, and communicated to software via the configuration registers.

The Linux kernel automatically parses the 64 bytes of standardized configuration registers for us.

QEMU devices register those regions with:

....
memory_region_init_io(&edu->mmio, OBJECT(edu), &edu_mmio_ops, edu,
                "edu-mmio", 1 << 20);
pci_register_bar(pdev, 0, PCI_BASE_ADDRESS_SPACE_MEMORY, &edu->mmio);
....

==== GPIO

TODO: broken. Was working before we moved `arm` from `-M versatilepb` to `-M virt` around af210a76711b7fa4554dcc2abd0ddacfc810dfd4. Either make it work on `-M virt` if that is possible, or document precisely how to make it work with `versatilepb`, or hopefully `vexpress` which is newer.

QEMU does not have a very nice mechanism to observe GPIO activity: https://raspberrypi.stackexchange.com/questions/56373/is-it-possible-to-get-the-state-of-the-leds-and-gpios-in-a-qemu-emulation-like-t/69267#69267

The best you can do is to hack our link:build[] script to add:

....
HOST_QEMU_OPTS='--extra-cflags=-DDEBUG_PL061=1'
....

where link:http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0190b/index.html[PL061] is the dominating ARM Holdings hardware that handles GPIO.

Then compile with:

....
./build-buildroot --arch arm --config-fragment buildroot_config/gpio
./build-linux --config-fragment linux_config/gpio
....

then test it out with:

....
./gpio.sh
....

Source: link:rootfs_overlay/lkmc/gpio.sh[]

Buildroot's Linux tools package provides some GPIO CLI tools: `lsgpio`, `gpio-event-mon`, `gpio-hammer`, TODO document them here.

==== LEDs

TODO: broken when `arm`  moved to `-M virt`, same as <<gpio>>.

Hack QEMU's `hw/misc/arm_sysctl.c` with a printf:

....
static void arm_sysctl_write(void *opaque, hwaddr offset,
                            uint64_t val, unsigned size)
{
    arm_sysctl_state *s = (arm_sysctl_state *)opaque;

    switch (offset) {
    case 0x08: /* LED */
        printf("LED val = %llx\n", (unsigned long long)val);
....

and then rebuild with:

....
./build-qemu --arch arm
./build-linux --arch arm --config-fragment linux_config/leds
....

But beware that one of the LEDs has a heartbeat trigger by default (specified on dts), so it will produce a lot of output.

And then activate it with:

....
cd /sys/class/leds/versatile:0
cat max_brightness
echo 255 >brightness
....

Relevant QEMU files:

* `hw/arm/versatilepb.c`
* `hw/misc/arm_sysctl.c`

Relevant kernel files:

* `arch/arm/boot/dts/versatile-pb.dts`
* `drivers/leds/led-class.c`
* `drivers/leds/leds-sysctl.c`

==== platform_device

Minimal platform device example coded into the `-M versatilepb` SoC of our QEMU fork.

Using this device now requires checking out to the branch:

....
git checkout platform-device
git submodule sync
....

before building, it does not work on master.

Rationale: we found out that the kernels that build for `qemu -M versatilepb` don't work on gem5 because `versatilepb` is an old pre-v7 platform, and gem5 requires armv7. So we migrated over to `-M virt` to have a single kernel for both gem5 and QEMU, and broke this since the single kernel was more important. TODO port to `-M virt`.

The module itself can be found at: https://github.com/cirosantilli/linux-kernel-module-cheat/blob/platform-device/kernel_modules/platform_device.c

Uses:

* `hw/misc/lkmc_platform_device.c` minimal device added in our QEMU fork to `-M versatilepb`
* the device tree entry we added to our Linux kernel fork: https://github.com/cirosantilli/linux/blob/361bb623671a52a36a077a6dd45843389a687a33/arch/arm/boot/dts/versatile-pb.dts#L42

Expected outcome after insmod:

* QEMU reports MMIO with printfs
* IRQs are generated and handled by this module, which logs to dmesg

Without insmoding this module, try writing to the register with <<dev-mem>>:

....
devmem 0x101e9000 w 0x12345678
....

We can also observe the interrupt with <<dummy-irq>>:

....
modprobe dummy-irq irq=34
insmod platform_device.ko
....

The IRQ number `34` was found by on the dmesg after:

....
insmod platform_device.ko
....

Bibliography: https://stackoverflow.com/questions/28315265/how-to-add-a-new-device-in-qemu-source-code/44612957#44612957

==== gem5 educational hardware models

TODO get some working!

http://gedare-csphd.blogspot.co.uk/2013/02/adding-simple-io-device-to-gem5.html

=== QEMU monitor

The QEMU monitor is a magic terminal that allows you to send text commands to the QEMU VM itself: https://en.wikibooks.org/wiki/QEMU/Monitor

While QEMU is running, on another terminal, run:

....
./qemu-monitor
....

or send one command such as `info qtree` and quit the monitor:

....
./qemu-monitor info qtree
....

or equivalently:

....
echo 'info qtree' | ./qemu-monitor
....

Source: link:qemu-monitor[]

`qemu-monitor` uses the `-monitor` QEMU command line option, which makes the monitor listen from a socket.

Alternatively, we can also enter the QEMU monitor from inside `-nographics` <<qemu-text-mode>> with:

....
Ctrl-A C
....

and go back to the terminal with:

....
Ctrl-A C
....

* http://stackoverflow.com/questions/14165158/how-to-switch-to-qemu-monitor-console-when-running-with-curses
* https://superuser.com/questions/488263/how-to-switch-to-the-qemu-control-panel-with-nographics

When in graphic mode, we can do it from the GUI:

....
Ctrl-Alt ?
....

where `?` is a digit `1`, or `2`, or, `3`, etc. depending on what else is available on the GUI: serial, parallel and frame buffer.

Finally, we can also access QEMU monitor commands directly from <<gdb>> with the `monitor` command:

....
./run-gdb
....

then inside that shell:

....
monitor info qtree
....

This way you can use both QEMU monitor and GDB commands to inspect the guest from inside a single shell! Pretty awesome.

In general, `./qemu-monitor` is the best option, as it:

* works on both modes
* allows to use the host Bash history to re-run one off commands
* allows you to search the output of commands on your host shell even when in graphic mode

Getting everything to work required careful choice of QEMU command line options:

* https://stackoverflow.com/questions/49716931/how-to-run-qemu-with-nographic-and-monitor-but-still-be-able-to-send-ctrlc-to/49751144#49751144
* https://unix.stackexchange.com/questions/167165/how-to-pass-ctrl-c-to-the-guest-when-running-qemu-with-nographic/436321#436321

==== QEMU monitor from guest

Peter Maydell said potentially not possible nicely as of August 2018: https://stackoverflow.com/questions/51747744/how-to-run-a-qemu-monitor-command-from-inside-the-guest/51764110#51764110

It is also worth looking into the QEMU Guest Agent tool `qemu-gq` that can be enabled with:

....
./build-buildroot --config 'BR2_PACKAGE_QEMU=y'
....

See also: https://superuser.com/questions/930588/how-to-pass-commands-noninteractively-to-running-qemu-from-the-guest-qmp-via-te

==== QEMU monitor from GDB

When doing <<gdb>> it is possible to send QEMU monitor commands through the GDB `monitor` command, which saves you the trouble of opening yet another shell.

Try for example:

....
monitor help
monitor info qtree
....

=== Debug the emulator

When you start hacking QEMU or gem5, it is useful to see what is going on inside the emulator themselves.

This is of course trivial since they are just regular userland programs on the host, but we make it a bit easier with:

....
./run --debug-vm
....

Then you could:

....
break edu_mmio_read
run
....

And in QEMU:

....
./qemu_edu.sh
....

Or for a faster development loop:

....
./run --debug-vm-args '-ex "break edu_mmio_read" -ex "run"'
....

When in <<qemu-text-mode>>, using `--debug-vm` makes Ctrl-C not get passed to the QEMU guest anymore: it is instead captured by GDB itself, so allow breaking. So e.g. you won't be able to easily quit from a guest program like:

....
sleep 10
....

In graphic mode, make sure that you never click inside the QEMU graphic while debugging, otherwise you mouse gets captured forever, and the only solution I can find is to go to a TTY with `Ctrl-Alt-F1` and `kill` QEMU.

You can still send key presses to QEMU however even without the mouse capture, just either click on the title bar, or alt tab to give it focus.

==== Debug gem5 Python scripts

Start pdb at the first instruction:

....
./run --emulator gem5 --gem5-exe-args='--pdb' --terminal
....

Requires `--terminal` as we must be on foreground.

Alternatively, you can add to the point of the code where you want to break the usual:

....
import ipdb; ipdb.set_trace()
....

and then run with:

....
./run --emulator gem5 --terminal
....

TODO test PyCharm: https://stackoverflow.com/questions/51982735/writing-gem5-configuration-scripts-with-pycharm

=== Tracing

QEMU can log several different events.

The most interesting are events which show instructions that QEMU ran, for which we have a helper:

....
./trace-boot --arch x86_64
....

Under the hood, this uses QEMU's `-trace` option.

You can then inspect the address of each instruction run:

....
less "$(./getvar --arch x86_64 run_dir)/trace.txt"
....

Sample output excerpt:

....
exec_tb 0.000 pid=10692 tb=0x7fb4f8000040 pc=0xfffffff0
exec_tb 35.391 pid=10692 tb=0x7fb4f8000180 pc=0xfe05b
exec_tb 21.047 pid=10692 tb=0x7fb4f8000340 pc=0xfe066
exec_tb 12.197 pid=10692 tb=0x7fb4f8000480 pc=0xfe06a
....

Get the list of available trace events:

....
./run --trace help
....

TODO: any way to show the actualy disassembled instruction executed directly from there? Possible with <<qemu-d-tracing>>.

Enable other specific trace events:

....
./run --trace trace1,trace2
./qemu-trace2txt -a "$arch"
less "$(./getvar -a "$arch" run_dir)/trace.txt"
....

This functionality relies on the following setup:

* `./configure --enable-trace-backends=simple`. This logs in a binary format to the trace file.
+
It makes 3x execution faster than the default trace backend which logs human readable data to stdout.
+
Logging with the default backend `log` greatly slows down the CPU, and in particular leads to this boot message:
+
....
All QSes seen, last rcu_sched kthread activity 5252 (4294901421-4294896169), jiffies_till_next_fqs=1, root ->qsmask 0x0
swapper/0       R  running task        0     1      0 0x00000008
 ffff880007c03ef8 ffffffff8107aa5d ffff880007c16b40 ffffffff81a3b100
 ffff880007c03f60 ffffffff810a41d1 0000000000000000 0000000007c03f20
 fffffffffffffedc 0000000000000004 fffffffffffffedc ffffffff00000000
Call Trace:
 <IRQ>  [<ffffffff8107aa5d>] sched_show_task+0xcd/0x130
 [<ffffffff810a41d1>] rcu_check_callbacks+0x871/0x880
 [<ffffffff810a799f>] update_process_times+0x2f/0x60
....
+
in which the boot appears to hang for a considerable time.
* patch  QEMU source to remove the `disable` from `exec_tb` in the `trace-events` file. See also: https://rwmj.wordpress.com/2016/03/17/tracing-qemu-guest-execution/

==== QEMU -d tracing

QEMU also has a second trace mechanism in addition to `-trace`, find out the events with:

....
./run -- -d help
....

Let's pick the one that dumps executed instructions, `in_asm`:

....
./run --eval './linux/poweroff.out' -- -D out/trace.txt -d in_asm
less out/trace.txt
....

Sample output excerpt:

....
----------------
IN:
0xfffffff0:  ea 5b e0 00 f0           ljmpw    $0xf000:$0xe05b

----------------
IN:
0x000fe05b:  2e 66 83 3e 88 61 00     cmpl     $0, %cs:0x6188
0x000fe062:  0f 85 7b f0              jne      0xd0e1
....

TODO: after `IN:`, symbol names are meant to show, which is awesome, but I don't get any. I do see them however when running a bare metal example from: https://github.com/cirosantilli/newlib-examples/tree/900a9725947b1f375323c7da54f69e8049158881

TODO: what is the point of having two mechanisms, `-trace` and `-d`? `-d` tracing is cool because it does not require a messy recompile, and it can also show symbols.

==== QEMU trace register values

TODO: is it possible to show the register values for each instruction?

This would include the memory values read into the registers.

Asked at: https://superuser.com/questions/1377764/how-to-trace-the-register-values-of-executed-instructions-in-qemu

Seems impossible due to optimizations that QEMU does:

* https://lists.gnu.org/archive/html/qemu-devel/2015-06/msg07479.html
* https://lists.gnu.org/archive/html/qemu-devel/2014-04/msg02856.html
* https://lists.gnu.org/archive/html/qemu-devel/2012-08/msg03057.html

PANDA can list memory addresses, so I bet it can also decode the instructions: https://github.com/panda-re/panda/blob/883c85fa35f35e84a323ed3d464ff40030f06bd6/panda/docs/LINE_Censorship.md I wonder why they don't just upstream those things to QEMU's tracing: https://github.com/panda-re/panda/issues/290

gem5 can do it: <<gem5-tracing>>.

==== Trace source lines

We can further use Binutils' `addr2line` to get the line that corresponds to each address:

....
./trace-boot --arch x86_64
./trace2line --arch x86_64
less "$(./getvar --arch x86_64 run_dir)/trace-lines.txt"
....

The last commands takes several seconds.

The format is as follows:

....
39368 _static_cpu_has arch/x86/include/asm/cpufeature.h:148
....

Where:

* `39368`: number of consecutive times that a line ran. Makes the output much shorter and more meaningful
* `_static_cpu_has`: name of the function that contains the line
* `arch/x86/include/asm/cpufeature.h:148`: file and line

This could of course all be done with GDB, but it would likely be too slow to be practical.

TODO do even more awesome offline post-mortem analysis things, such as:

* detect if we are in userspace or kernelspace. Should be a simple matter of reading the
* read kernel data structures, and determine the current thread. Maybe we can reuse / extend the kernel's GDB Python scripts??

==== QEMU record and replay

QEMU runs, unlike gem5, are not deterministic by default, however it does support a record and replay mechanism that allows you to replay a previous run deterministically.

This awesome feature allows you to examine a single run as many times as you would like until you understand everything:

....
# Record a run.
./run --eval-after './linux/rand_check.out;./linux/poweroff.out;' --record
# Replay the run.
./run --eval-after './linux/rand_check.out;./linux/poweroff.out;' --replay
....

A convenient shortcut to do both at once to test the feature is:

....
./qemu-rr --eval-after './linux/rand_check.out;./linux/poweroff.out;'
....

By comparing the terminal output of both runs, we can see that they are the exact same, including things which normally differ across runs:

* timestamps of dmesg output
* <<rand_check-out>> output

The record and replay feature was revived around QEMU v3.0.0. It existed earlier but it rot completely. As of v3.0.0 it is still flaky: sometimes we get deadlocks, and only a limited number of command line arguments are supported.

Documented at: https://github.com/qemu/qemu/blob/v2.12.0/docs/replay.txt

TODO: using `-r` as above leads to a kernel warning:

....
rcu_sched detected stalls on CPUs/tasks
....

TODO: replay deadlocks intermittently at disk operations, last kernel message:

....
EXT4-fs (sda): re-mounted. Opts: block_validity,barrier,user_xattr
....

TODO replay with network gets stuck:

....
./qemu-rr --eval-after 'ifup -a;wget -S google.com;./linux/poweroff.out;'
....

after the message:

....
adding dns 10.0.2.3
....

There is explicit network support on the QEMU patches, but either it is buggy or we are not using the correct magic options.

Solved on unmerged c42634d8e3428cfa60672c3ba89cabefc720cde9 from https://github.com/ispras/qemu/tree/rr-180725

TODO `arm` and `aarch64` only seem to work with initrd since I cannot plug a working IDE disk device? See also: https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg05245.html

Then, when I tried with <<initrd>> and no disk:

....
./build-buildroot --arch aarch64 --initrd
./qemu-rr --arch aarch64 --eval-after './linux/rand_check.out;./linux/poweroff.out;' --initrd
....

QEMU crashes with:

....
ERROR:replay/replay-time.c:49:replay_read_clock: assertion failed: (replay_file && replay_mutex_locked())
....

I had the same error previously on x86-64, but it was fixed: https://bugs.launchpad.net/qemu/+bug/1762179 so maybe the forgot to fix it for `aarch64`?

Solved on unmerged c42634d8e3428cfa60672c3ba89cabefc720cde9 from https://github.com/ispras/qemu/tree/rr-180725

===== QEMU reverse debugging

TODO get working.

QEMU replays support checkpointing, and this allows for a simplistic "reverse debugging" implementation proposed at https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg00478.html on the unmerged link:https://github.com/ispras/qemu/tree/rr-180725[]:

....
./run --eval-after './linux/rand_check.out;./linux/poweroff.out;' --record
./run --eval-after './linux/rand_check.out;./linux/poweroff.out;' --replay --gdb-wait
....

On another shell:

....
./run-gdb start_kernel
....

In GDB:

....
n
n
n
n
reverse-continue
....

and we are back at `start_kernel`

==== QEMU trace multicore

TODO: is there any way to distinguish which instruction runs on each core? Doing:

....
./run --arch x86_64 --cpus 2 --eval './linux/poweroff.out' --trace exec_tb
./qemu-trace2txt
....

just appears to output both cores intertwined without any clear differentiation.

==== gem5 tracing

gem5 provides also provides a tracing mechanism documented at: link:http://www.gem5.org/Trace_Based_Debugging[]:

....
./run --arch aarch64 --eval 'm5 exit' --emulator gem5 --trace Exec
less "$(./getvar --arch aarch64 run_dir)/trace.txt"
....

Output the trace to stdout instead of a file:

....
./run \
  --arch aarch64 \
  --emulator gem5 \
  --eval 'm5 exit' \
  --trace ExecAll \
  --trace-stdout \
;
....

We also have a shortcut for `--trace ExecAll -trace-stdout` with `--trace-insts-stdout`

....
./run \
  --arch aarch64 \
  --emulator gem5 \
  --eval 'm5 exit' \
  --trace-insts-stdout \
;
....

This would produce a lot of output however, so you will likely not want that when tracing a Linux kernel boot instructions. But it can be very convenient for smaller traces such as <<baremetal>>.

List all available debug flags:

....
./run --arch aarch64 --gem5-exe-args='--debug-help' --emulator gem5
....

but to understand most of them you have to look at the source code:

....
less "$(./getvar gem5_source_dir)/src/cpu/SConscript"
less "$(./getvar gem5_source_dir)/src/cpu/exetrace.cc"
....

The traces are generated from `DPRINTF(<trace-id>` calls scattered throughout the code.

As can be seen on the `Sconstruct`, `Exec` is just an alias that enables a set of flags.

Be warned, the trace is humongous, at 16Gb.

We can make the trace smaller by naming the trace file as `trace.txt.gz`, which enables GZIP compression, but that is not currently exposed on our scripts, since you usually just need something human readable to work on.

Enabling tracing made the runtime about 4x slower on the <<p51>>, with or without `.gz` compression.

The output format is of type:

....
25007000: system.cpu T0 : @start_kernel    : stp
25007000: system.cpu T0 : @start_kernel.0  :   addxi_uop   ureg0, sp, #-112 : IntAlu :  D=0xffffff8008913f90
25007500: system.cpu T0 : @start_kernel.1  :   strxi_uop   x29, [ureg0] : MemWrite :  D=0x0000000000000000 A=0xffffff8008913f90
25008000: system.cpu T0 : @start_kernel.2  :   strxi_uop   x30, [ureg0, #8] : MemWrite :  D=0x0000000000000000 A=0xffffff8008913f98
25008500: system.cpu T0 : @start_kernel.3  :   addxi_uop   sp, ureg0, #0 : IntAlu :  D=0xffffff8008913f90
....

There are two types of lines:

* full instructions, as the first line. Only shown if the `ExecMacro` flag is given.
* micro ops that constitute the instruction, the lines that follow. Yes, `aarch64` also has microops: link:https://superuser.com/questions/934752/do-arm-processors-like-cortex-a9-use-microcode/934755#934755[]. Only shown if the `ExecMicro` flag is given.

Breakdown:

* `25007500`: time count in some unit. Note how the microops execute at further timestamps.
* `system.cpu`: distinguishes between CPUs when there are more than one
* `T0`: thread number. TODO: link:https://superuser.com/questions/133082/hyper-threading-and-dual-core-whats-the-difference/995858#995858[hyperthread]? How to play with it?
* `@start_kernel`: we are in the `start_kernel` function. Awesome feature! Implemented with libelf https://sourceforge.net/projects/elftoolchain/ copy pasted in-tree `ext/libelf`. To get raw addresses, remove the `ExecSymbol`, which is enabled by `Exec`. This can be done with `Exec,-ExecSymbol`.
* `.1` as in `@start_kernel.1`: index of the microop
* `stp`: instruction disassembly. Seems to use `.isa` files dispersed per arch, which is an in house format: http://gem5.org/ISA_description_system
* `strxi_uop   x29, [ureg0]`: microop disassembly.
* `MemWrite :  D=0x0000000000000000 A=0xffffff8008913f90`: a memory write microop:
** `D` stands for data, and represents the value that was written to memory or to a register
** `A` stands for address, and represents the address to which the value was written. It only shows when data is being written to memory, but not to registers.

The best way to verify all of this is to write some <<baremetal,baremetal code>>

Trace the source lines just like <<trace-source-lines,for QEMU>> with:

....
./trace-boot --arch aarch64 --emulator gem5
./trace2line --arch aarch64 --emulator gem5
less "$(./getvar --arch aarch64 run_dir)/trace-lines.txt"
....

TODO: 7452d399290c9c1fc6366cdad129ef442f323564 `./trace2line` this is too slow and takes hours. QEMU's processing of 170k events takes 7 seconds. gem5's processing is analogous, but there are 140M events, so it should take 7000 seconds ~ 2 hours which seems consistent with what I observe, so maybe there is no way to speed this up... The workaround is to just use gem5's `ExecSymbol` to get function granularity, and then GDB individually if line detail is needed?

=== QEMU GUI is unresponsive

Sometimes in Ubuntu 14.04, after the QEMU SDL GUI starts, it does not get updated after keyboard strokes, and there are artifacts like disappearing text.

We have not managed to track this problem down yet, but the following workaround always works:

....
Ctrl-Shift-U
Ctrl-C
root
....

This started happening when we switched to building QEMU through Buildroot, and has not been observed on later Ubuntu.

Using text mode is another workaround if you don't need GUI features.

== gem5

Getting started at: <<gem5-buildroot-setup>>.

=== gem5 vs QEMU

* advantages of gem5:
** simulates a generic more realistic pipelined and optionally out of order CPU cycle by cycle, including a realistic DRAM memory access model with latencies, caches and page table manipulations. This allows us to:
+
--
*** do much more realistic performance benchmarking with it, which makes absolutely no sense in QEMU, which is purely functional
*** make certain functional observations that are not possible in QEMU, e.g.:
**** use Linux kernel APIs that flush cache memory like DMA, which are crucial for driver development. In QEMU, the driver would still work even if we forget to flush caches.
**** spectre / meltdown:
***** https://www.mail-archive.com/gem5-users@gem5.org/msg15319.html
***** https://github.com/jlpresearch/gem5/tree/spectre-test
--
+
It is not of course truly cycle accurate, as that:
+
--
** would require exposing proprietary information of the CPU designs: link:https://stackoverflow.com/questions/17454955/can-you-check-performance-of-a-program-running-with-qemu-simulator/33580850#33580850[]
** would make the simulation even slower TODO confirm, by how much
--
+
but the approximation is reasonable.
+
It is used mostly for microarchitecture research purposes: when you are making a new chip technology, you don't really need to specialize enormously to an existing microarchitecture, but rather develop something that will work with a wide range of future architectures.
** runs are deterministic by default, unlike QEMU which has a special <<qemu-record-and-replay>> mode, that requires first playing the content once and then replaying
** gem5 ARM at least appears to implement more low level CPU functionality than QEMU, e.g. QEMU only added EL2 in 2018: https://stackoverflow.com/questions/42824706/qemu-system-aarch64-entering-el1-when-emulating-a53-power-up See also: <<arm-exception-level>>
* disadvantage of gem5: slower than QEMU, see: <<benchmark-linux-kernel-boot>>
+
This implies that the user base is much smaller, since no Android devs.
+
Instead, we have only chip makers, who keep everything that really works closed, and researchers, who can't version track or document code properly >:-) And this implies that:
+
--
** the documentation is more scarce
** it takes longer to support new hardware features
--
+
Well, not that AOSP is that much better anyways.
* not sure: gem5 has BSD license while QEMU has GPL
+
This suits chip makers that want to distribute forks with secret IP to their customers.
+
On the other hand, the chip makers tend to upstream less, and the project becomes more crappy in average :-)

=== gem5 run benchmark

OK, this is why we used gem5 in the first place, performance measurements!

Let's see how many cycles https://en.wikipedia.org/wiki/Dhrystone[Dhrystone], which Buildroot provides, takes for a few different input parameters.

First build Dhrystone into the root filesystem:

....
./build-buildroot --config 'BR2_PACKAGE_DHRYSTONE=y'
....

Then, a flexible setup is demonstrated at:

....
./gem5-bench-dhrystone
cat out/gem5-bench-dhrystone.txt
....

Source: link:gem5-bench-dhrystone[]

Sample output:

....
n cycles
1000 12898577
10000 23441629
100000 128428617
....

so as expected, the Dhrystone run with a larger input parameter `100000` took more cycles than the ones with smaller input parameters.

The `gem5-stats` commands output the approximate number of CPU cycles it took Dhrystone to run.

Another interesting example can be found at: link:gem5-bench-cache[].

A more naive and simpler to understand approach would be a direct:

....
./run --arch aarch64 --emulator gem5 --eval 'm5 checkpoint;m5 resetstats;dhrystone 10000;m5 exit'
....

but the problem is that this method does not allow to easily run a different script without running the boot again, see: <<gem5-restore-new-script>>.

Now you can play a fun little game with your friends:

* pick a computational problem
* make a program that solves the computation problem, and outputs output to stdout
* write the code that runs the correct computation in the smallest number of cycles possible

To find out why your program is slow, a good first step is to have a look at <<stats-txt>> file.

==== Skip extra benchmark instructions

A few imperfections of our <<gem5-run-benchmark,benchmarking method>> are:

* when we do `m5 resetstats` and `m5 exit`, there is some time passed before the `exec` system call returns and the actual benchmark starts and ends
* the benchmark outputs to stdout, which means so extra cycles in addition to the actual computation. But TODO: how to get the output to check that it is correct without such IO cycles?

Solutions to these problems include:

* modify benchmark code with instrumentation directly, see <<m5ops-instructions>> for an example.
* monitor known addresses TODO possible? Create an example.

Discussion at: https://stackoverflow.com/questions/48944587/how-to-count-the-number-of-cpu-clock-cycles-between-the-start-and-end-of-a-bench/48944588#48944588

Those problems should be insignificant if the benchmark runs for long enough however.

==== gem5 system parameters

Besides optimizing a program for a given CPU setup, chip developers can also do the inverse, and optimize the chip for a given benchmark!

The rabbit hole is likely deep, but let's scratch a bit of the surface.

===== Number of cores

....
./run --arch arm --cpus 2 --emulator gem5
....

Check with:

....
cat /proc/cpuinfo
getconf _NPROCESSORS_CONF
....

====== gem5 arm more than 8 cores

https://stackoverflow.com/questions/50248067/how-to-run-a-gem5-arm-aarch64-full-system-simulation-with-fs-py-with-more-than-8

Build the kernel with the <<gem5-arm-linux-kernel-patches>>, and then run:

....
./run \
  --arch aarch64 \
  --linux-build-id gem5-v4.15 \
  --emulator gem5 \
  --cpus 16 \
  -- \
  --param 'system.realview.gic.gem5_extensions = True' \
;
....

===== gem5 cache size

https://stackoverflow.com/questions/49624061/how-to-run-gem5-simulator-in-fs-mode-without-cache/49634544#49634544

A quick `+./run --emulator gem5 -- -h+` leads us to the options:

....
--caches
--l1d_size=1024
--l1i_size=1024
--l2cache
--l2_size=1024
--l3_size=1024
....

But keep in mind that it only affects benchmark performance of the most detailed CPU types:

[options="header"]
|===
|arch |CPU type |caches used

|X86
|`AtomicSimpleCPU`
|no

|X86
|`DerivO3CPU`
|?*

|ARM
|`AtomicSimpleCPU`
|no

|ARM
|`HPI`
|yes

|===

{empty}*: couldn't test because of:

* https://stackoverflow.com/questions/49011096/how-to-switch-cpu-models-in-gem5-after-restoring-a-checkpoint-and-then-observe-t

Cache sizes can in theory be checked with the methods described at: link:https://superuser.com/questions/55776/finding-l2-cache-size-in-linux[]:

....
getconf -a | grep CACHE
lscpu
cat /sys/devices/system/cpu/cpu0/cache/index2/size
....

but for some reason the Linux kernel is not seeing the cache sizes:

* https://stackoverflow.com/questions/49008792/why-doesnt-the-linux-kernel-see-the-cache-sizes-in-the-gem5-emulator-in-full-sy
* http://gem5-users.gem5.narkive.com/4xVBlf3c/verify-cache-configuration

Behaviour breakdown:

* arm QEMU and gem5 (both `AtomicSimpleCPU` or `HPI`), x86 gem5: `/sys` files don't exist, and `getconf` and `lscpu` value empty
* x86 QEMU: `/sys` files exist, but `getconf` and `lscpu` values still empty

So we take a performance measurement approach instead:

....
./gem5-bench-cache --arch aarch64
cat "$(./getvar --arch aarch64 run_dir)/bench-cache.txt"
....

which gives:

....
cmd ./run --emulator gem5 --arch aarch64 --gem5-readfile "dhrystone 1000" --gem5-restore 1 -- --caches --l2cache --l1d_size=1024   --l1i_size=1024   --l2_size=1024   --l3_size=1024   --cpu-type=HPI --restore-with-cpu=HPI
time 23.82
exit_status 0
cycles 93284622
instructions 4393457

cmd ./run --emulator gem5 --arch aarch64 --gem5-readfile "dhrystone 1000" --gem5-restore 1 -- --caches --l2cache --l1d_size=1024kB --l1i_size=1024kB --l2_size=1024kB --l3_size=1024kB --cpu-type=HPI --restore-with-cpu=HPI
time 14.91
exit_status 0
cycles 10128985
instructions 4211458

cmd ./run --emulator gem5 --arch aarch64 --gem5-readfile "dhrystone 10000" --gem5-restore 1 -- --caches --l2cache --l1d_size=1024   --l1i_size=1024   --l2_size=1024   --l3_size=1024   --cpu-type=HPI --restore-with-cpu=HPI
time 51.87
exit_status 0
cycles 188803630
instructions 12401336

cmd ./run --emulator gem5 --arch aarch64 --gem5-readfile "dhrystone 10000" --gem5-restore 1 -- --caches --l2cache --l1d_size=1024kB --l1i_size=1024kB --l2_size=1024kB --l3_size=1024kB --cpu-type=HPI --restore-with-cpu=HPI
time 35.35
exit_status 0
cycles 20715757
instructions 12192527

cmd ./run --emulator gem5 --arch aarch64 --gem5-readfile "dhrystone 100000" --gem5-restore 1 -- --caches --l2cache --l1d_size=1024   --l1i_size=1024   --l2_size=1024   --l3_size=1024   --cpu-type=HPI --restore-with-cpu=HPI
time 339.07
exit_status 0
cycles 1176559936
instructions 94222791

cmd ./run --emulator gem5 --arch aarch64 --gem5-readfile "dhrystone 100000" --gem5-restore 1 -- --caches --l2cache --l1d_size=1024kB --l1i_size=1024kB --l2_size=1024kB --l3_size=1024kB --cpu-type=HPI --restore-with-cpu=HPI
time 240.37
exit_status 0
cycles 125666679
instructions 91738770
....

We make the following conclusions:

* the number of instructions almost does not change: the CPU is waiting for memory all the extra time. TODO: why does it change at all?
* the wall clock execution time is not directionally proportional to the number of cycles: here we had a 10x cycle increase, but only 2x time increase. This suggests that the simulation of cycles in which the CPU is waiting for memory to come back is faster.

===== gem5 memory latency

TODO These look promising:

....
--list-mem-types
--mem-type=MEM_TYPE
--mem-channels=MEM_CHANNELS
--mem-ranks=MEM_RANKS
--mem-size=MEM_SIZE
....

TODO: now to verify this with the Linux kernel? Besides raw performance benchmarks.

===== Memory size

....
./run --arch arm --memory 512M
....

and verify inside the guest with:

....
free -m
....

===== gem5 disk and network latency

TODO These look promising:

....
--ethernet-linkspeed
--ethernet-linkdelay
....

and also: `gem5-dist`: https://publish.illinois.edu/icsl-pdgem5/

===== gem5 clock frequency

Clock frequency: TODO how does it affect performance in benchmarks?

....
./run --arch aarch64 --emulator gem5 -- --cpu-clock 10000000
....

Check with:

....
m5 resetstats
sleep 10
m5 dumpstats
....

and then:

....
./gem5-stat --arch aarch64
....

TODO: why doesn't this exist:

....
ls /sys/devices/system/cpu/cpu0/cpufreq
....

==== Interesting benchmarks

Buildroot built-in libraries, mostly under Libraries > Other:

* Armadillo `C++`: linear algebra
* fftw: Fourier transform
* Flann
* GSL: various
* liblinear
* libspacialindex
* libtommath
* qhull

There are not yet enabled, but it should be easy to so, see: <<add-new-buildroot-packages>>

===== BST vs heap

https://stackoverflow.com/questions/6147242/heap-vs-binary-search-tree-bst/29548834#29548834

Usage:

....
./build-userland \
  --arch aarch64 \
  --ccflagg='-DLKMC_M5OPS_ENABLE=1' \
  --force-build cpp/bst_vs_heap \
  --static \
;
./run \
  --arch aarch64 \
  --emulator gem5 \
  --static \
  --userland userland/cpp/bst_vs_heap.cpp \
  --userland-args='1000' \
;
./bst-vs-heap --arch aarch64 > bst_vs_heap.dat
./bst-vs-heap.gnuplot
xdg-open bst-vs-heap.tmp.png
....

Sources:

* link:userland/cpp/bst_vs_heap.cpp[]
* link:bst-vs-heap[]
* link:bst-vs-heap.gnuplot[]

Tested on e70103b9b32e6e33dbab9eaf2ff00c358f55d8db + 1 with the workaround patch mentioned at: <<fatal-kernel-too-old>>.

===== BLAS

Buildroot supports it, which makes everything just trivial:

....
./build-buildroot --config 'BR2_PACKAGE_OPENBLAS=y'
./build-userland --package openblas -- userland/libs/openblas/hello.c
./run --eval-after './libs/openblas/hello.out; echo $?'
....

Outcome: the test passes:

....
0
....

Source: link:userland/libs/openblas/hello.c[]

The test performs a general matrix multiplication:

....
    |  1.0 -3.0 |   |  1.0  2.0  1.0 |       |  0.5  0.5  0.5 |   |  11.0 - 9.0  5.0 |
1 * |  2.0  4.0 | * | -3.0  4.0 -1.0 | + 2 * |  0.5  0.5  0.5 | = | - 9.0  21.0 -1.0 |
    |  1.0 -1.0 |                            |  0.5  0.5  0.5 |   |   5.0 - 1.0  3.0 |
....

This can be deduced from the Fortran interfaces at

....
less "$(./getvar buildroot_build_build_dir)"/openblas-*/reference/dgemmf.f
....

which we can map to our call as:

....
C := alpha*op( A )*op( B ) + beta*C,
SUBROUTINE DGEMMF(               TRANA,        TRANB,     M,N,K,  ALPHA,A,LDA,B,LDB,BETA,C,LDC)
cblas_dgemm(      CblasColMajor, CblasNoTrans, CblasTrans,3,3,2  ,1,    A,3,  B,3,  2   ,C,3  );
....

===== Eigen

Header only linear algebra library with a mainline Buildroot package:

....
./build-buildroot --config 'BR2_PACKAGE_EIGEN=y'
./build-userland --package eigen -- userland/libs/eigen/hello.cpp
....

Just create an array and print it:

....
./run --eval-after './libs/eigen/hello.out'
....

Output:

....
  3  -1
2.5 1.5
....

Source: link:userland/libs/eigen/hello.cpp[]

This example just creates a matrix and prints it out.

Tested on: link:http://github.com/cirosantilli/linux-kernel-module-cheat/commit/a4bdcf102c068762bb1ef26c591fcf71e5907525[a4bdcf102c068762bb1ef26c591fcf71e5907525]

===== PARSEC benchmark

We have ported parts of the link:http://parsec.cs.princeton.edu[PARSEC benchmark] for cross compilation at: https://github.com/cirosantilli/parsec-benchmark See the documentation on that repo to find out which benchmarks have been ported. Some of the benchmarks were are segfaulting, they are documented in that repo.

There are two ways to run PARSEC with this repo:

* <<parsec-benchmark-without-parsecmgmt,without `pasecmgmt`>>, most likely what you want
* <<parsec-benchmark-with-parsecmgmt,with `pasecmgmt`>>

====== PARSEC benchmark without parsecmgmt

....
./build --arch arm --download-dependencies gem5-buildroot parsec-benchmark
./build-buildroot --arch arm --config 'BR2_PACKAGE_PARSEC_BENCHMARK=y'
./run --arch arm --emulator gem5
....

Once inside the guest, launch one of the `test` input sized benchmarks manually as in:

....
cd /parsec/ext/splash2x/apps/fmm/run
../inst/arm-linux.gcc/bin/fmm 1 < input_1
....

To find run out how to run many of the benchmarks, have a look at the `test.sh` script of the `parse-benchmark` repo.

From the guest, you can also run it as:

....
cd /parsec
./test.sh
....

but this might be a bit time consuming in gem5.

====== PARSEC change the input size

Running a benchmark of a size different than `test`, e.g. `simsmall`, requires a rebuild with:

....
./build-buildroot \
  --arch arm \
  --config 'BR2_PACKAGE_PARSEC_BENCHMARK=y' \
  --config 'BR2_PACKAGE_PARSEC_BENCHMARK_INPUT_SIZE="simsmall"' \
  -- parsec_benchmark-reconfigure \
;
....

Large input may also require tweaking:

* <<br2_target_rootfs_ext2_size>> if the unpacked inputs are large
* <<memory-size>>, unless you want to meet the OOM killer, which is admittedly kind of fun

`test.sh` only contains the run commands for the `test` size, and cannot be used for `simsmall`.

The easiest thing to do, is to link:https://superuser.com/questions/231002/how-can-i-search-within-the-output-buffer-of-a-tmux-shell/1253137#1253137[scroll up on the host shell] after the build, and look for a line of type:

....
Running /root/linux-kernel-module-cheat/out/aarch64/buildroot/build/parsec-benchmark-custom/ext/splash2x/apps/ocean_ncp/inst/aarch64-linux.gcc/bin/ocean_ncp -n2050 -p1 -e1e-07 -r20000 -t28800
....

and then tweak the command found in `test.sh` accordingly.

Yes, we do run the benchmarks on host just to unpack / generate inputs. They are expected fail to run since they were build for the guest instead of host, including for x86_64 guest which has a different interpreter than the host's (see `file myexecutable`).

The rebuild is required because we unpack input files on the host.

Separating input sizes also allows to create smaller images when only running the smaller benchmarks.

This limitation exists because `parsecmgmt` generates the input files just before running via the Bash scripts, but we can't run `parsecmgmt` on gem5 as it is too slow!

One option would be to do that inside the guest with QEMU.

Also, we can't generate all input sizes at once, because many of them have the same name and would overwrite one another...

PARSEC simply wasn't designed with non native machines in mind...

====== PARSEC benchmark with parsecmgmt

Most users won't want to use this method because:

* running the `parsecmgmt` Bash scripts takes forever before it ever starts running the actual benchmarks on gem5
+
Running on QEMU is feasible, but not the main use case, since QEMU cannot be used for performance measurements
* it requires putting the full `.tar` inputs on the guest, which makes the image twice as large (1x for the `.tar`, 1x for the unpacked input files)

It would be awesome if it were possible to use this method, since this is what Parsec supports officially, and so:

* you don't have to dig into what raw command to run
* there is an easy way to run all the benchmarks in one go to test them out
* you can just run any of the benchmarks that you want

but it simply is not feasible in gem5 because it takes too long.

If you still want to run this, try it out with:

....
./build-buildroot \
  --arch aarch64 \
  --config 'BR2_PACKAGE_PARSEC_BENCHMARK=y' \
  --config 'BR2_PACKAGE_PARSEC_BENCHMARK_PARSECMGMT=y' \
  --config 'BR2_TARGET_ROOTFS_EXT2_SIZE="3G"' \
  -- parsec_benchmark-reconfigure \
;
....

And then you can run it just as you would on the host:

....
cd /parsec/
bash
. env.sh
parsecmgmt -a run -p splash2x.fmm -i test
....

====== PARSEC uninstall

If you want to remove PARSEC later, Buildroot doesn't provide an automated package removal mechanism: <<remove-buildroot-packages>>, but the following procedure should be satisfactory:

....
rm -rf \
  "$(./getvar buildroot_download_dir)"/parsec-* \
  "$(./getvar buildroot_build_dir)"/build/parsec-* \
  "$(./getvar buildroot_build_dir)"/build/packages-file-list.txt \
  "$(./getvar buildroot_build_dir)"/images/rootfs.* \
  "$(./getvar buildroot_build_dir)"/target/parsec-* \
;
./build-buildroot --arch arm
....

====== PARSEC benchmark hacking

If you end up going inside link:submodules/parsec-benchmark[] to hack up the benchmark (you will!), these tips will be helpful.

Buildroot was not designed to deal with large images, and currently cross rebuilds are a bit slow, due to some image generation and validation steps.

A few workarounds are:

* develop in host first as much as you can. Our PARSEC fork supports it.
+
If you do this, don't forget to do a:
+
....
cd "$(./getvar parsec_source_dir)"
git clean -xdf .
....
before going for the cross compile build.
+
* patch Buildroot to work well, and keep cross compiling all the way. This should be totally viable, and we should do it.
+
Don't forget to explicitly rebuild PARSEC with:
+
....
./build-buildroot \
  --arch arm \
  --config 'BR2_PACKAGE_PARSEC_BENCHMARK=y' \
  -- parsec_benchmark-reconfigure \
;
....
+
You may also want to test if your patches are still functionally correct inside of QEMU first, which is a faster emulator.
* sell your soul, and compile natively inside the guest. We won't do this, not only because it is evil, but also because Buildroot explicitly does not support it: https://buildroot.org/downloads/manual/manual.html#faq-no-compiler-on-target ARM employees have been known to do this: https://github.com/arm-university/arm-gem5-rsk/blob/aa3b51b175a0f3b6e75c9c856092ae0c8f2a7cdc/parsec_patches/qemu-patch.diff

=== gem5 kernel command line parameters

Analogous <<kernel-command-line-parameters,to QEMU>>:

....
./run --arch arm --kernel-cli 'init=/lkmc/linux/poweroff.out' --emulator gem5
....

Internals: when we give `--command-line=` to gem5, it overrides default command lines, including some mandatory ones which are required to boot properly.

Our run script hardcodes the require options in the default `--command-line` and appends extra options given by `-e`.

To find the default options in the first place, we removed `--command-line` and ran:

....
./run --arch arm --emulator gem5
....

and then looked at the line of the Linux kernel that starts with:

....
Kernel command line:
....

[[gem5-gdb]]
=== gem5 GDB step debug

==== gem5 GDB step debug kernel
Analogous <<gdb,to QEMU>>, on the first shell:

....
./run --arch arm --emulator gem5 --gdb-wait
....

On the second shell:

....
./run-gdb --arch arm --emulator gem5
....

On a third shell:

....
./gem5-shell
....

When you want to break, just do a `Ctrl-C` on GDB shell, and then `continue`.

And we now see the boot messages, and then get a shell. Now try the `./count.sh` procedure described for QEMU: <<gdb-step-debug-kernel-post-boot>>.

==== gem5 GDB step debug userland process

We are unable to use `gdbserver` because of networking: <<gem5-host-to-guest-networking>>

The alternative is to do as in <<gdb-step-debug-userland-processes>>.

Next, follow the exact same steps explained at <<gdb-step-debug-userland-non-init-without--d>>, but passing `-g` to every command as usual.

But then TODO (I'll still go crazy one of those days): for `arm`, while debugging `./linux/myinsmod.out hello.ko`, after then line:

....
23     if (argc < 3) {
24         params = "";
....

I press `n`, it just runs the program until the end, instead of stopping on the next line of execution. The module does get inserted normally.

TODO:

....
./run-gdb-user --arch arm --emulator gem5 gem5-1.0/gem5/util/m5/m5 main
....

breaks when `m5` is run on guest, but does not show the source code.

=== gem5 checkpoint

Analogous to QEMU's <<snapshot>>, but better since it can be started from inside the guest, so we can easily checkpoint after a specific guest event, e.g. just before `init` is done.

Documentation: http://gem5.org/Checkpoints

....
./run --arch arm --emulator gem5
....

In the guest, wait for the boot to end and run:

....
m5 checkpoint
....

where <<m5>> is a guest utility present inside the gem5 tree which we cross-compiled and installed into the guest.

To restore the checkpoint, kill the VM and run:

....
./run --arch arm --emulator gem5 --gem5-restore 1
....

The `--gem5-restore` option restores the checkpoint that was created most recently.

Let's create a second checkpoint to see how it works, in guest:

....
date >f
m5 checkpoint
....

Kill the VM, and try it out:

....
./run --arch arm --emulator gem5 --gem5-restore 1
....

Here we use `--gem5-restore 1` again, since the second snapshot we took is now the most recent one

Now in the guest:

....
cat f
....

contains the `date`. The file `f` wouldn't exist had we used the first checkpoint with `--gem5-restore 2`, which is the second most recent snapshot taken.

If you automate things with <<kernel-command-line-parameters>> as in:

....
./run --arch arm --eval 'm5 checkpoint;m5 resetstats;dhrystone 1000;m5 exit' --emulator gem5
....

Then there is no need to pass the kernel command line again to gem5 for replay:

....
./run --arch arm --emulator gem5 --gem5-restore 1
....

since boot has already happened, and the parameters are already in the RAM of the snapshot.

==== gem5 checkpoint internals

Checkpoints are stored inside the <<m5out-directory>> at:

....
"$(./getvar --emulator gem5 m5out_dir)/cpt.<checkpoint-time>"
....

where `<checkpoint-time>` is the cycle number at which the checkpoint was taken.

`fs.py` exposes the `-r N` flag to restore checkpoints, which N-th checkpoint with the largest `<checkpoint-time>`: https://github.com/gem5/gem5/blob/e02ec0c24d56bce4a0d8636a340e15cd223d1930/configs/common/Simulation.py#L118

However, that interface is bad because if you had taken previous checkpoints, you have no idea what `N` to use, unless you memorize which checkpoint was taken at which cycle.

Therefore, just use our superior `--gem5-restore` flag, which uses directory timestamps to determine which checkpoint you created most recently.

The `-r N` integer value is just pure `fs.py` sugar, the backend at `m5.instantiate` just takes the actual tracepoint directory path as input.

[[gem5-restore-new-script]]
==== gem5 checkpoint restore and run a different script

You want to automate running several tests from a single pristine post-boot state.

The problem is that boot takes forever, and after the checkpoint, the memory and disk states are fixed, so you can't for example:

* hack up an existing rc script, since the disk is fixed
* inject new kernel boot command line options, since those have already been put into memory by the bootloader

There is however a few loopholes, <<m5-readfile>> being the simplest, as it reads whatever is present on the host.

So we can do it like:

....
# Boot, checkpoint and exit.
printf 'echo "setup run";m5 exit' > "$(./getvar gem5_readfile)"
./run --emulator gem5 --eval 'm5 checkpoint;m5 readfile > a.sh;sh a.sh'

# Restore and run the first benchmark.
printf 'echo "first benchmark";m5 exit' > "$(./getvar gem5_readfile)"
./run --emulator gem5 --gem5-restore 1

# Restore and run the second benchmark.
printf 'echo "second benchmark";m5 exit' > "$(./getvar gem5_readfile)"
./run --emulator gem5 --gem5-restore 1

# If something weird happened, create an interactive shell to examine the system.
printf 'sh' > "$(./getvar gem5_readfile)"
./run --emulator gem5 --gem5-restore 1
....

Since this is such a common setup, we provide some helpers for it as described at <<gem5-run-benchmark>>:

* link:rootfs_overlay/lkmc/gem5.sh[]. This script is analogous to gem5's in-tree link:https://github.com/gem5/gem5/blob/2b4b94d0556c2d03172ebff63f7fc502c3c26ff8/configs/boot/hack_back_ckpt.rcS[hack_back_ckpt.rcS], but with less noise.
* `./run --gem5-readfile` is a convenient way to set the `m5 readfile`

Other loophole possibilities include:

* <<9p>>
* <<secondary-disk>>
* `expect` as mentioned at: https://stackoverflow.com/questions/7013137/automating-telnet-session-using-bash-scripts
+
....
#!/usr/bin/expect
spawn telnet localhost 3456
expect "# $"
send "pwd\r"
send "ls /\r"
send "m5 exit\r"
expect eof
....
+
This is ugly however as it is not deterministic.

https://www.mail-archive.com/gem5-users@gem5.org/msg15233.html

==== gem5 restore checkpoint with a different CPU

gem5 can switch to a different CPU model when restoring a checkpoint.

A common combo is to boot Linux with a fast CPU, make a checkpoint and then replay the benchmark of interest with a slower CPU.

An illustrative interactive run:

....
./run --arch arm --emulator gem5
....

In guest:

....
m5 checkpoint
....

And then restore the checkpoint with a different CPU:

....
./run --arch arm --emulator gem5 --gem5-restore 1 -- --caches --restore-with-cpu=HPI
....

=== Pass extra options to gem5

Pass options to the `fs.py` script:

* get help:
+
....
./run --emulator gem5 -- -h
....
* boot with the more detailed and slow `HPI` CPU model:
+
....
./run --arch arm --emulator gem5 -- --caches --cpu-type=HPI
....

Pass options to the `gem5` executable itself:

* get help:
+
....
./run --gem5-exe-args='-h' --emulator gem5
....

=== gem5 exit after a number of instructions

Quit the simulation after `1024` instructions:

....
./run --emulator gem5 -- -I 1024
....

Can be nicely checked with <<gem5-tracing>>.

Cycles instead of instructions:

....
./run --emulator gem5 -- --memory 1024
....

Otherwise the simulation runs forever by default.

=== m5ops

m5ops are magic instructions which lead gem5 to do magic things, like quitting or dumping stats.

Documentation: http://gem5.org/M5ops

There are two main ways to use m5ops:

* <<m5>>
* <<m5ops-instructions>>

`m5` is convenient if you only want to take snapshots before or after the benchmark, without altering its source code. It uses the <<m5ops-instructions>> as its backend.

`m5` cannot should / should not be used however:

* in bare metal setups
* when you want to call the instructions from inside interest points of your benchmark. Otherwise you add the syscall overhead to the benchmark, which is more intrusive and might affect results.
+
Why not just hardcode some <<m5ops-instructions>> as in our example instead, since you are going to modify the source of the benchmark anyways?

==== m5

`m5` is a guest command line utility that is installed and run on the guest, that serves as a CLI front-end for the <<m5ops>>

Its source is present in the gem5 tree: https://github.com/gem5/gem5/blob/6925bf55005c118dc2580ba83e0fa10b31839ef9/util/m5/m5.c

It is possible to guess what most tools do from the corresponding <<m5ops>>, but let's at least document the less obvious ones here.

===== m5 exit

End the simulation.

Sane Python scripts will exit gem5 with status 0, which is what `fs.py` does.

===== m5 fail

End the simulation with a failure exit event:

....
m5 fail 1
....

Sane Python scripts would use that as the exit status of gem5, which would be useful for testing purposes, but `fs.py` at 200281b08ca21f0d2678e23063f088960d3c0819 just prints an error message:

....
Simulated exit code not 0! Exit code is 1
....

and exits with status 0.

We then parse that string ourselves in link:run[] and exit with the correct status...

TODO: it used to be like that, but it actually got changed to just print the message. Why? https://gem5-review.googlesource.com/c/public/gem5/+/4880

`m5 fail` is just a superset of `m5 exit`, which is just:

....
m5 fail 0
....

as can be seen from the source: https://github.com/gem5/gem5/blob/50a57c0376c02c912a978c4443dd58caebe0f173/src/sim/pseudo_inst.cc#L303

===== m5 writefile

Send a guest file to the host. <<9p>> is a more advanced alternative.

Guest:

....
echo mycontent > myfileguest
m5 writefile myfileguest myfilehost
....

Host:

....
cat "$(./getvar --arch aarch64 --emulator gem5 m5out_dir)/myfilehost"
....

Does not work for subdirectories, gem5 crashes:

....
m5 writefile myfileguest mydirhost/myfilehost
....

===== m5 readfile

Read a host file pointed to by the `fs.py --script` option to stdout.

https://stackoverflow.com/questions/49516399/how-to-use-m5-readfile-and-m5-execfile-in-gem5/49538051#49538051

Host:

....
date > "$(./getvar gem5_readfile)"
....

Guest:

....
m5 readfile
....

Outcome: date shows on guest.

===== m5 initparam

Ermm, just another <<m5-readfile>> that only takes integers and only from CLI options? Is this software so redundant?

Host:

....
./run --emulator gem5 --gem5-restore 1 -- --initparam 13
./run --emulator gem5 --gem5-restore 1 -- --initparam 42
....

Guest:

....
m5 initparm
....

Outputs the given paramter.

===== m5 execfile

Trivial combination of `m5 readfile` + execute the script.

Host:

....
printf '#!/bin/sh
echo asdf
' > "$(./getvar gem5_readfile)"
....

Guest:

....
touch /tmp/execfile
chmod +x /tmp/execfile
m5 execfile
....

Outcome:

....
adsf
....

==== m5ops instructions

gem5 allocates some magic instructions on unused instruction encodings for convenient guest instrumentation.

Those instructions are exposed through the <<m5>> in tree executable.

To make things simpler to understand, you can play around with our own minimized educational `m5` subset link:userland/c/m5ops.c[].

The instructions used by `./c/m5ops.out` are present in link:lkmc/m5ops.h[] in a very simple to understand and reuse inline assembly form.

To use that file, first rebuild `m5ops.out` with the m5ops instructions enabled and install it on the root filesystem:

....
./build-userland \
  --arch aarch64 \
  --ccflags='-DLKMC_M5OPS_ENABLE=1' \
  --force-build c/m5ops \
  --static \
;
./build-buildroot --arch aarch64
....

We don't enable `-DLKMC_M5OPS_ENABLE=1` by default on userland executables because we try to use a single image for both gem5, QEMU and <<userland-setup-getting-started-natively,native>>, and those instructions would break the latter two. We enable it in the <<baremetal-setup>> by default since we already have different images for QEMU and gem5 there.

Then, from inside <<gem5-buildroot-setup>>, test it out with:

....
# checkpoint
./c/m5ops.out c

# dumpstats
./c/m5ops.out d

# exit
./c/m5ops.out e

# dump resetstats
./c/m5ops.out r
....

In theory, the cleanest way to add m5ops to your benchmarks would be to do exactly what the `m5` tool does:

* include link:https://github.com/gem5/gem5/blob/05c4c2b566ce351ab217b2bd7035562aa7a76570/include/gem5/asm/generic/m5ops.h[`include/gem5/asm/generic/m5ops.h`]
* link with the `.o` file under `util/m5` for the correct arch, e.g. `m5op_arm_A64.o` for aarch64.

However, I think it is usually not worth the trouble of hacking up the build system of the benchmark to do this, and I recommend just hardcoding in a few raw instructions here and there, and managing it with version control + `sed`.

Related: https://www.mail-archive.com/gem5-users@gem5.org/msg15418.html

===== m5ops instructions interface

Let's study how <<m5>> uses them:

* link:https://github.com/gem5/gem5/blob/05c4c2b566ce351ab217b2bd7035562aa7a76570/include/gem5/asm/generic/m5ops.h[`include/gem5/asm/generic/m5ops.h`]: defines the magic constants that represent the instructions
* link:https://github.com/gem5/gem5/blob/05c4c2b566ce351ab217b2bd7035562aa7a76570/util/m5/m5op_arm_A64.S[`util/m5/m5op_arm_A64.S`]: use the magic constants that represent the instructions using C preprocessor magic
* link:https://github.com/gem5/gem5/blob/05c4c2b566ce351ab217b2bd7035562aa7a76570/util/m5/m5.c[`util/m5/m5.c`]: the actual executable. Gets linked to `m5op_arm_A64.S` which defines a function for each m5op.

We notice that there are two different implementations for each arch:

* magic instructions, which don't exist in the corresponding arch
* magic memory addresses on a given page

TODO: what is the advantage of magic memory addresses? Because you have to do more setup work by telling the kernel never to touch the magic page. For the magic instructions, the only thing that could go wrong is if you run some crazy kind of fuzzing workload that generates random instructions.

Then, in aarch64 magic instructions for example, the lines:

....
.macro  m5op_func, name, func, subfunc
        .globl \name
        \name:
        .long 0xff000110 | (\func << 16) | (\subfunc << 12)
        ret
....

define a simple function function for each m5op. Here we see that:

* `0xff000110` is a base mask for the magic non-existing instruction
* `\func` and `\subfunc` are OR-applied on top of the base mask, and define m5op this is.
+
Those values will loop over the magic constants defined in `m5ops.h` with the deferred preprocessor idiom.
+
For example, `exit` is `0x21` due to:
+
....
#define M5OP_EXIT               0x21
....

Finally, `m5.c` calls the defined functions as in:

....
m5_exit(ints[0]);
....

Therefore, the runtime "argument" that gets passed to the instruction, e.g. the delay in ticks until the exit for `m5 exit`, gets passed directly through the link:https://en.wikipedia.org/wiki/Calling_convention#ARM_(A64)[aarch64 calling convention].

Keep in mind that for all archs, `m5.c` does the calls with 64-bit integers:

....
uint64_t ints[2] = {0,0};
parse_int_args(argc, argv, ints, argc);
m5_fail(ints[1], ints[0]);
....

Therefore, for example:

* aarch64 uses `x0` for the first argument and `x1` for the second, since each is 64 bits log already
* arm uses `r0` and `r1` for the first argument, and `r2` and `r3` for the second, since each register is only 32 bits long

That convention specifies that `x0` to `x7` contain the function arguments, so `x0` contains the first argument, and `x1` the second.

In our `m5ops` example, we just hardcode everything in the assembly one-liners we are producing.

We ignore the `\subfunc` since it is always 0 on the ops that interest us.

===== m5op annotations

`include/gem5/asm/generic/m5ops.h` also describes some annotation instructions.

What they mean: https://stackoverflow.com/questions/50583962/what-are-the-gem5-annotations-mops-magic-instructions-and-how-to-use-them

=== gem5 arm Linux kernel patches

https://gem5.googlesource.com/arm/linux/ contains an ARM Linux kernel forks with a few gem5 specific Linux kernel patches on top of mainline created by ARM Holdings on top of a few upstream kernel releases.

The patches are optional: the vanilla kernel does boot. But they add some interesting gem5-specific optimizations, instrumentations and device support.

The patches also <<notable-alternate-gem5-kernel-configs,add defconfigs>> that are known to work well with gem5.

E.g. for arm v4.9 there is: link:https://gem5.googlesource.com/arm/linux/+/917e007a4150d26a0aa95e4f5353ba72753669c7/arch/arm/configs/gem5_defconfig[].

In order to use those patches and their associated configs, and, we recommend using <<linux-kernel-build-variants>> as:

....
git -C "$(./getvar linux_source_dir)" fetch https://gem5.googlesource.com/arm/linux gem5/v4.15:gem5/v4.15
git -C "$(./getvar linux_source_dir)" checkout gem5/v4.15
./build-linux \
  --arch aarch64 \
  --custom-config-file-gem5 \
  --linux-build-id gem5-v4.15 \
;
git -C "$(./getvar linux_source_dir)" checkout -
./run \
  --arch aarch64 \
  --emulator gem5 \
  --linux-build-id gem5-v4.15 \
;
....

QEMU also boots that kernel successfully:

....
./run \
  --arch aarch64 \
  --linux-build-id gem5-v4.15 \
;
....

but glibc kernel version checks make init fail with:

....
FATAL: kernel too old
....

because that kernel version is too old.

It is obviously not possible to understand what they actually do from their commit message, so let's explain them one by one here as we understand them:

* `drm: Add component-aware simple encoder` allows you to see images through VNC: <<gem5-graphic-mode>>
* `gem5: Add support for gem5's extended GIC mode` adds support for more than 8 cores: <<gem5-arm-more-than-8-cores>>

Tested on 649d06d6758cefd080d04dc47fd6a5a26a620874 + 1.

==== gem5 arm Linux kernel patches boot speedup

We have observed that with the kernel patches, boot is 2x faster, falling from 1m40s to 50s.

With link:https://stackoverflow.com/questions/49797246/how-to-monitor-for-how-much-time-each-line-of-stdout-was-the-last-output-line-in/49797547#49797547[`ts`], we see that a large part of the difference is at the message:

....
clocksource: Switched to clocksource arch_sys_counter
....

which takes 4s on the patched kernel, and 30s on the unpatched one! TODO understand why, especially if it is a config difference, or if it actually comes from a patch.

=== m5out directory

When you run gem5, it generates an `m5out` directory at:

....
echo $(./getvar --arch arm --emulator gem5 m5out_dir)"
....

The location of that directory can be set with `./gem5.opt -d`, and defaults to `./m5out`.

The files in that directory contains some very important information about the run, and you should become familiar with every one of them.

==== system.terminal

Contains UART output, both from the Linux kernel or from the baremetal system.

Can also be seen live on <<m5term>>.

==== stats.txt

This file contains important statistics about the run:

....
cat "$(./getvar --arch aarch64 m5out_dir)/stats.txt"
....

Whenever we run `m5 dumpstats` or `m5 exit`, a section with the following format is added to that file:

....
---------- Begin Simulation Statistics ----------
[the stats]
---------- End Simulation Statistics   ----------
....

That file contains several important execution metrics, e.g. number of cycles and several types of cache misses:

....
system.cpu.numCycles
system.cpu.dtb.inst_misses
system.cpu.dtb.inst_hits
....

For x86, it is interesting to try and correlate `numCycles` with:

==== config.ini

The `config.ini` file, contains a very good high level description of the system:

....
less $(./getvar --arch arm --emulator gem5 m5out_dir)"
....

That file contains a tree representation of the system, sample excerpt:

....
[root]
type=Root
children=system
full_system=true

[system]
type=ArmSystem
children=cpu cpu_clk_domain
auto_reset_addr_64=false
semihosting=Null

[system.cpu]
type=AtomicSimpleCPU
children=dstage2_mmu dtb interrupts isa istage2_mmu itb tracer
branchPred=Null

[system.cpu_clk_domain]
type=SrcClockDomain
clock=500
....

Each node has:

* a list of child nodes, e.g. `system` is a child of `root`, and both `cpu` and `cpu_clk_domain` are children of `system`
* a list of parameters, e.g. `system.semihosting` is `Null`, which means that <<semihosting>> was turned off
** the `type` parameter shows is present on every node, and it maps to a `Python` object that inherits from `SimObject`.
+
For example, `AtomicSimpleCPU` maps is defined at link:https://github.com/gem5/gem5/blob/05c4c2b566ce351ab217b2bd7035562aa7a76570/src/cpu/simple/AtomicSimpleCPU.py#L45[src/cpu/simple/AtomicSimpleCPU.py].

You can also get a simplified graphical view of the tree with:

....
xdg-open "$(./getvar --arch arm --emulator gem5 m5out_dir)/config.dot.pdf"
....

Modifying the `config.ini` file manually does nothing since it gets overwritten every time.

Set custom configs with the `--param` option of `fs.py`, e.g. we can make gem5 wait for GDB to connect with:

....
fs.py --param 'system.cpu[0].wait_for_remote_gdb = True'
....

More complex settings involving new classes however require patching the config files, although it is easy to hack this up. See for example: link:patches/manual/gem5-semihost.patch[].

=== m5term

We use the `m5term` in-tree executable to connect to the terminal instead of a direct `telnet`.

If you use `telnet` directly, it mostly works, but certain interactive features don't, e.g.:

* up and down arrows for history navigation
* tab to complete paths
* `Ctrl-C` to kill processes

TODO understand in detail what `m5term` does differently than `telnet`.

=== gem5 Python scripts without rebuild

We have made a crazy setup that allows you to just `cd` into `submodules/gem5`, and edit Python scripts directly there.

This is not normally possible with Buildroot, since normal Buildroot packages first copy files to the output directory (`$(./getvar -a <arch> buildroot_build_build_dir)/<pkg>`), and then build there.

So if you modified the Python scripts with this setup, you would still need to `./build` to copy the modified files over.

For gem5 specifically however, we have hacked up the build so that we `cd` into the `submodules/gem5` tree, and then do an link:https://stackoverflow.com/questions/54343515/how-to-build-gem5-out-of-tree/54343516#54343516[out of tree] build to `out/common/gem5`.

Another advantage of this method is the we factor out the `arm` and `aarch64` gem5 builds which are identical and large, as well as the smaller arch generic pieces.

Using Buildroot for gem5 is still convenient because we use it to:

* to cross build `m5` for us
* check timestamps and skip the gem5 build when it is not requested

The out of build tree is required, because otherwise Buildroot would copy the output build of all archs to each arch directory, resulting in `arch^2` build copies, which is significant.

=== gem5 fs_bigLITTLE

By default, we use `configs/example/fs.py` script.

The `--gem5-script biglittle` option enables the alternative `configs/example/arm/fs_bigLITTLE.py` script instead.

First apply:

....
patch -d "$(./getvar gem5_source_dir)" -p 1 < patches/manual/gem5-biglittle.patch
....

then:

....
./run --arch aarch64 --emulator gem5 --gem5-script biglittle
....

Advantages over `fs.py`:

* more representative of mobile ARM SoCs, which almost always have  big little cluster
* simpler than `fs.py`, and therefore easier to understand and modify

Disadvantages over `fs.py`:

* only works for ARM, not other archs
* not as many configuration options as `fs.py`, many things are hardcoded

We setup 2 big and 2 small CPUs, but `cat /proc/cpuinfo` shows 4 identical CPUs instead of 2 of two different types, likely because gem5 does not expose some informational register much like the caches: https://www.mail-archive.com/gem5-users@gem5.org/msg15426.html <<config-ini>> does show that the two big ones are `DerivO3CPU` and the small ones are `MinorCPU`.

TODO: why is the `--dtb` required despite `fs_bigLITTLE.py` having a DTB generation capability? Without it, nothing shows on terminal, and the simulation terminates with `simulate() limit reached  @  18446744073709551615`. The magic `vmlinux.vexpress_gem5_v1.20170616` works however without a DTB.

Tested on: link:http://github.com/cirosantilli/linux-kernel-module-cheat/commit/18c1c823feda65f8b54cd38e261c282eee01ed9f[18c1c823feda65f8b54cd38e261c282eee01ed9f]

=== gem5 unit tests

https://stackoverflow.com/questions/52279971/how-to-run-the-gem5-unit-tests

These are just very small GTest tests that test a single class in isolation, they don't run any executables.

Build the unit tests and run them:

....
./build-gem5 --unit-tests
....

Running individual unit tests is not yet exposed, but it is easy to do: while running the full tests, GTest prints each test command being run, e.g.:

....
/path/to/build/ARM/base/circlebuf.test.opt --gtest_output=xml:/path/to/build/ARM/unittests.opt/base/circlebuf.test.xml
[==========] Running 4 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 4 tests from CircleBufTest
[ RUN      ] CircleBufTest.BasicReadWriteNoOverflow
[       OK ] CircleBufTest.BasicReadWriteNoOverflow (0 ms)
[ RUN      ] CircleBufTest.SingleWriteOverflow
[       OK ] CircleBufTest.SingleWriteOverflow (0 ms)
[ RUN      ] CircleBufTest.MultiWriteOverflow
[       OK ] CircleBufTest.MultiWriteOverflow (0 ms)
[ RUN      ] CircleBufTest.PointerWrapAround
[       OK ] CircleBufTest.PointerWrapAround (0 ms)
[----------] 4 tests from CircleBufTest (0 ms total)

[----------] Global test environment tear-down
[==========] 4 tests from 1 test case ran. (0 ms total)
[  PASSED  ] 4 tests.
....

so you can just copy paste the command.

Building individual tests is possible with:

....
./build-gem5 --unit-test base/circlebuf.test
....

This does not run the test however.

Note that the command and it's corresponding results don't need to show consecutively on stdout because tests are run in parallel. You just have to match them based on the class name `CircleBufTest` to the file `circlebuf.test.cpp`.

Running the larger regression tests is exposed with:

....
./build-gem5 --regression-test quick/fs
....

but TODO: those require magic blobs on `M5_PATH` that we don't currently automate.

=== gem5 clang build

TODO test properly, benchmark vs GCC.

....
sudo apt-get install clang
./build-gem5 --clang
./run --clang --emulator gem5
....

== Buildroot

=== Introduction to Buildroot

link:https://en.wikipedia.org/wiki/Buildroot[Buildroot] is a set of Make scripts that download and compile from source compatible versions of:

* GCC
* Linux kernel
* C standard library: Buildroot supports several implementations, see: <<libc-choice>>
* link:https://en.wikipedia.org/wiki/BusyBox[BusyBox]: provides the shell and basic command line utilities

It therefore produces a pristine, blob-less, debuggable setup, where all moving parts are configured to work perfectly together.

Perhaps the awesomeness of Buildroot only sinks in once you notice that all it takes is 4 commands as explained at https://stackoverflow.com/questions/47557262/how-to-download-the-torvalds-linux-kernel-master-recompile-it-and-boot-it-wi/49349237#49349237

....
git clone https://github.com/buildroot/buildroot
cd buildroot
git checkout 2018.02
make qemu_aarch64_virt_defconfig
make olddefconfig
time make BR2_JLEVEL="$(nproc)"
qemu-system-aarch64 -M virt -cpu cortex-a57 -nographic -smp 1 -kernel output/images/Image -append "root=/dev/vda console=ttyAMA0" -netdev user,id=eth0 -device virtio-net-device,netdev=eth0 -drive file=output/images/rootfs.ext4,if=none,format=raw,id=hd0 -device virtio-blk-device,drive=hd0
....

This repo basically wraps around that, and tries to make everything even more awesome for kernel developers.

The downsides of Buildroot are:

* the first build takes a while, but it is well worth it
* the selection of software packages is relatively limited if compared to Debian, e.g. no Java or Python package in guest out of the box.
+
In theory, any software can be packaged, and the Buildroot side is easy.
+
The hard part is dealing with crappy third party build systems and huge dependency chains.

=== Custom Buildroot configs

We provide the following mechanisms:

* `./build-buildroot --config-fragment data/br2`: append the Buildroot configuration file `data/br2` to a single build. Must be passed every time you run `./build`. The format is the same as link:buildroot_config/default[].
* `./build-buildroot --config 'BR2_SOME_OPTION="myval"'`: append a single option to a single build.

For example, if you decide to <<enable-buildroot-compiler-optimizations>> after an initial build is finished, you must <<clean-the-build>> and rebuild:

....
./build-buildroot \
  --config 'BR2_OPTIMIZE_3=y' \
  --config 'BR2_PACKAGE_SAMPLE_PACKAGE=y' \
  --
  sample_package-dirclean \
  sample_package-reconfigure \
;
....

as explained at: https://buildroot.org/downloads/manual/manual.html#rebuild-pkg

The clean is necessary because the source files didn't change, so `make` would just check the timestamps and not build anything.

You will then likely want to make those more permanent with: <<default-command-line-arguments>>

==== Enable Buildroot compiler optimizations

If you are benchmarking compiled programs instead of hand written assembly, remember that we configure Buildroot to disable optimizations by default with:

....
BR2_OPTIMIZE_0=y
....

to improve the debugging experience.

You will likely want to change that to:

....
BR2_OPTIMIZE_3=y
....

Our link:kernel_modules/user[] package correctly forwards the Buildroot options to the build with `$(TARGET_CONFIGURE_OPTS)`, so you don't have to do any extra work.

Don't forget to do that if you are <<add-new-buildroot-packages,adding a new package>> with your own build system.

Then, you have two choices:

* if you already have a full `-O0` build, you can choose to rebuild just your package of interest to save some time as described at: <<custom-buildroot-configs>>
+
....
./build-buildroot \
  --config 'BR2_OPTIMIZE_3=y' \
  --config 'BR2_PACKAGE_SAMPLE_PACKAGE=y' \
  -- \
  sample_package-dirclean \
  sample_package-reconfigure \
;
....
+
However, this approach might not be representative since calls to an unoptimized libc and other libraries will have a negative performance impact.
+
Maybe you can get away with rebuilding libc, but I'm not sure that it will work properly.
+
Kernel-wise it should be fine though due to: <<kernel-o0>>
* <<clean-the-build,clean the build>> and rebuild from scratch:
+
....
mv out out~
./build-buildroot --config 'BR2_OPTIMIZE_3=y'
....

=== Find Buildroot options with make menuconfig

`make menuconfig` is a convenient way to find Buildroot configurations:

....
cd "$(./getvar buildroot_build_dir)"
make menuconfig
....

Hit `/` and search for the settings.

Save and quit.

....
diff -u .config.olg .config
....

Then copy and paste the diff additions to link:buildroot_config/default[] to make them permanent.

=== Change user

At startup, we login automatically as the `root` user.

If you want to switch to another user to test some permissions, we have already created an `user0` user through the link:user_table[] file, and you can just login as that user with:

....
login user0
....

and password:

....
a
....

Then test that the user changed with:

....
id
....

which gives:

....
uid=1000(user0) gid=1000(user0) groups=1000(user0)
....

==== Login as a non-root user without password

Replace on `inittab`:

....
::respawn:-/bin/sh
....

with:

....
::respawn:-/bin/login -f user0
....

`-f` forces login without asking for the password.

=== Add new Buildroot packages

First, see if you can't get away without actually adding a new package, for example:

* if you have a standalone C file with no dependencies besides the C standard library to be compiled with GCC, just add a new file under link:kernel_modules/user[] and you are done
* if you have a dependency on a library, first check if Buildroot doesn't have a package for it already with `ls buildroot/package`. If yes, just enable that package as explained at: <<custom-buildroot-configs>>

If none of those methods are flexible enough for you, you can just fork or hack up link:buildroot_packages/sample_package[] the sample package to do what you want.

For how to use that package, see: <<buildroot_packages-directory>>.

Then iterate trying to do what you want and reading the manual until it works: https://buildroot.org/downloads/manual/manual.html

=== Remove Buildroot packages

Once you've built a package in to the image, there is no easy way to remove it.

Documented at: link:https://github.com/buildroot/buildroot/blob/2017.08/docs/manual/rebuilding-packages.txt#L90[]

Also mentioned at: https://stackoverflow.com/questions/47320800/how-to-clean-only-target-in-buildroot

See this for a sample manual workaround: <<parsec-uninstall>>.

=== BR2_TARGET_ROOTFS_EXT2_SIZE

When adding new large package to the Buildroot root filesystem, it may fail with the message:

....
Maybe you need to increase the filesystem size (BR2_TARGET_ROOTFS_EXT2_SIZE)
....

The solution is to simply add:

....
./build-buildroot --config 'BR2_TARGET_ROOTFS_EXT2_SIZE="512M"'
....

where 512Mb is "large enough".

Note that dots cannot be used as in `1.5G`, so just use Megs as in `1500M` instead.

Unfortunately, TODO we don't have a perfect way to find the right value for `BR2_TARGET_ROOTFS_EXT2_SIZE`. One good heuristic is:

....
du -hsx "$(./getvar --arch arm buildroot_target_dir)"
....

Some promising ways to overcome this problem include:

* <<squashfs>>
TODO benchmark: would gem5 suffer a considerable disk read performance hit due to decompressing SquashFS?
* libguestfs: link:https://serverfault.com/questions/246835/convert-directory-to-qemu-kvm-virtual-disk-image/916697#916697[], in particular link:http://libguestfs.org/guestfish.1.html#vfs-minimum-size[`vfs-minimum-size`]
* use methods described at: <<gem5-restore-new-script>> instead of putting builds on the root filesystem

Bibliography: https://stackoverflow.com/questions/49211241/is-there-a-way-to-automatically-detect-the-minimum-required-br2-target-rootfs-ex

==== SquashFS

link:https://en.wikipedia.org/wiki/SquashFS[SquashFS] creation with `mksquashfs` does not take fixed sizes, and I have successfully booted from it, but it is readonly, which is unacceptable.

But then we could mount link:https://wiki.debian.org/ramfs[ramfs] on top of it with <<overlayfs>> to make it writable, but my attempts failed exactly as mentioned at <<overlayfs>>.

This is the exact unanswered question: https://unix.stackexchange.com/questions/343484/mounting-squashfs-image-with-read-write-overlay-for-rootfs

[[rpath]]
=== Buildroot rebuild is slow when the root filesystem is large

Buildroot is not designed for large root filesystem images, and the rebuild becomes very slow when we add a large package to it.

This is due mainly to the `pkg-generic` `GLOBAL_INSTRUMENTATION_HOOKS` sanitation which go over the entire tree doing complex operations... I no like, in particular `check_bin_arch` and `check_host_rpath`

We have applied link:https://github.com/cirosantilli/buildroot/commit/983fe7910a73923a4331e7d576a1e93841d53812[983fe7910a73923a4331e7d576a1e93841d53812] to out Buildroot fork which removes part of the pain by not running:

....
>>>   Sanitizing RPATH in target tree
....

which contributed to a large part of the slowness.

Test how Buildroot deals with many files with:

....
./build-buildroot \
  --config 'BR2_PACKAGE_LKMC_MANY_FILES=y' \
  -- \
  lkmc_many_files-reconfigure \
  |& \
  ts -i '%.s' \
;
./build-buildroot |& ts -i '%.s'
....

and notice how the second build, which does not rebuilt the package at all, still gets stuck in the `RPATH` check forever without our Buildroot patch.

=== Report upstream bugs

When asking for help on upstream repositories outside of this repository, you will need to provide the commands that you are running in detail without referencing our scripts.

For example, QEMU developers will only want to see the final QEMU command that you are running.

For the configure and build, search for the `Building` and `Configuring` parts of the build log, then try to strip down all Buildroot related paths, to keep only options that seem to matter.

We make that easy by building commands as strings, and then echoing them before evaling.

So for example when you run:

....
./run --arch arm
....

the very first stdout output of that script is the actual QEMU command that is being run.

The command is also saved to a file for convenience:

....
cat "$(./getvar --arch arm run_cmd_file)"
....

which you can manually modify and execute during your experiments later:

....
vim "$(./getvar --arch arm run_cmd_file)"
./"$(./getvar --arch arm run_cmd_file)"
....

If you are not already on the master of the given component, you can do that neatly with <<build-variants>>.

E.g., to check if a QEMU bug is still present on `master`, you can do as explained at <<qemu-build-variants>>:

....
git -C "$(./getvar qemu_source_dir)" checkout master
./build-qemu --clean --qemu-build-id master
./build-qemu --qemu-build-id master
git -C "$(./getvar qemu_source_dir)" checkout -
./run --qemu-build-id master
....

Then, you will also want to do a <<bisection>> to pinpoint the exact commit to blame, and CC that developer.

Finally, give the images you used save upstream developpers time: <<release-zip>>.

For Buildroot problems, you should wither provide the config you have:

....
./getvar buildroot_config_file
....

or try to reproduce with a minimal config, see: https://github.com/cirosantilli/buildroot/tree/in-tree-package-master

=== libc choice

Buildroot supports several libc implementations, including:

* link:https://en.wikipedia.org/wiki/GNU_C_Library[glibc]
* link:https://en.wikipedia.org/wiki/UClibc[uClibc]

We currently use glibc, which is selected by:

....
BR2_TOOLCHAIN_BUILDROOT_GLIBC=y
....

Ideally we would like to use uClibc, as it is more minimal and easier to understand, but unfortunately there are some very few packages that use some weird glibc extension that uClibc hasn't implemented yet, e.g.:

* <<selinux>>. Trivial unmerged fix at: http://lists.busybox.net/pipermail/buildroot/2017-July/197793.html just missing the uClibc option to expose `fts.h`...
* <<stress>>

The full list of unsupported packages can be found by grepping the Buildroot source:

....
git -C "$(./getvar buildroot_source_dir)" grep 'depends on BR2_TOOLCHAIN_USES_GLIBC'
....

One "downside" of glibc is that it exercises much more kernel functionality on its more bloated pre-main init, which breaks user mode C hello worlds more often, see: <<user-mode-simulation-with-glibc>>. I quote "downside" because glibc is actually exposing emulator bugs which we should actually go and fix.

== Userland content

Getting started at: <<userland-setup>>

The quickest way to run the arch agnostic examples is natively with: <<userland-setup-getting-started-natively>>

This sectionwas were originally moved in here from: https://github.com/cirosantilli/cpp-cheat

=== C

Programs under link:userland/c/[] are examples of link:https://en.wikipedia.org/wiki/ANSI_C[ANSI C] programming:

* Standard library
** assert.h
*** link:userland/c/assert_fail.c[]

Userland assembly content is located at: <<userland-assembly>>. It was split from this section basically becase we were hitting the HTML `h6` limit, stupid web :-)

==== GCC C extensions

===== C empty struct

Example: link:userland/gcc/empty_struct.c[]

Documentation: https://gcc.gnu.org/onlinedocs/gcc-8.2.0/gcc/Empty-Structures.html#Empty-Structures

Question: https://stackoverflow.com/questions/24685399/c-empty-struct-what-does-this-mean-do

===== OpenMP

GCC implements the <<OpenMP>> threading implementation: https://stackoverflow.com/questions/3949901/pthreads-vs-openmp

Example: link:userland/gcc/openmp.c[]

The implementation is built into GCC itself. It is enabled at GCC compile time by `BR2_GCC_ENABLE_OPENMP=y` on Buildroot, and at program compile time by `-fopenmp`.

It seems to be easier to use for compute parallelism and more language agnostic than POSIX threads.

pthreads are more versatile though and allow for a superset of OpenMP.

The implementation lives under `libgomp` in the GCC tree, and is documented at: https://gcc.gnu.org/onlinedocs/libgomp/

`strace` shows that OpenMP makes `clone()` syscalls in Linux. TODO: does it actually call `pthread_` functions, or does it make syscalls directly? Or in other words, can it work on <<freestanding-programs>>? A quick grep shows many references to pthreads.

[[cpp]]
=== C++

Programs under link:userland/cpp/[] are examples of link:https://en.wikipedia.org/wiki/C%2B%2B#Standardization[ISO C] programming.

=== POSIX

Programs under link:userland/posix/[] are examples of POSIX C programming.

What is POSIX:

* https://stackoverflow.com/questions/1780599/what-is-the-meaning-of-posix/31865755#31865755
* https://unix.stackexchange.com/questions/11983/what-exactly-is-posix/220877#220877

== Userland assembly

Programs under `userland/arch/<arch>/` are examples of userland assembly programming.

This section will document ISA agnostic concepts.

ISA specifics are covered at:

* <<x86-userland-assembly>> under link:userland/arch/x86_64/[], originally migrated from: https://github.com/cirosantilli/x86-assembly-cheat
* <<arm-userland-assembly>> under originally migrated from https://github.com/cirosantilli/arm-assembly-cheat under:
** link:userland/arch/arm/[]
** link:userland/arch/aarch64/[]

Like other userland programs, these programs can be run as explained at: <<userland-setup>>.

As a quick reminder, the fastest setups to get started are:

* <<userland-setup-getting-started-natively>> if your host can run the examples, e.g. x86 example on an x86 host
* <<userland-setup-getting-started-with-prebuilt-toolchain-and-qemu-user-mode>> otherwise

However, as usual, it is saner to build your toolchain as explained at: <<qemu-user-mode-getting-started>>.

The first example that you want to run for each arch is:

....
./run --userland userland/arch/<arch>/add.S
....

e.g.:

....
./run --userland userland/arch/x86_64/add.S
....

Sources:

* link:userland/arch/x86_64/add.S[]
* link:userland/arch/arm/add.S[]
* link:userland/arch/aarch64/add.S[]

These examples use the venerable ADD instruction to:

* introduce the basics of how a given assembly works: how many inputs / outputs, who is input and output, can it use memory or just registers, etc.
+
It is then a big copy paste for most other data instructions.
* verify that the venerable `add` instruction and our assertions are working

Then, modify that program to make the assertion fail:

....
ASSERT_EQ(%rax, $4)
....

because 1 + 2 tends to equal 3 instead of 4.

And then watch the assertion fail:

....
./build-userland
./run --userland userland/arch/x86_64/add.S
....

with error message:

....
assert_eq_64 failed
val1 0x3
val2 0x4
error: asm_main returned 1 at line 8
....

and notice how the error message gives both:

* the actual assembly source line number where the failing assert was
* the actual and expected values

Other infrastructure sanity checks that you might want to look into include:

* link:userland/arch/empty.S[]
* `FAIL` tests
** link:userland/arch/fail.S[]
* `ASSERT_MEMCMP` tests
** link:userland/arch/x86_64/lkmc_assert_memcmp_fail.S[]

=== Assembly registers

After seeing an <<userland-assembly,ADD hello world>>, you need to learn the general registers:

* arm
** link:userland/arch/arm/registers.S[]
* aarch64
** link:userland/arch/aarch64/registers.S[]
** link:userland/arch/aarch64/pc.S[]

Bibliography: <<armarm7>> A2.3 "ARM core registers".

==== ARMv8 aarch64 x31 register

Example: link:userland/arch/aarch64/x31.S[]

There is no `x31` name, and the encoding can have two different names depending on the instruction:

* `xzr`: zero register:
** https://stackoverflow.com/questions/42788696/why-might-one-use-the-xzr-register-instead-of-the-literal-0-on-armv8
** https://community.arm.com/processors/f/discussions/3185/wzr-xzr-register-s-purpose
* `sp`: stack pointer

To make things more confusing, some aliases can take either name, which makes them alias to different things, e.g. `mov` accepts both:

....
mov x0, sp
mov x0, xzr
....

and the first one is an alias to `add` while the second an alias to `orr`.

The difference is documented on a per instruction basis. Instructions that encode 31 as SP say:

....
if d == 31 then
  SP[] = result;
else
  X[d] = result;
....

And then those that don't say that, B1.2.1 "Registers in AArch64 state" implies the zero register:

____
In instruction encodings, the value 0b11111 (31) is used to indicate the ZR (zero register). This
indicates that the argument takes the value zero, but does not indicate that the ZR is implemented
as a physical register.
____

This is also described on <<armarm8>> C1.2.5 "Register names":

____
There is no register named W31 or X31.

The name SP represents the stack pointer for 64-bit operands where an encoding of the value 31 in the
corresponding register field is interpreted as a read or write of the current stack pointer. When instructions
do not interpret this operand encoding as the stack pointer, use of the name SP is an error.

The name XZR represents the zero register for 64-bit operands where an encoding of the value 31 in the
corresponding register field is interpreted as returning zero when read or discarding the result when written.
When instructions do not interpret this operand encoding as the zero register, use of the name XZR is an error
____

=== Floating point assembly

Keep in mind that many ISAs started floating point as an optional thing, and it later got better integrated into the main CPU, side by side with SIMD.

For this reason, there are sometimes multiple ways to do floating point operations in each ISA.

Let's start as usual with floating point addition + register file:

* arm
** <<arm-vadd-instruction>>
** <<arm-vfp-registers>>
* aarch64
** <<armv8-aarch64-fadd-instruction>>
** <<armv8-aarch64-floating-point-registers>>

=== SIMD assembly

Much like ADD for non-SIMD, start learning SIMD instructions by looking at the integer and floating point SIMD ADD instructions of each ISA:

* x86
** <<x86-addpd-instruction>>
** <<x86-paddq-instruction>>
* arm
** <<arm-vadd-instruction>>
* aarch64
** <<armv8-aarch64-add-vector-instruction>>
** <<armv8-aarch64-fadd-instruction>>

Then it is just a huge copy paste of infinite boring details:

* <<x86-simd>>
* <<arm-simd>>

=== User vs system assembly

By "userland assembly", we mean "the parts of the ISA which can be freely used from userland".

Most ISAs are divided into a system and userland part, and to running the system part requires elevated privileges such as <<ring0>> in x86.

One big difference between both is that we can run userland assembly on <<userland-setup>>, which is easier to get running and debug.

In particular, most userland assembly examples link to the C standard library: <<userland-assembly-c-standard-library>>.

Userland assembly is generally simpler, and a pre-requisite for <<baremetal-setup>>.

System-land assembly cheats will be put under: <<baremetal-setup>>.

=== Userland assembly C standard library

All examples except the <<freestanding-programs>> link to the C standard library.

This allows using the C standard library for IO, which is very convenient and portable across host OSes.

It also exposes other non-IO functionality that is very convenient such as `memcmp`.

The C standard library infrastructure is implemented in the following files:

* link:userland/arch/main.c[]
* link:userland/arch/common.h[]
* link:userland/arch/x86_64/common_arch.h[]
* link:userland/arch/arm/common_arch.h[]
* link:userland/arch/aarch64/common_arch.h[]

==== Freestanding programs

Unlike most our other assembly examples, which use the C standard library for portability, examples under `freestanding/` directories don't link to the C standard library.

As a result, those examples cannot do IO portably, and so they make raw system calls and only be run on one given OS, e.g. Linux: <<linux-system-calls>>.

Such executables are called freestanding because they don't execute the glibc initialization code, but rather start directly on our custom hand written assembly.

In order to GDB step debug those executables, you will want to use `--no-continue`, e.g.:

....
./run --arch aarch64 --userland userland/arch/aarch64/freestanding/linux/hello.S --gdb-wait
./run-gdb --arch aarch64 --no-continue --userland userland/arch/aarch64/freestanding/linux/hello.S
....

You are now left on the very first instruction of our tiny executable!

=== GCC inline assembly

Examples under `arch/<arch>/c/` directories show to how use inline assembly from higher level languages such as C:

* x86_64
** link:userland/arch/x86_64/c/inc.c[]
** link:userland/arch/x86_64/c/add.c[]
* arm
** link:userland/arch/arm/c/inc.c[]
** link:userland/arch/arm/c/inc_memory.c[]
** link:userland/arch/arm/c/inc_memory_global.c[]
** link:userland/arch/arm/c/add.c[]
* aarch64
** link:userland/arch/aarch64/c/earlyclobber.c[]
** link:userland/arch/aarch64/c/inc.c[]
** link:userland/arch/aarch64/c/multiline.cpp[]

==== GCC inline assembly register variables

Used notably in some of the <<linux-system-calls>> setups:

* link:userland/arch/arm/reg_var.c[]
* link:userland/arch/aarch64/reg_var.c[]
* link:userland/arch/aarch64/reg_var_float.c[]

In x86, makes it possible to access variables not exposed with the one letter register constraints.

In arm, it is the only way to achieve this effect: https://stackoverflow.com/questions/10831792/how-to-use-specific-register-in-arm-inline-assembler

This feature notably useful for making system calls from C, see: <<linux-system-calls>>.

Documentation: https://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Explicit-Reg-Vars.html

==== GCC inline assembly scratch registers

How to use temporary registers in inline assembly:

* x86_64
** link:userland/arch/x86_64/c/scratch.c[]
** link:userland/arch/x86_64/c/scratch_hardcode.c[]

Bibliography: https://stackoverflow.com/questions/6682733/gcc-prohibit-use-of-some-registers/54963829#54963829

==== GCC inline assembly early-clobbers

An example of using the `&` early-clobber modifier: link:userland/arch/aarch64/earlyclobber.c

More details at: https://stackoverflow.com/questions/15819794/when-to-use-earlyclobber-constraint-in-extended-gcc-inline-assembly/54853663#54853663

The assertion may fail without it. It actually does fail in GCC 8.2.0.

==== GCC inline assembly floating point ARM

Not documented as of GCC 8.2, but possible: https://stackoverflow.com/questions/53960240/armv8-floating-point-output-inline-assembly

* link:userland/arch/arm/c/inc_float.c[]
* link:userland/arch/aarch64/c/inc_float.c[]

=== Linux system calls

The following <<userland-setup>> programs illustrate how to make system calls:

* x86_64
** link:userland/arch/x86_64/freestanding/linux/hello.S[]
** link:userland/arch/x86_64/c/freestanding/linux/hello.c[]
** link:userland/arch/x86_64/c/freestanding/linux/hello_regvar.c[]
* arm
** link:userland/arch/arm/freestanding/linux/hello.S[]
** link:userland/arch/arm/c/freestanding/linux/hello.c[]
* aarch64
** link:userland/arch/aarch64/freestanding/linux/hello.S[]
** link:userland/arch/aarch64/c/freestanding/linux/hello.c[]
** link:userland/arch/aarch64/c/freestanding/linux/hello_clobbers.c[]

Determining the ARM syscall numbers:

* https://reverseengineering.stackexchange.com/questions/16917/arm64-syscalls-table
* arm: https://github.com/torvalds/linux/blob/v4.17/arch/arm/tools/syscall.tbl
* aarch64: https://github.com/torvalds/linux/blob/v4.17/include/uapi/asm-generic/unistd.h

Determining the ARM syscall interface:

* https://stackoverflow.com/questions/12946958/what-is-the-interface-for-arm-system-calls-and-where-is-it-defined-in-the-linux
* https://stackoverflow.com/questions/45742869/linux-syscall-conventions-for-armv8

Questions about the C inline assembly examples:

* x86_64: https://stackoverflow.com/questions/9506353/how-to-invoke-a-system-call-via-sysenter-in-inline-assembly/54956854#54956854
* ARM: https://stackoverflow.com/questions/21729497/doing-a-syscall-without-libc-using-arm-inline-assembly

=== Calling conventions

==== x86_64 calling convention

Examples:

* link:userland/arch/x86_64/common_arch.h[] `ENTRY` and `EXIT`

==== ARM calling convention

Call C standard library functions from assembly and vice versa.

* arm
** link:userland/arch/arm/common_arch.h[] `ENTRY` and `EXIT`
** link:userland/arch/arm/linux/c_from_asm.S[]
* aarch64
** link:userland/arch/aarch64/common_arch.h[] `ENTRY` and `EXIT`
** link:userland/arch/aarch64/c/linux/asm_from_c.c[]

ARM Architecture Procedure Call Standard (AAPCS) is the name that ARM Holdings gives to the calling convention.

Official specification: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042f/IHI0042F_aapcs.pdf

Bibliography:

* https://en.wikipedia.org/wiki/Calling_convention#ARM_(A32) Wiki contains the master list as usual.
* http://stackoverflow.com/questions/8422287/calling-c-functions-from-arm-assembly
* http://stackoverflow.com/questions/261419/arm-to-c-calling-convention-registers-to-save
* https://stackoverflow.com/questions/10494848/arm-whats-the-difference-between-apcs-and-aapcs-abi

=== GNU GAS assembler

link:https://en.wikipedia.org/wiki/GNU_Assembler[GNU GAS] is the default assembler used by GDB, and therefore it completely dominates in Linux.

The Linux kernel in particular uses GNU GAS assembly extensively for the arch specific parts under `arch/`.

==== GNU GAS assembler comments

In this tutorial, we use exclusively C Preprocessor `/**/` comments because:

* they are the same for all archs
* we are already stuck to the C Preprocessor because GNU GAS macros are unusable so we need `#define`
* mixing `#` GNU GAS comments and `#define` is a bad idea ;-)

But just in case you want to suffer, see this full explanation of GNU GAS comments: https://stackoverflow.com/questions/15663280/how-to-make-the-gnu-assembler-use-a-slash-for-comments/51991349#51991349

Examples:

* link:userland/arch/arm/comments.S[]
* link:userland/arch/aarch64/comments.S[]

==== GNU GAS assembler immediates

Summary:

* x86 always dollar `$` everywhere.
* ARM: can use either `#`, `$` or nothing depending on v7 vs v8 and <<gnu-gas-assembler-arm-unified-syntax,`.syntax unified`>>.
+
Fuller explanation at: https://stackoverflow.com/questions/21652884/is-the-hash-required-for-immediate-values-in-arm-assembly/51987780#51987780

Examples:

* link:userland/arch/arm/immediates.S[]
* link:userland/arch/aarch64/immediates.S[]

==== GNU GAS assembler data sizes

Let's see how many bytes go into each data type:

* link:userland/arch/x86_64/gas_data_sizes.S[]
* link:userland/arch/arm/gas_data_sizes.S[]
* link:userland/arch/aarch64/gas_data_sizes.S[]

Conclusion:

[options="header"]
|===
|.byte |.word |.long |.quad |.octa

|x86
|1
|2
|4
|8
|16

|arm
|1
|4
|4
|8
|16

|aarch64
|1
|4
|4
|8
|16

|===

and also keep in mind that according to the manual:

* `.int` is the same as `.long`
* `.hword` is the same as `.short` which is usually the same as `.word`

Bibliography:

* https://sourceware.org/binutils/docs-2.32/as/Pseudo-Ops.html#Pseudo-Ops
* https://stackoverflow.com/questions/43005411/how-does-the-quad-directive-work-in-assembly/43006616
* https://gist.github.com/steakknife/d47d0b19a24817f48027

===== GNU GAS assembler ARM specifics

====== GNU GAS assembler ARM unified syntax

There are two types of ARMv7 assemblies:

* `.syntax divided`
* `.syntax unified`

They are very similar, but unified is the new and better one, which we use in this tutorial.

Unfortunately, for backwards compatibility, GNU AS 2.31.1 and GCC 8.2.0 still use `.syntax divided` by default.

The concept of unified assembly is mentioned in ARM's official assembler documentation: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0473c/BABJIHGJ.html and is often called Unified Assembly Language (UAL).

Some of the differences include:

* `#` is optional in unified syntax int literals, see <<gnu-gas-assembler-immediates>>
* many mnemonics changed:
** most of them are condition code position changes, e.g. `andseq` vs `andeqs`: https://stackoverflow.com/questions/51184921/wierd-gcc-behaviour-with-arm-assembler-andseq-instruction
** but there are some more drastic ones, e.g. `swi` vs `svc`: https://stackoverflow.com/questions/8459279/are-arm-instructuons-swi-and-svc-exactly-same-thing/54078731#54078731
* cannot have implicit destination with shift, see: <<arm-shift-suffixes>>

===== GNU GAS assembler ARM .n and .w suffixes

When reading disassembly, many instructions have either a `.n` or `.w` suffix.

`.n` means narrow, and stands for the <<arm-instruction-encodings,Thumb encoding>> of an instructions, while `.w` means wide and stands for the ARM encoding.

Bibliography: https://stackoverflow.com/questions/27147043/n-suffix-to-branch-instruction

== x86 userland assembly

Arch agnostic infrastructure getting started at: <<userland-assembly>>.

=== x86 userland assembly getting started

These are the main concepts and instructions that you should learn to be able to understand what is going on.

Once those are done, everything else left on userland is just to learn a huge list of instructions: <<x86-userland-assembly-instructions>>

=== x86 userland assembly instructions

==== x86 SIMD

History:

* link:https://en.wikipedia.org/wiki/MMX_(instruction_set)[MMX]: 1997
* link:https://en.wikipedia.org/wiki/Streaming_SIMD_Extensions[SSE]: Streaming SIMD Extensions. 1999. 128-bit XMM registers.
* link:https://en.wikipedia.org/wiki/SSE2[SSE2]: 2004
* link:https://en.wikipedia.org/wiki/SSE3[SSE3]: 2006
* link:https://en.wikipedia.org/wiki/SSE4[SSE4]: 2006
* link:https://en.wikipedia.org/wiki/Advanced_Vector_Extensions[AVX]: Advanced Vector Extensions. 2011. 256-bit YMM registers. Extension of XMM.
* AVX2:2013
* AVX-512: 2016. 512-bit ZMM registers. Extension of YMM.

===== x86 SSE2

====== x86 addpd instruction

link:userland/arch/x86_64/addpd.S[]: `addps`, `addpd`

Good first instruction to learn SIMD: <<simd-assembly>>

====== x86 paddq instruction

link:userland/arch/x86_64/paddq.S[]: `paddq`, `paddl`, `paddw`, `paddb`

Good first instruction to learn SIMD: <<simd-assembly>>

=== x86 rdtsc instruction

TODO: review this section, make a more controlled userland experiment with <<m5ops>> instrumentation.

Let's have some fun and try to correlate the gem5 <<stats-txt>> `system.cpu.numCycles` cycle count with the link:https://en.wikipedia.org/wiki/Time_Stamp_Counter[x86 `rdtsc` instruction] that is supposed to do the same thing:

....
./build-userland --static userland/arch/x86_64/c/rdtsc.c
./run --eval './arch/x86_64/c/rdtsc.out;m5 exit;' --emulator gem5
./gem5-stat
....

Source: link:userland/rdtsc.c[]

`rdtsc` outputs a cycle count which we compare with gem5's `gem5-stat`:

* `3828578153`: `rdtsc`
* `3830832635`: `gem5-stat`

which gives pretty close results, and serve as a nice sanity check that the cycle counter is coherent.

It is also nice to see that `rdtsc` is a bit smaller than the `stats.txt` value, since the latter also includes the exec syscall for `m5`.

Bibliography:

* https://en.wikipedia.org/wiki/Time_Stamp_Counter
* https://stackoverflow.com/questions/9887839/clock-cycle-count-wth-gcc/9887979

==== ARM pmccntr

TODO We didn't manage to find a working ARM analogue to <<x86-rdtsc-instruction>>: link:kernel_modules/pmccntr.c[] is oopsing, and even it if weren't, it likely won't give the cycle count since boot since it needs to be activate before it starts counting anything:

* https://stackoverflow.com/questions/40454157/is-there-an-equivalent-instruction-to-rdtsc-in-arm
* https://stackoverflow.com/questions/31620375/arm-cortex-a7-returning-pmccntr-0-in-kernel-mode-and-illegal-instruction-in-u/31649809#31649809
* https://blog.regehr.org/archives/794

== ARM userland assembly

Arch general getting started at: <<userland-assembly>>.

Instructions here loosely grouped based on that of the <<armarm7>> Chapter A4 "The Instruction Sets".

We cover here mostly ARMv7, and then treat aarch64 differentially, since much of the ARMv7 userland is the same in aarch32.

=== Introduction to the ARM architecture

The link:https://en.wikipedia.org/wiki/ARM_architecture[ARM architecture] is has been used on the vast majority of mobile phones in the 2010's, and on a large fraction of micro controllers.

It competes with <<x86-userland-assembly>> because its implementations are designed for low power consumption, which is a major requirement of the cell phone market.

ARM is generally considered a RISC instruction set, although there are some more complex instructions which would not generally be classified as purely RISC.

ARM is developed by the British funded company ARM Holdings: https://en.wikipedia.org/wiki/Arm_Holdings which originated as a joint venture between Acorn Computers, Apple  and VLSI Technology in 1990.

ARM Holdings was bought by the Japanese giant SoftBank in 2016.

==== ARMv8 vs ARMv7 vs AArch64 vs AArch32

ARMv7 is the older architecture described at: <<armarm7>>.

ARMv8 is the newer architecture ISA link:https://developer.arm.com/docs/den0024/latest/preface[released in 2013] and described at: <<armarm8>>. It can be in either of two states:

* <<aarch32>>
* aarch64

In the lose terminology of this repository:

* `arm` means basically AArch32
* `aarch64` means ARMv8 AArch64

ARMv8 has link:https://en.wikipedia.org/wiki/ARM_architecture#ARMv8-A[had several updates] since its release:

* v8.1: 2014
* v8.2: 2016
* v8.3: 2016
* v8.4: TODO
* v8.5: 2018

They are described at: <<armarm8>> A1.7 "ARMv8 architecture extensions".

===== AArch32

32-bit mode of operation of ARMv8.

Userland is highly / fully backwards compatible with ARMv7:

* https://stackoverflow.com/questions/42972096/armv8-backward-compatibility-with-armv7-snapdragon-820-vs-cortex-a15
* https://stackoverflow.com/questions/31848185/does-armv8-aarch32-mode-has-backward-compatible-with-armv4-armv5-or-armv6

For this reason, QEMU and GAS seems to enable both AArch32 and ARMv7 under `arm` rather than `aarch64`.

There are however some extensions over ARMv7, many of them are functionality that ARMv8 has and that designers decided to backport on AArch32 as well, e.g.:

* <<armv8-aarch32-vcvta-instruction>>

===== AArch32 vs AArch64

A great summary of differences can be found at: https://en.wikipedia.org/wiki/ARM_architecture#AArch64_features

Some random ones:

* aarch32 has two encodings: Thumb and ARM: <<arm-instruction-encodings>>
* in ARMv8, the stack has to 16-byte aligned. Therefore, the main way to push things to stack is with 8-byte pair pushes with the <<armv8-aarch64-ldp-and-stp-instructions>>

==== Free ARM implementations

The ARM instruction set is itself protected by patents / copyright / whatever, and you have to pay ARM Holdings a licence to implement it, even if you are creating your own custom Verilog code.

ARM has already sued people in the past for implementing ARM ISA: http://www.eetimes.com/author.asp?section_id=36&doc_id=1287452

http://semiengineering.com/an-alternative-to-x86-arm-architectures/ mentions that:

____
Asanovic joked that the shortest unit of time is not the moment between a traffic light turning green in New York City and the cab driver behind the first vehicle blowing the horn; it’s someone announcing that they have created an open-source, ARM-compatible core and receiving a “cease and desist” letter from a law firm representing ARM.
____

This licensing however does have the following fairness to it: ARM Holdings invents a lot of money in making a great open source software environment for the ARM ISA, so it is only natural that it should be able to get some money from hardware manufacturers for using their ISA.

Patents for very old ISAs however have expired, Amber is one implementation of those: https://en.wikipedia.org/wiki/Amber_%28processor_core%29 TODO does it have any application?


Generally, it is mostly large companies that implement the CPUs themselves. For example, the link:https://en.wikipedia.org/wiki/Apple_A12[Apple A12 chip], which is used in iPhones, has verilog designs:

____
The A12 features an Apple-designed 64-bit ARMv8.3-A six-core CPU, with two high-performance cores running at 2.49 GHz called Vortex and four energy-efficient cores called Tempest.
____

ARM designed CPUs however are mostly called `Coretx-A<id>`: https://en.wikipedia.org/wiki/List_of_applications_of_ARM_cores Vortex and Tempest are Apple designed ones.
Bibliography: https://www.quora.com/Why-is-it-that-you-need-a-license-from-ARM-to-design-an-ARM-CPU-How-are-the-instruction-sets-protected

==== ARM instruction encodings

Understanding the basics of instruction encodings is fundamental to help you to remember what instructions do and why some things are possible or not, notably the <<arm-ldr-pseudo-instruction>> and the <<arm-adr-instruction,`adrp` instruction>>.

aarch32 has two "instruction sets", which to look just like encodings.

Some control bit must determine which one we are currently on, and userland can switch between them with the <<arm-bx-instruction>> TODO: details.

The encodings are:

* A32: every instruction is 4 bytes long. Can encode every instruction.
* T32: most common instructions are 2 bytes long. Many others less common ones are 4 bytes long.
+
T stands for "Thumb", which is the original name for the technology. The word "Thumb" does not appear on <<armarm8>> however. It does appear on <<armarm7>> though.
+
Example: link:userland/arch/arm/thumb.S[]
+
See also: <<armarm8>> F2.1.3 "Instruction encodings".

Within each instruction set, there can be multiple encodings for a given function, and they are noted simply as:

* A1, A2, ...: A32 encodings
* T1, T2, ..m: T32 encodings

This RISC-y mostly fixed instruction length design likely makes processor design easier and allows for certain optimizations, at the cost of slightly more complex assembly, as you can't encode 4 / 8 byte addresses in a single instruction. Totally worth it IMHO.

This design can be contrasted with x86, which has widely variable instruction length.

Bibliography:

* https://stackoverflow.com/questions/28669905/what-is-the-difference-between-the-arm-thumb-and-thumb-2-instruction-encodings
* https://reverseengineering.stackexchange.com/questions/6080/how-to-detect-thumb-mode-in-arm-disassembly

=== ARM branch instructions

==== ARM b instruction

Unconditional branch.

Example: link:userland/arch/arm/b.S[]

The encoding stores `pc` offsets in 24 bits. The destination must be a multiple of 4, which is easy since all instructions are 4 bytes.

This allows for 26 bit long jumps, which is 64 MiB.

TODO: what to do if we want to jump longer than that?

==== ARM beq instruction

Branch if equal based on the status registers.

Examples:

* link:userland/arch/arm/beq.S[].
* link:userland/arch/aarch64/beq.S[].

The family of instructions includes:

* `beq`: branch if equal
* `bne`: branch if not equal
* `ble`: less or equal
* `bge`: greater or equal
* `blt`: less than
* `bgt`: greater than

==== ARM bl instruction

Branch with link, i.e. branch and store the return address on the `rl` register.

Example: link:userland/arch/arm/bl.S[]

This is the major way to make function calls.

The current ARM / Thumb mode is encoded in the least significant bit of lr.

===== ARM bx instruction

`bx`: branch and switch between ARM / Thumb mode, encoded in the least significant bit of the given register.

`bx lr` is the main way to return from function calls after a `bl` call.

Since `bl` encodes the current ARM / Thumb in the register, `bx` keeps the mode unchanged by default.

===== ARMv8 aarch64 ret instruction

Example: link:userland/arch/aarch64/ret.S[]

ARMv8 AArch64 only:

* there is no `bx` in AArch64 since no Thumb to worry about, so it is called just `br`
* the `ret` instruction was added in addition to `br`, with the following differences:
** provides a hint that this is a function call return
** has a default argument `x30` if none is given. This is where `bl` puts the return value.

See also: https://stackoverflow.com/questions/32304646/arm-assembly-branch-to-address-inside-register-or-memory/54145818#54145818

==== ARM cbz instruction

Compare and branch if zero.

Example: link:userland/arch/aarch64/cbz.S[]

Only in ARMv8 and ARMv7 Thumb mode, not in armv7 ARM mode.

Very handy!

==== ARM conditional execution

Weirdly, <<arm-b-instruction>> and family are not the only instructions that can execute conditionally on the flags: the same also applies to most instructions, e.g. `add`.

Example: link:userland/arch/arm/cond.S[]

Just add the usual `eq`, `ne`, etc. suffixes just as for `b`.

The list of all extensions is documented at <<armarm7>> "A8.3 Conditional execution".

=== ARM load and store instructions

In ARM, there are only two instruction families that do memory access: <<arm-ldr-instruction>>  to load and <<arm-str-instruction>> to store.

Everything else works on register and immediates.

This is part of the RISC-y beauty of the ARM instruction set, unlike x86 in which several operations can read from memory, and helps to predict how to optimize for a given CPU pipeline.

This kind of architecture is called a link:https://en.wikipedia.org/wiki/Load/store_architecture[Load/store architecture].

==== ARM ldr instruction

===== ARM ldr pseudo-instruction

`ldr` can be either a regular instruction that loads stuff into memory, or also a pseudo-instruction (assembler magic): http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0041c/Babbfdih.html

The pseudo instruction version is when an equal sign appears on one of the operators.

The `ldr` pseudo instruction can automatically create hidden variables in a place called the "literal pool", and load them from memory with PC relative loads.

Example: link:userland/arch/arm/ldr_pseudo.S[]

This is done basically because all instructions are 32-bit wide, and there is not enough space to encode 32-bit addresses in them.

Bibliography:

* https://stackoverflow.com/questions/37840754/what-does-an-equals-sign-on-the-right-side-of-a-ldr-instruction-in-arm-mean
* https://stackoverflow.com/questions/17214962/what-is-the-difference-between-label-equals-sign-and-label-brackets-in-ar
* https://stackoverflow.com/questions/14046686/why-use-ldr-over-mov-or-vice-versa-in-arm-assembly

===== ARM addressing modes

Example: link:userland/arch/arm/address_modes.S[]

Load and store instructions can update the source register with the following modes:

* offset: add an offset, don't change the address register. Notation:
+
....
ldr r1, [r0, 4]
....
* pre-indexed: change the address register, and then use it modified. Notation:
+
....
ldr r1, [r0, 4]!
....
* post-indexed: use the address register unmodified, and then modify it. Notation:
+
....
ldr r1, [r0], 4
....

The offset itself can come from the following sources:

* immediate
* register
* scaled register: left shift the register and use that as an offset

The indexed modes are convenient to loop over arrays.

Bibliography: <<armarm7>>:

* A4.6.5 "Addressing modes"
* A8.5 "Memory accesses"

====== ARM loop over array

As an application of the post-indexed addressing mode, let's increment an array.

Example: link:userland/arch/arm/inc_array.S[]

===== ARM ldrh and ldrb instructions

There are `ldr` variants that load less than full 4 bytes:

* link:userland/arch/arm/ldrb.S[]: load byte
* link:userland/arch/arm/ldrh.S[]: load half word

==== ARM str instruction

Store from memory into registers.

Example: link:userland/arch/arm/str.S[]

Basically everything that applies to <<arm-ldr-instruction>> also applies here so we won't go into much detail.

===== ARMv8 aarch64 str instruction

PC-relative `str` is not possible in aarch64.

For `ldr` it works <<arm-ldr-instruction,as in aarch32>>.

As a result, it is not possible to load from the literal pool for `str`.

Example: link:userland/arch/aarch64/str.S[]

This can be seen from <<armarm8>> C3.2.1 "Load/Store register": `ldr` simply has on extra PC encoding that `str` does not.

===== ARMv8 aarch64 ldp and stp instructions

Push a pair of registers to the stack.

TODO minimal example. Currently used on link:v8/commmon_arch.h[] since it is the main way to restore register state.

==== ARM ldmia instruction

Pop values form stack into the register and optionally update the address register.

`stmdb` is the push version.

Example: link:userland/arch/arm/ldmia.S[]

The mnemonics stand for:

* `stmdb`: STore Multiple Decrement Before
* `ldmia`: LoaD Multiple Increment After

Example: link:userland/arch/arm/push.S[]

`push` and `pop` are just mnemonics `stdmdb` and `ldmia` using the stack pointer `sp` as address register:

....
stmdb sp!, reglist
ldmia sp!, reglist
....

The `!` indicates that we want to update the register.

The registers are encoded as single bits inside the instruction: each bit represents one register.

As a consequence, the push order is fixed no matter how you write the assembly instruction: there is just not enough space to encode ordering.

AArch64 loses those instructions, likely because it was not possible anymore to encode all registers: http://stackoverflow.com/questions/27941220/push-lr-and-pop-lr-in-arm-arch64 and replaces them with the <<armv8-aarch64-ldp-and-stp-instructions>>

=== ARM data processing instructions

Arithmetic:

* link:userland/arch/arm/mul.S[]: multiply
* link:userland/arch/arm/sub.S[]: subtract
* link:userland/arch/arm/rbit.S[]: reverse bit order
* link:userland/arch/arm/rev.S[]: reverse byte order
* link:userland/arch/arm/tst.S[]

==== ARM cset instruction

Example: link:userland/arch/aarch64/cset.S[]

Set a register conditionally depending on the condition flags:

ARMv8-only, likely because in ARMv8 you can't have conditional suffixes for every instruction.

==== ARM bitwise instructions

* link:userland/arch/arm/and.S[]
* `eor`: exclusive OR
* `orr`: OR
* link:userland/arch/arm/clz.S[]: count leading zeroes

===== ARM bic instruction

Bitwise Bit Clear: clear some bits.

....
dest = `left & ~right`
....

Example: link:userland/arch/arm/bic.S[]

===== ARM ubfm instruction

Unsigned Bitfield Move.

____
copies any number of low-order bits from a source register into the same number of adjacent bits at any position in the destination register, with zeros in the upper and lower bits.
____

Example: link:userland/arch/aarch64/ubfm.S[]

TODO: explain full behaviour. Very complicated. Has several simpler to understand aliases.

====== ARM ubfx instruction

Alias for:

....
UBFM <Wd>, <Wn>, #<lsb>, #(<lsb>+<width>-1)
....

Example: link:userland/arch/aarch64/ubfx.S[]

The operation:

....
UBFX dest, src, lsb, width
....

does:

....
dest = (src & ((1 << width) - 1)) >> lsb;
....

Bibliography: https://stackoverflow.com/questions/8366625/arm-bit-field-extract

===== ARM bfm instruction

TODO: explain. Similar to <<arm-ubfm-instruction,`ubfm`>> but leave untouched bits unmodified.

====== ARM bfi instruction

Examples:

* link:userland/arch/arm/bfi.S[]
* link:userland/arch/aarch64/bfi.S[]

Move the lower bits of source register into any position in the destination:

* ARMv8: an alias for <<arm-bfm-instruction,`bfm`>>
* ARMv7: a real instruction

==== ARM mov instruction

Move an immediate to a register, or a register to another register.

Cannot load from or to memory, since only the `ldr` and `str` instruction families can do that in ARM: <<arm-load-and-store-instructions>>

Example: link:userland/arch/arm/mov.S[]

Since every instruction <<arm-instruction-encodings,has a fixed 4 byte size>>, there is not enough space to encode arbitrary 32-bit immediates in a single instruction, since some of the bits are needed to actually encode the instruction itself.

The solutions to this problem are mentioned at:

* https://stackoverflow.com/questions/38689886/loading-32-bit-values-to-a-register-in-arm-assembly
* https://community.arm.com/processors/b/blog/posts/how-to-load-constants-in-assembly-for-arm-architecture

Summary of solutions:

* <<arm-movw-and-movt-instructions>>
* place it in memory. But then how to load the address, which is also a 32-bit value?
** use pc-relative addressing if the memory is close enough
** use `orr` encodable shifted immediates

The blog article summarizes nicely which immediates can be encoded and the design rationale:

____
An Operand 2 immediate must obey the following rule to fit in the instruction: an 8-bit value rotated right by an even number of bits between 0 and 30 (inclusive). This allows for constants such as 0xFF (0xFF rotated right by 0), 0xFF00 (0xFF rotated right by 24) or 0xF000000F (0xFF rotated right by 4).

In software - especially in languages like C - constants tend to be small. When they are not small they tend to be bit masks. Operand 2 immediates provide a reasonable compromise between constant coverage and encoding space; most common constants can be encoded directly.
____

Assemblers however support magic memory allocations which may hide what is truly going on: https://stackoverflow.com/questions/14046686/why-use-ldr-over-mov-or-vice-versa-in-arm-assembly Always ask your friendly disassembly for a good confirmation.

===== ARM movw and movt instructions

Set the higher or lower 16 bits of a register to an immediate in one go.

Example: link:userland/arch/arm/movw.S[]

The armv8 version analogue is <<armv8-aarch64-movk-instruction>>.

===== ARMv8 aarch64 movk instruction

Fill a 64 bit register with 4 16-bit instructions one at a time.

Similar to <<arm-movw-and-movt-instructions>> in v7.

Example: link:userland/arch/aarch64/movk.S[]

Bibliography: https://stackoverflow.com/questions/27938768/moving-a-32-bit-constant-in-arm-arch64-register

===== ARMv8 aarch64 movn instruction

Set 16-bits negated and the rest to `1`.

Example: link:userland/arch/aarch64/movn.S[]

==== ARM data processing instruction suffixes

===== ARM shift suffixes

Most data processing instructions can also optionally shift the second register operand.

Example: link:userland/arch/arm/shift.S[]

The shift types are:

* `lsr` and `lfl`: Logical Shift Right / Left. Insert zeroes.
* `ror`: Rotate Right / Left. Wrap bits around.
* `asr`: Arithmetic Shift Right. Keep sign.

Documented at: <<armarm7>> "A4.4.1 Standard data-processing instructions"

===== ARM S suffix

Example: link:userland/arch/arm/s_suffix.S[]

The `S` suffix, present on most <<arm-data-processing-instructions>>, makes the instruction also set the Status register flags that control conditional jumps.

If the result of the operation is `0`, then it triggers `beq`, since comparison is a subtraction, with success on 0.

`cmp` sets the flags by default of course.

==== ARM adr instruction

Similar rationale to the <<arm-ldr-pseudo-instruction>>, allowing to easily store a PC-relative reachable address into a register in one go, to overcome the 4-byte fixed instruction size.

Examples:

* link:userland/arch/arm/adr.S[]
* link:userland/arch/aarch64/adr.S[]
* link:userland/arch/aarch64/adrp.S[]

More details: https://stackoverflow.com/questions/41906688/what-are-the-semantics-of-adrp-and-adrl-instructions-in-arm-assembly/54042899#54042899

===== ARM adrl instruction

See: <<arm-adr-instruction>>.

=== ARM miscellaneous instructions

==== ARM nop instruction

There are a few different ways to encode `nop`, notably `mov` a register into itself, and a dedicated miscellaneous instruction.

Example: link:userland/arch/arm/nop.S[]

Try disassembling the executable to see what the assembler is emitting:

....
gdb-multiarch -batch -ex 'arch arm' -ex "file v7/nop.out" -ex "disassemble/rs asm_main_after_prologue"
....

Bibliography: https://stackoverflow.com/questions/1875491/nop-for-iphone-binaries

=== ARM SIMD

==== ARM VFP

The name for the ARMv7 and AArch32 floating point and SIMD instructions / registers.

Vector Floating Point extension.

TODO I think it was optional in ARMv7, find quote.

VFP has several revisions, named as VFPv1, VFPv2, etc. TODO: announcement dates.

As mentioned at: https://stackoverflow.com/questions/37790029/what-is-difference-between-arm64-and-armhf/48954012#48954012 the Linux kernel shows those capabilities in `/proc/cpuinfo` with flags such as `vfp`, `vfpv3` and others, see:

* https://github.com/torvalds/linux/blob/v4.18/arch/arm/kernel/setup.c#L1199
* https://github.com/torvalds/linux/blob/v4.18/arch/arm64/kernel/cpuinfo.c#L95

When a certain version of VFP is present on a CPU, the compiler prefix typically contains the `hf` characters which stands for Hard Float, e.g.: `arm-linux-gnueabihf`. This means that the compiler will emit VFP instructions instead of just using software implementations.

Bibliography:

* <<armarm7>> Appendix D6 "Common VFP Subarchitecture Specification". It is not part of the ISA, but just an extension. TODO: that spec does not seem to have the instructions documented, and instruction like `VMOV` just live with the main instructions. Is `VMOV` part of VFP?
* https://mindplusplus.wordpress.com/2013/06/25/arm-vfp-vector-programming-part-1-introduction/
* https://en.wikipedia.org/wiki/ARM_architecture#Floating-point_(VFP)

===== ARM VFP registers

TODO example

<<armarm8>> E1.3.1 "The SIMD and floating-point register file" Figure E1-1 "SIMD and floating-point register file, AArch32 operation":

....
+-----+-----+-----+
| S0  |     |     |
+-----+ D0  +     |
| S1  |     |     |
+-----+-----+ Q0  |
| S2  |     |     |
+-----+ D1  +     |
| S3  |     |     |
+-----+-----+-----+
| S4  |     |     |
+-----+ D2  +     |
| S5  |     |     |
+-----+-----+ Q1  |
| S6  |     |     |
+-----+ D3  +     |
| S7  |     |     |
+-----+-----+-----+
....

Note how Sn is weirdly packed inside Dn, and Dn weirdly packed inside Qn, likely for historical reasons.

And you can't access the higher bytes at D16 or greater with Sn.

===== ARM vadd instruction

* link:userland/arch/arm/vadd_scalar.S[]: see also: <<floating-point-assembly>>
* link:userland/arch/arm/vadd_vector.S[]: see also: <<simd-assembly>>

===== ARM vcvt instruction

Example: link:userland/arch/arm/vcvt.S[]

Convert between integers and floating point.

<<armarm7>> on rounding:

____
The floating-point to fixed-point operation uses the Round towards Zero rounding mode. The fixed-point to floating-point operation uses the Round to Nearest rounding mode.
____

Notice how the opcode takes two types.

E.g., in our 32-bit float to 32-bit unsigned example we use:

....
vld1.32.f32
....

====== ARM vcvtr instruction

Example: link:userland/arch/arm/vcvtr.S[]

Like <<arm-vcvt-instruction>>, but the rounding mode is selected by the FPSCR.RMode field.

Selecting rounding mode explicitly per instruction was apparently not possible in ARMv7, but was made possible in <<aarch32>> e.g. with <<armv8-aarch32-vcvta-instruction>>.

Rounding mode selection is exposed in the ANSI C standard through link:https://en.cppreference.com/w/c/numeric/fenv/feround[`fesetround`].

TODO: is the initial rounding mode specified by the ELF standard? Could not find a reference.

====== ARMv8 AArch32 vcvta instruction

Example: link:userland/arch/arm/vcvt.S[]

Added in ARMv8 <<aarch32>> only, not present in ARMv7.

In ARMv7, to use a non-round-to-zero rounding mode, you had to set the rounding mode with FPSCR and use the R version of the instruction e.g. <<arm-vcvtr-instruction>>.

Now in AArch32 it is possible to do it explicitly per-instruction.

Also there was no ties to away mode in ARMv7. This mode does not exist in C99 either.

==== ARMv8 Advanced SIMD and floating-point support

The <<armarm8>> specifies floating point and SIMD support in the main architecture at A1.5 "Advanced SIMD and floating-point support".

The feature is often refered to simply as "SIMD&FP" throughout the manual.

The Linux kernel shows `/proc/cpuinfo` compatibility as `neon`, which is yet another intermediate name that came up at some point: <<arm-neon>>

Vs <<arm-vfp>>: https://stackoverflow.com/questions/4097034/arm-cortex-a8-whats-the-difference-between-vfp-and-neon

===== ARMv8 floating point availability

Support is semi-mandatory. <<armarm8>> A1.5 "Advanced SIMD and floating-point support":

____
ARMv8 can support the following levels of support for Advanced SIMD and floating-point instructions:

- Full SIMD and floating-point support without exception trapping.
- Full SIMD and floating-point support with exception trapping.
- No floating-point or SIMD support. This option is licensed only for implementations targeting specialized markets.

Note: All systems that support standard operating systems with rich application environments provide hardware
support for Advanced SIMD and floating-point. It is a requirement of the ARM Procedure Call Standard for
AArch64, see Procedure Call Standard for the ARM 64-bit Architecture.
____

Therefore it is in theory optional, but highly available.

This is unlike ARMv7, where floating point is completely optional through <<arm-vfp>>.

===== ARM NEON

Just an informal name for the "Advanced SIMD instructions"? Very confusing.

<<armarm8>> F2.9 "Additional information about Advanced SIMD and floating-point instructions" says:

____
The Advanced SIMD architecture, its associated implementations, and supporting software, are commonly referred to as NEON technology.
____

https://developer.arm.com/technologies/neon mentions that is is present on both ARMv7 and ARMv8:

____
NEON technology was introduced to the Armv7-A and Armv7-R profiles. It is also now an extension to the Armv8-A and Armv8-R profiles.
____

==== ARMv8 AArch64 floating point registers

TODO example.

<<armarm8>> B1.2.1 "Registers in AArch64 state" describes the registers:

____
32 SIMD&FP registers, `V0` to `V31`. Each register can be accessed as:

* A 128-bit register named `Q0` to `Q31`.
* A 64-bit register named `D0` to `D31`.
* A 32-bit register named `S0` to `S31`.
* A 16-bit register named `H0` to `H31`.
* An 8-bit register named `B0` to `B31`.
____

Notice how Sn is very different between v7 and v8! In v7 it goes across Dn, and in v8 inside each Dn.

===== ARMv8 aarch64 add vector instruction

link:userland/arch/aarch64/add_vector.S[]

Good first instruction to learn SIMD: <<simd-assembly>>

===== ARMv8 aarch64 fadd instruction

* link:userland/arch/aarch64/fadd_vector.S[]: see also: <<simd-assembly>>
* link:userland/arch/aarch64/fadd_scalar.S[]: see also: <<floating-point-assembly>>

====== ARM fadd vs vadd

It is very confusing, but `fadds` and `faddd` in Aarch32 are <<gnu-gas-assembler-arm-unified-syntax,pre-UAL>> for `vadd.f32` and `vadd.f64` which we use in this tutorial: <<arm-vadd-instruction>>

The same goes for most ARMv7 mnemonics: `f*` is old, and `v*` is the newer better syntax.

But then, in ARMv8, they decided to use <<armv8-aarch64-fadd-instruction>> as the main floating point add name, and get rid of `vadd`!

Also keep in mind that fused multiply add is `fmadd`.

Examples at: <<simd-assembly>>

===== ARMv8 aarch64 ld2 instruction

Example: link:userland/arch/aarch64/ld2.S[]

We can load multiple vectors interleaved from memory in one single instruction!

This is why the `ldN` instructions take an argument list denoted by `{}` for the registers, much like armv7 <<arm-ldmia-instruction>>.

There are analogous `ld3` and `ld4` instruction.

==== ARM SIMD bibliography

* GNU GAS tests under link:https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=tree;f=gas/testsuite/gas/aarch64;hb=00f223631fa9803b783515a2f667f86997e2cdbe[`gas/testsuite/gas/aarch64`]
* https://stackoverflow.com/questions/2851421/is-there-a-good-reference-for-arm-neon-intrinsics
* assembly optimized libraries:
** https://github.com/projectNe10/Ne10

=== ARM assembly bibliography

==== ARM non-official bibliography

Good getting started tutorials:

* http://www.davespace.co.uk/arm/introduction-to-arm/
* https://azeria-labs.com/writing-arm-assembly-part-1/
* https://thinkingeek.com/arm-assembler-raspberry-pi/
* http://bob.cs.sonoma.edu/IntroCompOrg-RPi/app-make.html

==== ARM official bibliography

The official manuals were stored in http://infocenter.arm.com but as of 2017 they started to slowly move to link:https://developer.arm.com[].

Each revision of a document has a "ARM DDI" unique document identifier.

The "ARM Architecture Reference Manuals" are the official canonical ISA documentation document. In this repository, we always reference the following revisions:

Bibliography: https://www.quora.com/Where-can-I-find-the-official-documentation-of-ARM-instruction-set-architectures-ISAs

[[armarm7]]
===== ARMv7 architecture reference manual

https://developer.arm.com/products/architecture/a-profile/docs/ddi0406/latest/arm-architecture-reference-manual-armv7-a-and-armv7-r-edition

The official comprehensive ARMv7 reference.

We use by default: DDI 0406C.d: https://static.docs.arm.com/ddi0406/cd/DDI0406C_d_armv7ar_arm.pdf

[[armarm8]]
===== ARMv8 architecture reference manual

https://static.docs.arm.com/ddi0487/ca/DDI0487C_a_armv8_arm.pdf

Latest version: https://developer.arm.com/docs/ddi0487/latest/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile

The official comprehensive ARMv8 reference.

ISA quick references can be found in some places:

* https://web.archive.org/web/20161009122630/http://infocenter.arm.com/help/topic/com.arm.doc.qrc0001m/QRC0001_UAL.pdf

[[armv8-programmers-guide]]
===== Programmer's Guide for ARMv8-A

https://static.docs.arm.com/den0024/a/DEN0024A_v8_architecture_PG.pdf

A more terse human readable introduction to the ARM architecture than the reference manuals.

Does not have as many assembly code examples as you'd hope however...

Latest version at: https://developer.arm.com/docs/den0024/latest/preface

== Baremetal

Getting started at: <<baremetal-setup>>

=== Baremetal GDB step debug

GDB step debug works on baremetal exactly as it does on the Linux kernel: <<gdb>>.

Except that is is even cooler here since we can easily control and understand every single instruction that is being run!

For example, on the first shell:

....
./run --arch arm --baremetal baremetal/hello.c --gdb-wait
....

then on the second shell:

....
./run-gdb --arch arm --baremetal baremetal/hello.c -- main
....

Or if you are a <<tmux,tmux pro>>, do everything in one go with:

....
./run --arch arm --baremetal baremetal/hello.c --gdb
....

Alternatively, to start from the very first executed instruction of our tiny <<baremetal-bootloaders>>:

....
./run \
  --arch arm \
  --baremetal baremetal/hello.c \
  --gdb-wait \
  --tmux-args=--no-continue \
;
....

Now you can just `stepi` to when jumping into main to go to the C code in link:baremetal/hello.c[].

This is specially interesting for the executables that don't use the bootloader from under `baremetal/arch/<arch>/no_bootloader/*.S`, e.g.:

....
./run \
  --arch arm \
  --baremetal baremetal/arch/arm/no_bootloader/semihost_exit.S \
  --gdb-wait \
  --tmux-args=--no-continue \
;
....

The cool thing about those examples is that you start at the very first instruction of your program, which gives more control.

=== Baremetal bootloaders

As can be seen from <<baremetal-gdb-step-debug>>, all examples under link:baremetal/[], with the exception of `baremetal/arch/<arch>/no_bootloader`, start from our tiny bootloaders:

* link:baremetal/lib/arm.S[]
* link:baremetal/lib/aarch64.S[]

Out simplistic bootloaders basically setup up just enough system state to allow calling:

* C functions such as `exit` from the assembly examples
* the `main` of C examples itself

The most important things that we setup in the bootloaders are:

* the stack pointer
* NEON: <<aarch64-baremetal-neon-setup>>
* TODO: we don't do this currently but maybe we should setup BSS

The C functions that become available as a result are:

* Newlib functions implemented at link:baremetal/lib/syscalls.c[]
* non-Newlib functions implemented at link:kwargs['c'][]

It is not possible to call those C functions from the examples that don't use a bootloader.

For this reason, we tend to create examples with bootloaders, as it is easier to write them portably.

=== Semihosting

Semihosting is a publicly documented interface specified by ARM Holdings that allows us to do some magic operations very useful in development.

Semihosting is implemented both on some real devices and on simulators such as QEMU and <<gem5-semihosting>>.

It is documented at: https://developer.arm.com/docs/100863/latest/introduction

For example, the following code makes QEMU exit:

....
./run --arch arm --baremetal baremetal/arch/arm/semihost_exit.S
....

Source: link:baremetal/arch/arm/no_bootloader/semihost_exit.S[]

That program program contains the code:

....
mov r0, #0x18
ldr r1, =#0x20026
svc 0x00123456
....

and we can see from the docs that `0x18` stands for the `SYS_EXIT` command.

This is also how we implement the `exit(0)` system call in C for QEMU for link:baremetal/exit0.c[] through the Newlib via the function `_exit` at link:baremetal/lib/kwargs['c'][].

Other magic operations we can do with semihosting besides exiting the on the host include:

* read and write to host stdin and stdout
* read and write to host files

Alternatives exist for some semihosting operations, e.g.:

* UART IO for host stdin and stdout in both emulators and real hardware
* <<m5ops>> for <<gem5>>, e.g. `m5 exit` makes the emulator quit

The big advantage of semihosting is that it is standardized across all ARM boards, and therefore allows you to make a single image that does those magic operations instead of having to compile multiple images with different magic addresses.

The downside of semihosting is that it is ARM specific. TODO is it an open standard that other vendors can implement?

In QEMU, we enable semihosting with:

....
-semihosting
....

Newlib 9c84bfd47922aad4881f80243320422b621c95dc already has a semi-hosting implementation at:

....
newlib/libc/sys/arm/syscalls.c
....

TODO: how to use it? Possible through crosstool-NG? In the worst case we could just copy it.

Bibliography:

* https://stackoverflow.com/questions/31990487/how-to-cleanly-exit-qemu-after-executing-bare-metal-program-without-user-interve/40957928#40957928
* https://balau82.wordpress.com/2010/11/04/qemu-arm-semihosting/

==== gem5 semihosting

For gem5, you need:

....
patch -d "$(./getvar gem5_source_dir)" -p 1 < patches/manual/gem5-semihost.patch
....

https://stackoverflow.com/questions/52475268/how-to-enable-arm-semihosting-in-gem5/52475269#52475269

=== gem5 baremetal carriage return

TODO: our example is printing newlines without automatic carriage return `\r` as in:

....
enter a character
                 got: a
....

We use `m5term` by default, and if we try `telnet` instead:

....
telnet localhost 3456
....

it does add the carriage returns automatically.

=== Baremetal host packaged toolchain

For `arm`, some baremetal examples compile fine with:

....
sudo apt-get install gcc-arm-none-eabi qemu-system-arm
./build-baremetal --arch arm --gcc-which host-baremetal
./run --arch arm --baremetal baremetal/hello.c --qemu-which host
....

However, there are as usual limitations to using prebuilts:

* certain examples fail to build with the Ubuntu packaged toolchain. E.g.: link:baremetal/exit0.c[] fails with:
+
....
/usr/lib/gcc/arm-none-eabi/6.3.1/../../../arm-none-eabi/lib/libg.a(lib_a-fini.o): In function `__libc_fini_array':
/build/newlib-8gJlYR/newlib-2.4.0.20160527/build/arm-none-eabi/newlib/libc/misc/../../../../../newlib/libc/misc/fini.c:33: undefined reference to `_fini'
collect2: error: ld returned 1 exit status
....
+
with the prebuilt toolchain, and I'm lazy to debug.
* there seems to to be no analogous `aarch64` Ubuntu package to `gcc-arm-none-eabi`: https://askubuntu.com/questions/1049249/is-there-a-package-with-the-aarch64-version-of-gcc-arm-none-eabi-for-bare-metal

=== C++ baremetal

TODO I tried by there was an error. Not yet properly reported. Should not be hard in theory since `libstdc++` is just part of GCC, as shown at: https://stackoverflow.com/questions/21872229/how-to-edit-and-re-build-the-gcc-libstdc-c-standard-library-source/51946224#51946224

=== GDB builtin CPU simulator

It is incredible, but GDB also has a CPU simulator inside of it as documented at: https://sourceware.org/gdb/onlinedocs/gdb/Target-Commands.html

TODO: any advantage over QEMU? I doubt it, mostly using it as as toy for now:

Without running `./run`, do directly:

....
./run-gdb --arch arm --baremetal baremetal/hello.c --sim
....

Then inside GDB:

....
load
starti
....

and now you can debug normally.

Enabled with the crosstool-NG configuration:

....
CT_GDB_CROSS_SIM=y
....

which by grepping crosstool-NG we can see does on GDB:

....
./configure --enable-sim
....

Those are not set by default on `gdb-multiarch` in Ubuntu 16.04.

Bibliography:

* https://stackoverflow.com/questions/49470659/arm-none-eabi-gdb-undefined-target-command-sim
* http://cs107e.github.io/guides/gdb/

==== GDB builtin CPU simulator userland

Since I had this compiled, I also decided to try it out on userland.

I was also able to run a freestanding Linux userland example on it: https://github.com/cirosantilli/arm-assembly-cheat/blob/cd232dcaf32c0ba6399b407e0b143d19b6ec15f4/v7/linux/hello.S

It just ignores the `swi` however, and does not forward syscalls to the host like QEMU does.

Then I tried a glibc example: https://github.com/cirosantilli/arm-assembly-cheat/blob/cd232dcaf32c0ba6399b407e0b143d19b6ec15f4/v7/mov.S

First it wouldn't break, so I added `-static` to the `Makefile`, and then it started failing with:

....
Unhandled v6 thumb insn
....

Doing:

....
help architecture
....

shows ARM version up to `armv6`, so maybe `armv6` is not implemented?

=== ARM baremetal

In this section we will focus on learning ARM architecture concepts that can only learnt on baremetal setups.

Userland information can be found at: https://github.com/cirosantilli/arm-assembly-cheat

==== ARM exception level

ARM exception levels are analogous to x86 <<ring0,rings>>.

Print the EL at the beginning of a baremetal simulation:

....
./run --arch arm --baremetal baremetal/arch/arm/el.c
./run --arch aarch64 --baremetal baremetal/arch/aarch64/el.c
....

Sources:

* link:baremetal/arch/arm/el.c[]
* link:baremetal/arch/aarch64/el.c[]

The instructions that find the ARM EL are explained at: https://stackoverflow.com/questions/31787617/what-is-the-current-execution-mode-exception-level-etc

The lower ELs are not mandated by the architecture, and can be controlled through command line options in QEMU and gem5.

In QEMU, you can configure the lowest EL as explained at https://stackoverflow.com/questions/42824706/qemu-system-aarch64-entering-el1-when-emulating-a53-power-up

....
./run --arch arm --baremetal baremetal/arch/arm/el.c
./run --arch arm --baremetal baremetal/arch/arm/el.c -- -machine virtualization=on
./run --arch arm --baremetal baremetal/arch/arm/el.c -- -machine secure=on
./run --arch aarch64 --baremetal baremetal/arch/aarch64/el.c
./run --arch aarch64 --baremetal baremetal/arch/aarch64/el.c -- -machine virtualization=on
./run --arch aarch64 --baremetal baremetal/arch/aarch64/el.c -- -machine secure=on
....

outputs respectively:

....
19
19
19
1
2
3
....

TODO: why is `arm` stuck at `19` which equals Supervisor mode?

In gem5, you can configure the lowest EL with:

....
./run --arch arm --baremetal baremeta/arch/arm/el.c --emulator gem5
cat "$(./getvar --arch arm --emulator gem5 gem5_guest_terminal_file)"
./run --arch arm --baremetal baremetal/arch/arm/el.c --emulator gem5 -- --param 'system.have_virtualization = True'
cat "$(./getvar --arch arm --emulator gem5 gem5_guest_terminal_file)"
./run --arch arm --baremetal baremetal/arch/arm/el.c --emulator gem5 -- --param 'system.have_security = True'
cat "$(./getvar --arch arm --emulator gem5 gem5_guest_terminal_file)"
./run --arch aarch64 --baremetal baremetal/arch/aarch64/el.c --emulator gem5
cat "$(./getvar --arch aarch64 --emulator gem5 gem5_guest_terminal_file)"
./run --arch aarch64 --baremetal baremetal/arch/aarch64/el.c --emulator gem5 -- --param 'system.have_virtualization = True'
cat "$(./getvar --arch aarch64 --emulator gem5 gem5_guest_terminal_file)"
./run --arch aarch64 --baremetal baremetal/arch/aarch64/el.c --emulator gem5 -- --param 'system.have_security = True'
cat "$(./getvar --arch aarch64 --emulator gem5 gem5_guest_terminal_file)"
....

output:

....
19
26
19
1
2
3
....

==== svc

This is the most basic example of exception handling we have.

We a handler for `svc`, do an `svc`, and observe that the handler got called and returned from C and assembly:

....
./run --arch aarch64 --baremetal baremetal/arch/aarch64/svc.c
./run --arch aarch64 --baremetal baremetal/arch/aarch64/svc_asm.S
....

Sources:

* link:baremetal/arch/aarch64/svc.c[]
* link:baremetal/arch/aarch64/svc_asm.S[]

Sample output for the C one:

....
daif 0x3c0
spsel 0x1
vbar_el1 0x40000800
lkmc_vector_trap_handler
exc_type 0x11
exc_type is LKMC_VECTOR_SYNC_SPX
ESR 0x56000042
SP 0x4200bba8
ELR 0x40002470
SPSR 0x600003c5
x0 0x0
x1 0x1
x2 0x14
x3 0x14
x4 0x40008390
x5 0xfffffff8
x6 0x4200ba28
x7 0x0
x8 0x0
x9 0x13
x10 0x0
x11 0x0
x12 0x0
x13 0x0
x14 0x0
x15 0x0
x16 0x0
x17 0x0
x18 0x0
x19 0x0
x20 0x0
x21 0x0
x22 0x0
x23 0x0
x24 0x0
x25 0x0
x26 0x0
x27 0x0
x28 0x0
x29 0x4200bba8
x30 0x4000246c
....

Both QEMU and gem5 are able to trace interrupts in addition to instructions, and it is instructive to enable both and have a look at the traces:

....
./run \
  --arch aarch64 \
  --baremetal baremetal/arch/aarch64/svc_asm.S
  -- -d in_asm,int \
;
....

contains:

....
----------------
IN:
0x40002060:  d4000001  svc      #0

Taking exception 2 [SVC]
...from EL1 to EL1
...with ESR 0x15/0x56000000
...with ELR 0x40002064
...to EL1 PC 0x40000a00 PSTATE 0x3c5
----------------
IN:
0x40000a00:  14000225  b        #0x40001294
....

and:

....
./run \
  --arch aarch64 \
  --baremetal baremetal/arch/aarch64/svc_asm.S \
  --trace ExecAll,Faults \
  --trace-stdout \
;
....

contains:

....
   4000: system.cpu A0 T0 : @main+8    :   svc   #0x0               : IntAlu :   flags=(IsSerializeAfter|IsNonSpeculative|IsSyscall)
   4000: Supervisor Call: Invoking Fault (AArch64 target EL):Supervisor Call cpsr:0x3c5 PC:0x80000808 elr:0x8000080c newVec: 0x80001200
   4500: system.cpu A0 T0 : @vector_table+512    :   b   <_curr_el_spx_sync>  : IntAlu :   flags=(IsControl|IsDirectControl|IsUncondControl)
....

So we see in both cases that the `svc` is done, then an exception happens, and then we just continue running from the exception handler address.

The vector table format is described on <<armarm8>> Table D1-7 "Vector offsets from vector table base address".

A good representation of the format of the vector table can also be found at <<armv8-programmers-guide>> Table 10-2 "Vector table offsets from vector table base address".

The first part of the table contains:

[options="header"]
|===
|Address |Exception type |Description

|VBAR_ELn + 0x000
|Synchronous
|Current EL with SP0

|VBAR_ELn + 0x080
|IRQ/vIRQ + 0x100
|Current EL with SP0

|VBAR_ELn + 0x100
|FIQ/vFIQ
|Current EL with SP0

|VBAR_ELn + 0x180
|SError/vSError
|Current EL with SP0

|===

and the following other parts are analogous, but referring to `SPx` and lower ELs.

We are going to do everything in <<arm-exception-level,EL1>> for now.

On the terminal output, we observe the initial values of:

* `DAIF`: `0x3c0`, i.e. 4 bits (6 to 9) set to 1, which means that exceptions are masked for each exception type: Synchronous, System error, IRQ and FIQ.
+
This reset value is defined by <<armarm8>> C5.2.2 "DAIF, Interrupt Mask Bits".
* `SPSel`: `0x1`, which means: use `SPx` instead of `SP0`.
+
This reset value is defined by <<armarm8>> C5.2.16 "SPSel, Stack Pointer Select".
* `VBAR_EL1`: `0x0` holds the base address of the vector table
+
This reset value is defined `UNKNOWN` by <<armarm8>> D10.2.116 "VBAR_EL1, Vector Base Address Register (EL1)", so we must set it to something ourselves to have greater portability.

Bibliography:

* https://github.com/torvalds/linux/blob/v4.20/arch/arm64/kernel/entry.S#L430 this is where the kernel defines the vector table
* https://github.com/dwelch67/qemu_arm_samples/tree/07162ba087111e0df3f44fd857d1b4e82458a56d/swi01
* https://github.com/NienfengYao/armv8-bare-metal/blob/572c6f95880e70aa92fe9fed4b8ad7697082a764/vector.S#L168
* https://stackoverflow.com/questions/51094092/how-to-make-timer-irq-work-on-qemu-machine-virt-cpu-cortex-a57
* https://stackoverflow.com/questions/44991264/armv8-exception-vectors-and-handling
* https://stackoverflow.com/questions/44198483/arm-timers-and-interrupts

==== ARM multicore

....
./run --arch aarch64 --baremetal baremetal/arch/aarch64/multicore.S --cpus 2
./run --arch aarch64 --baremetal baremetal/arch/aarch64/multicore.S --cpus 2 --emulator gem5
./run --arch arm --baremetal baremetal/arch/aarch64/multicore.S --cpus 2
./run --arch arm --baremetal baremetal/arch/aarch64/multicore.S --cpus 2 --emulator gem5
....

Sources:

* link:baremetal/arch/aarch64/multicore.S[]
* link:baremetal/arch/arm/multicore.S[]

CPU 0 of this program enters a spinlock loop: it repeatedly checks if a given memory address is `1`.

So, we need CPU 1 to come to the rescue and set that memory address to `1`, otherwise CPU 0 will be stuck there forever!

Don't believe me? Then try:

....
./run --arch aarch64 --baremetal baremetal/arch/aarch64/multicore.S --cpus 1
....

and watch it hang forever.

Note that if you try the same thing on gem5:

....
./run --arch aarch64 --baremetal baremetal/arch/aarch64/multicore.S --cpus 1 --emulator gem5
....

then the gem5 actually exits, but with a different message:

....
Exiting @ tick 18446744073709551615 because simulate() limit reached
....

as opposed to the expected:

....
Exiting @ tick 36500 because m5_exit instruction encountered
....

since gem5 is able to detect when nothing will ever happen, and exits.

When GDB step debugging, switch between cores with the usual `thread` commands, see also: <<gdb-step-debug-multicore-userland>>.

Bibliography: https://stackoverflow.com/questions/980999/what-does-multicore-assembly-language-look-like/33651438#33651438

===== WFE and SEV

The `WFE` and `SEV` instructions are just hints: a compliant implementation can treat them as NOPs.

However, likely no implementation likely does (TODO confirm), since:

* `WFE` puts the core in a low power mode
* `SEV` wakes up cores from a low power mode

and power consumption is key in ARM applications.

In QEMU 3.0.0, `SEV` is a NOPs, and `WFE` might be, but I'm not sure, see: https://github.com/qemu/qemu/blob/v3.0.0/target/arm/translate-a64.c#L1423

....
    case 2: /* WFE */
        if (!(tb_cflags(s->base.tb) & CF_PARALLEL)) {
            s->base.is_jmp = DISAS_WFE;
        }
        return;
    case 4: /* SEV */
    case 5: /* SEVL */
        /* we treat all as NOP at least for now */
        return;
....

TODO: what does the WFE code do? How can it not be a NOP if SEV is a NOP? https://github.com/qemu/qemu/blob/v3.0.0/target/arm/translate.c#L4609 might explain why, but it is Chinese to me (I only understand 30% ;-)):

....
 * For WFI we will halt the vCPU until an IRQ. For WFE and YIELD we
 * only call the helper when running single threaded TCG code to ensure
 * the next round-robin scheduled vCPU gets a crack. In MTTCG mode we
 * just skip this instruction. Currently the SEV/SEVL instructions
 * which are *one* of many ways to wake the CPU from WFE are not
 * implemented so we can't sleep like WFI does.
 */
....

For gem5 however, if we comment out the `SVE` instruction, then it actually exits with `simulate() limit reached`, so the CPU truly never wakes up, which is a more realistic behaviour.

The following Raspberry Pi bibliography helped us get this sample up and running:

* https://github.com/bztsrc/raspi3-tutorial/tree/a3f069b794aeebef633dbe1af3610784d55a0efa/02_multicorec
* https://github.com/dwelch67/raspberrypi/tree/a09771a1d5a0b53d8e7a461948dc226c5467aeec/multi00
* https://github.com/LdB-ECM/Raspberry-Pi/blob/3b628a2c113b3997ffdb408db03093b2953e4961/Multicore/SmartStart64.S
* https://github.com/LdB-ECM/Raspberry-Pi/blob/3b628a2c113b3997ffdb408db03093b2953e4961/Multicore/SmartStart32.S

===== PSCI

In QEMU, CPU 1 starts in a halted state. This can be observed from GDB, where:

....
info threads
....

shows something like:

....
* 1    Thread 1 (CPU#0 [running]) mystart
  2    Thread 2 (CPU#1 [halted ]) mystart
....

To wake up CPU 1 on QEMU, we must use the Power State Coordination Interface (PSCI) which is documented at: link:https://developer.arm.com/docs/den0022/latest/arm-power-state-coordination-interface-platform-design-document[].

This interface uses `HVC` calls, and the calling convention is documented at "SMC CALLING CONVENTION" link:https://developer.arm.com/docs/den0028/latest[].

If we boot the Linux kernel on QEMU and <<get-device-tree-from-a-running-kernel,dump the auto-generated device tree>>, we observe that it contains the address of the PSCI CPU_ON call:

....
        psci {
                method = "hvc";
                compatible = "arm,psci-0.2", "arm,psci";
                cpu_on = <0xc4000003>;
                migrate = <0xc4000005>;
                cpu_suspend = <0xc4000001>;
                cpu_off = <0x84000002>;
        };
....

The Linux kernel wakes up the secondary cores in this exact same way at: https://github.com/torvalds/linux/blob/v4.19/drivers/firmware/psci.c#L122 We first actually got it working here by grepping the kernel and step debugging that call :-)

In gem5, CPU 1 starts woken up from the start, so PSCI is not needed. TODO gem5 actually blows up if we try to do the `hvc` call, understand why.

Bibliography: https://stackoverflow.com/questions/20055754/arm-start-wakeup-bringup-the-other-cpu-cores-aps-and-pass-execution-start-addre/53473447#53473447

===== DMB

TODO: create and study a minimal examples in gem5 where the `DMB` instruction leads to less cycles: https://stackoverflow.com/questions/15491751/real-life-use-cases-of-barriers-dsb-dmb-isb-in-arm

==== ARM baremetal bibliography

First, also consider the userland bibliography: <<arm-assembly-bibliography>>.

The most useful ARM baremetal example sets we've seen so far are:

* https://github.com/dwelch67/raspberrypi real hardware
* https://github.com/dwelch67/qemu_arm_samples QEMU `-m vexpress`
* https://github.com/bztsrc/raspi3-tutorial real hardware + QEMU `-m raspi`
* https://github.com/LdB-ECM/Raspberry-Pi real hardware

===== armv8-bare-metal

https://github.com/NienfengYao/armv8-bare-metal

The only QEMU `-m virt` aarch64 example set that I can find on the web. Awesome.

A large part of the code is taken from the awesome educational OS under 2-clause BSD as can be seen from file headers: https://github.com/takeharukato/sample-tsk-sw/tree/ce7973aa5d46c9eedb58309de43df3b09d4f8d8d/hal/aarch64 but Nienfeng largely minimized it.

I needed the following minor patches: https://github.com/NienfengYao/armv8-bare-metal/pull/1

=== How we got some baremetal stuff to work

It is nice when thing just work.

But you can also learn a thing or two from how I actually made them work in the first place.

==== Find the UART address

Enter the QEMU console:

....
Ctrl-X C
....

Then do:

....
info mtree
....

And look for `pl011`:

....
    0000000009000000-0000000009000fff (prio 0, i/o): pl011
....

On gem5, it is easy to find it on the source. We are using the machine `RealView_PBX`, and a quick grep leads us to: https://github.com/gem5/gem5/blob/a27ce59a39ec8fa20a3c4e9fa53e9b3db1199e91/src/dev/arm/RealView.py#L615

....
class RealViewPBX(RealView):
    uart = Pl011(pio_addr=0x10009000, int_num=44)
....

==== aarch64 baremetal NEON setup

Inside link:baremetal/lib/aarch64.S[] there is a chunk of code called "NEON setup".

Without that, the `printf`:

....
printf("got: %c\n", c);
....

compiled to a:

....
str    q0, [sp, #80]
....

which uses NEON registers, and goes into an exception loop.

It was a bit confusing because there was a previous `printf`:

....
printf("enter a character\n");
....

which did not blow up because GCC compiles it into `puts` directly since it has no arguments, and that does not generate NEON instructions.

The last instructions ran was found with:

....
while(1)
stepi
end
....

or by hacking the QEMU CLI to contain:

.....
-D log.log -d in_asm
.....

I could not find any previous NEON instruction executed so this led me to suspect that some NEON initialization was required:

* http://infocenter.arm.com/help/topic/com.arm.doc.dai0527a/DAI0527A_baremetal_boot_code_for_ARMv8_A_processors.pdf "Bare-metal Boot Code for ARMv8-A Processors"
* https://community.arm.com/processors/f/discussions/5409/how-to-enable-neon-in-cortex-a8
* https://stackoverflow.com/questions/19231197/enable-neon-on-arm-cortex-a-series

We then tried to copy the code from the "Bare-metal Boot Code for ARMv8-A Processors" document:

....
// Disable trapping of accessing in EL3 and EL2.
MSR CPTR_EL3, XZR
MSR CPTR_EL3, XZR
// Disable access trapping in EL1 and EL0.
MOV X1, #(0x3 << 20) // FPEN disables trapping to EL1.
MSR CPACR_EL1, X1
ISB
....

but it entered an exception loop at `MSR CPTR_EL3, XZR`.

We then found out that QEMU starts in EL1, and so we kept just the EL1 part, and it worked. Related:

* https://stackoverflow.com/questions/42824706/qemu-system-aarch64-entering-el1-when-emulating-a53-power-up
* https://stackoverflow.com/questions/37299524/neon-support-in-armv8-system-mode-qemu

=== Baremetal tests

Automatically run all non-interactive baremetal tests:

....
./test-baremetal --arch aarch64
....

Source: link:test-baremetal[]

Analogously to <<user-mode-tests>>, we can select individual tests or directories with:

....
./test-baremetal --arch aarch64 baremetal/hello.c baremetal/arch/aarch64/no_bootloader/
....

which would run all of:

* link:baremetal/hello.c[]
* all tests under the directory: link:baremetal/arch/aarch64/no_bootloader/[]

We detect if tests failed by parsing logs for the <<magic-failure-string>>.

We also skip tests that cannot work on certain conditions based on their basenames, e.g.:

* tests that start with `gem5_` only run in `gem5`
* tests that start with `semihost_` only run in QEMU, until we find a better way to automate <<gem5-semihosting>>

See: <<test-this-repo>> for more useful testing tips.

=== Baremetal bibliography

https://stackoverflow.com/questions/43682311/uart-communication-in-gem5-with-arm-bare-metal

https://github.com/tukl-msd/gem5.bare-metal contains an alternative working baremetal setup. Our setup has more features at the time of writing however. Usage:

....
# Build gem5.
git clone https://gem5.googlesource.com/public/gem5
cd gem5
git checkout 60600f09c25255b3c8f72da7fb49100e2682093a
scons --ignore-style -j`nproc` build/ARM/gem5.opt
cd ..

# Build example.
sudo apt-get install gcc-arm-none-eabi
git clone https://github.com/tukl-msd/gem5.bare-metal
cd gem5.bare-metal
git checkout 6ad1069d4299b775b5491e9252739166bfac9bfe
cd Simple
make CROSS_COMPILE_DIR=/usr/bin

# Run example.
../../gem5/default/build/ARM/gem5.opt' \
  ../../gem5/configs/example/fs.py' \
  --bare-metal \
  --disk-image="$(pwd)/../common/fake.iso" \
  --kernel="$(pwd)/main.elf" \
  --machine-type=RealView_PBX \
  --mem-size=256MB \
;
....

== Android

Remember: Android AOSP is a huge undocumented piece of bloatware. It's integration into this repo will likely never be super good.

Verbose setup description: https://stackoverflow.com/questions/1809774/how-to-compile-the-android-aosp-kernel-and-test-it-with-the-android-emulator/48310014#48310014

Download, build and run with the prebuilt AOSP QEMU emulator and the AOSP kernel:

....
./build-android \
  --android-base-dir /path/to/your/hd \
  --android-version 8.1.0_r60 \
  download \
  build \
;
./run-android \
  --android-base-dir /path/to/your/hd \
  --android-version 8.1.0_r60 \
;
....

Sources:

* link:build-android[]
* link:run-android[]

TODO how to hack the AOSP kernel, userland and emulator?

Other archs work as well as usual with `--arch` parameter. However, running in non-x86 is very slow due to the lack of KVM.

Tested on: `8.1.0_r60`.

=== Android image structure

https://source.android.com/devices/bootloader/partitions-images

The messy AOSP generates a ton of images instead of just one.

When the emulator launches, we can see them through QEMU `-drive` arguments:

....
emulator: argv[21] = "-initrd"
emulator: argv[22] = "/data/aosp/8.1.0_r60/out/target/product/generic_x86_64/ramdisk.img"
emulator: argv[23] = "-drive"
emulator: argv[24] = "if=none,index=0,id=system,file=/path/to/aosp/8.1.0_r60/out/target/product/generic_x86_64/system-qemu.img,read-only"
emulator: argv[25] = "-device"
emulator: argv[26] = "virtio-blk-pci,drive=system,iothread=disk-iothread,modern-pio-notify"
emulator: argv[27] = "-drive"
emulator: argv[28] = "if=none,index=1,id=cache,file=/path/to/aosp/8.1.0_r60/out/target/product/generic_x86_64/cache.img.qcow2,overlap-check=none,cache=unsafe,l2-cache-size=1048576"
emulator: argv[29] = "-device"
emulator: argv[30] = "virtio-blk-pci,drive=cache,iothread=disk-iothread,modern-pio-notify"
emulator: argv[31] = "-drive"
emulator: argv[32] = "if=none,index=2,id=userdata,file=/path/to/aosp/8.1.0_r60/out/target/product/generic_x86_64/userdata-qemu.img.qcow2,overlap-check=none,cache=unsafe,l2-cache-size=1048576"
emulator: argv[33] = "-device"
emulator: argv[34] = "virtio-blk-pci,drive=userdata,iothread=disk-iothread,modern-pio-notify"
emulator: argv[35] = "-drive"
emulator: argv[36] = "if=none,index=3,id=encrypt,file=/path/to/aosp/8.1.0_r60/out/target/product/generic_x86_64/encryptionkey.img.qcow2,overlap-check=none,cache=unsafe,l2-cache-size=1048576"
emulator: argv[37] = "-device"
emulator: argv[38] = "virtio-blk-pci,drive=encrypt,iothread=disk-iothread,modern-pio-notify"
emulator: argv[39] = "-drive"
emulator: argv[40] = "if=none,index=4,id=vendor,file=/path/to/aosp/8.1.0_r60/out/target/product/generic_x86_64/vendor-qemu.img,read-only"
emulator: argv[41] = "-device"
emulator: argv[42] = "virtio-blk-pci,drive=vendor,iothread=disk-iothread,modern-pio-notify"
....

The root directory is the <<initrd>> given on the QEMU CLI, which `/proc/mounts` reports at:

....
rootfs on / type rootfs (ro,seclabel,size=886392k,nr_inodes=221598)
....

This contains the <<android-init>>, which through `.rc` must be mounting mounts the drives int o the right places TODO find exact point.

The drive order is:

....
system
cache
userdata
encryptionkey
vendor-qemu
....

Then, on the terminal:

....
mount | grep vd
....

gives:

....
/dev/block/vda1 on /system type ext4 (ro,seclabel,relatime,data=ordered)
/dev/block/vde1 on /vendor type ext4 (ro,seclabel,relatime,data=ordered)
/dev/block/vdb on /cache type ext4 (rw,seclabel,nosuid,nodev,noatime,errors=panic,data=ordered)
....

and we see that the order of `vda`, `vdb`, etc. matches that in which `-drive` were given to QEMU.

Tested on: `8.1.0_r60`.

==== Android images read-only

From `mount`, we can see that some of the mounted images are `ro`.

Basically, every image that was given to QEMU as qcow2 is writable, and that qcow2 is an overlay over the actual original image.

In order to make `/system` and `/vendor` writable by using qcow2 for them as well, we must use the `-writable-system` option:

....
./run-android -- -writable-system
....

* https://android.stackexchange.com/questions/110927/how-to-mount-system-rewritable-or-read-only-rw-ro/207200#207200
* https://stackoverflow.com/questions/13089694/adb-remount-permission-denied-but-able-to-access-super-user-in-shell-android/43163693#43163693

then:

....
su
mount -o rw,remount /system
date >/system/a
....

Now reboot, and relaunch with `-writable-system` once again to pick up the modified qcow2 images:

....
./run-android -- -writable-system
....

and the newly created file is still there:

....
date >/system/a
....

`/system` and `/vendor` can be nuked quickly with:

....
./build-android --extra-args snod
./build-android --extra-args vnod
....

as mentioned at: https://stackoverflow.com/questions/29023406/how-to-just-build-android-system-image and on:

....
./build-android --extra-args help
....

Tested on: `8.1.0_r60`.

==== Android /data partition

When I install an app like F-Droid, it goes under `/data` according to:

....
find / -iname '*fdroid*'
....

and it <<disk-persistency,persists across boots>>.

`/data` is behind a RW LVM device:

....
/dev/block/dm-0 on /data type ext4 (rw,seclabel,nosuid,nodev,noatime,errors=panic,data=ordered)
....

but TODO I can't find where it comes from since I don't have the CLI tools mentioned at:

* https://superuser.com/questions/131519/what-is-this-dm-0-device
* https://unix.stackexchange.com/questions/185057/where-does-lvm-store-its-configuration

However, by looking at:

....
./run-android -- -help
....

we see:

....
-data <file>                   data image (default <datadir>/userdata-qemu.img
....

which confirms the suspicion that this data goes in `userdata-qemu.img`.

To reset images to their original state, just remove the qcow2 overlay and regenerate it: https://stackoverflow.com/questions/54446680/how-to-reset-the-userdata-image-when-building-android-aosp-and-running-it-on-the

Tested on: `8.1.0_r60`.

=== Install Android apps

I don't know how to download files from the web on Vanilla android, the default browser does not download anything, and there is no `wget`:

* https://android.stackexchange.com/questions/6984/how-to-download-files-from-the-web-in-the-android-browser
* https://stackoverflow.com/questions/26775079/wget-in-android-terminal

Installing with `adb install` does however work: https://stackoverflow.com/questions/7076240/install-an-apk-file-from-command-prompt

link:https://f-droid.org[F-Droid] installed fine like that, however it does not have permission to install apps: https://www.maketecheasier.com/install-apps-from-unknown-sources-android/

And the `Settings` app crashes so I can't change it, logcat contains:

....
No service published for: wifip2p
....

which is mentioned at: https://stackoverflow.com/questions/47839955/android-8-settings-app-crashes-on-emulator-with-clean-aosp-build

We also tried to enable it from the command line with:

....
settings put secure install_non_market_apps 1
....

as mentioned at: https://android.stackexchange.com/questions/77280/allow-unknown-sources-from-terminal-without-going-to-settings-app but it didn't work either.

No person alive seems to know how to pre-install apps on AOSP: https://stackoverflow.com/questions/6249458/pre-installing-android-application

Tested on: `8.1.0_r60`.

=== Android init

For Linux in general, see: <<init>>.

The `/init` executable interprets the `/init.rc` files, which is in a custom Android init system language: https://android.googlesource.com/platform/system/core/+/ee0e63f71d90537bb0570e77aa8a699cc222cfaf/init/README.md

The top of that file then sources other `.rc` files present on the root directory:

....
import /init.environ.rc
import /init.usb.rc
import /init.${ro.hardware}.rc
import /vendor/etc/init/hw/init.${ro.hardware}.rc
import /init.usb.configfs.rc
import /init.${ro.zygote}.rc
....

TODO: how is `ro.hardware` determined? https://stackoverflow.com/questions/20572781/android-boot-where-is-the-init-hardware-rc-read-in-init-c-where-are-servic It is a system property and can be obtained with:

....
getprop ro.hardware
....

This gives:

....
ranchu
....

which is the codename for the QEMU virtual platform we are running on: https://www.oreilly.com/library/view/android-system-programming/9781787125360/9736a97c-cd09-40c3-b14d-955717648302.xhtml

TODO: is it possible to add a custom `.rc` file without modifying the initrd that <<android-image-structure,gets mounted on root>>? https://stackoverflow.com/questions/9768103/make-persistent-changes-to-init-rc

Tested on: `8.1.0_r60`.

== Benchmark this repo

TODO: didn't fully port during refactor after 3b0a343647bed577586989fb702b760bd280844a. Reimplementing should not be hard.

In this section document how benchmark builds and runs of this repo, and how to investigate what the bottleneck is.

Ideally, we should setup an automated build server that benchmarks those things continuously for us, but our <<travis>> attempt failed.

So currently, we are running benchmarks manually when it seems reasonable and uploading them to: https://github.com/cirosantilli/linux-kernel-module-cheat-regression

All benchmarks were run on the <<p51>> machine, unless stated otherwise.

Run all benchmarks and upload the results:

....
cd ..
git clone https://github.com/cirosantilli/linux-kernel-module-cheat-regression
cd -
./bench-all -A
....

=== Travis

We tried to automate it on Travis with link:.travis.yml[] but it hits the current 50 minute job timeout: https://travis-ci.org/cirosantilli/linux-kernel-module-cheat/builds/296454523 And I bet it would likely hit a disk maxout either way if it went on.

=== Benchmark this repo benchmarks

==== Benchmark Linux kernel boot

Run all kernel boot benchmarks for one arch:

....
./build-test-boot --size 3 && ./test-boot --size 3
cat "$(./getvar test_boot_benchmark_file)"
....

Sample results at 8fb9db39316d43a6dbd571e04dd46ae73915027f:

....
cmd ./run --arch x86_64 --eval './linux/poweroff.out'
time 8.25
exit_status 0

cmd ./run --arch x86_64 --eval './linux/poweroff.out' --kvm
time 1.22
exit_status 0

cmd ./run --arch x86_64 --eval './linux/poweroff.out' --trace exec_tb
time 8.83
exit_status 0
instructions 2244297

cmd ./run --arch x86_64 --eval 'm5 exit' --emulator gem5
time 213.39
exit_status 0
instructions 318486337

cmd ./run --arch arm --eval './linux/poweroff.out'
time 6.62
exit_status 0
cmd ./run --arch arm --eval './linux/poweroff.out' --trace exec_tb
time 6.90
exit_status 0
instructions 776374

cmd ./run --arch arm --eval 'm5 exit' --emulator gem5
time 118.46
exit_status 0
instructions 153023392

cmd ./run --arch arm --eval 'm5 exit' --emulator gem5 -- --cpu-type=HPI --caches --l2cache --l1d_size=1024kB --l1i_size=1024kB --l2_size=1024kB --l3_size=1024kB
time 2250.40
exit_status 0
instructions 151981914

cmd ./run --arch aarch64 --eval './linux/poweroff.out'
time 4.94
exit_status 0

cmd ./run --arch aarch64 --eval './linux/poweroff.out' --trace exec_tb
time 5.04
exit_status 0
instructions 233162

cmd ./run --arch aarch64 --eval 'm5 exit' --emulator gem5
time 70.89
exit_status 0
instructions 124346081

cmd ./run --arch aarch64 --eval 'm5 exit' --emulator gem5 -- --cpu-type=HPI --caches --l2cache --l1d_size=1024kB --l1i_size=1024kB --l2_size=1024kB --l3_size=1024kB
time 381.86
exit_status 0
instructions 124564620

cmd ./run --arch aarch64 --eval 'm5 exit' --emulator gem5 --gem5-build-type fast
time 58.00
exit_status 0
instructions 124346081

cmd ./run --arch aarch64 --eval 'm5 exit' --emulator gem5 --gem5-build-type debug
time 1022.03
exit_status 0
instructions 124346081
....

TODO: aarch64 gem5 and QEMU use the same kernel, so why is the gem5 instruction count so much much higher?

===== gem5 arm HPI boot takes much longer than aarch64

TODO 62f6870e4e0b384c4bd2d514116247e81b241251 takes 33 minutes to finish at 62f6870e4e0b384c4bd2d514116247e81b241251:

....
cmd ./run --arch arm --eval 'm5 exit' --emulator gem5 -- --caches --cpu-type=HPI
....

while aarch64 only 7 minutes.

I had previously documented on README 10 minutes at: 2eff007f7c3458be240c673c32bb33892a45d3a0 found with `git log` search for `10 minutes`. But then I checked out there, run it, and kernel panics before any messages come out. Lol?

Logs of the runs can be found at: https://github.com/cirosantilli-work/gem5-issues/tree/0df13e862b50ae20fcd10bae1a9a53e55d01caac/arm-hpi-slow

The cycle count is higher for `arm`, 350M vs 250M for `aarch64`, not nowhere near the 5x runtime time increase.

A quick look at the boot logs show that they are basically identical in structure: the same operations appear more ore less on both, and there isn't one specific huge time pit in arm: it is just that every individual operation seems to be taking a lot longer.

===== gem5 x86_64 DerivO3CPU boot panics

https://github.com/cirosantilli-work/gem5-issues/issues/2

....
Kernel panic - not syncing: Attempted to kill the idle task!
....

==== Benchmark builds

The build times are calculated after doing `./configure` and link:https://buildroot.org/downloads/manual/manual.html#_offline_builds[`make source`], which downloads the sources, and basically benchmarks the <<benchmark-internets,Internet>>.

Sample build time at 2c12b21b304178a81c9912817b782ead0286d282: 28 minutes, 15 with full ccache hits. Breakdown: 19% GCC, 13% Linux kernel, 7% uclibc, 6% host-python, 5% host-qemu, 5% host-gdb, 2% host-binutils

Buildroot automatically stores build timestamps as milliseconds since Epoch. Convert to minutes:

....
awk -F: 'NR==1{start=$1}; END{print ($1 - start)/(60000.0)}' "$(./getvar buildroot_build_build_dir)/build-time.log"
....

Or to conveniently do a clean build without affecting your current one:

....
./bench-all -b
cat ../linux-kernel-module-cheat-regression/*/build-time.log
....

===== Find which packages are making the build slow and big

....
./build-buildroot -- graph-build graph-size graph-depends
cd "$(./getvar buildroot_build_dir)/graphs"
xdg-open build.pie-packages.pdf
xdg-open graph-depends.pdf
xdg-open graph-size.pdf
....

[[prebuilt-toolchain]]
====== Buildroot use prebuilt host toolchain

The biggest build time hog is always GCC, and it does not look like we can use a precompiled one: https://stackoverflow.com/questions/10833672/buildroot-environment-with-host-toolchain

===== Benchmark Buildroot build baseline

This is the minimal build we could expect to get away with.

We will run this whenever the Buildroot submodule is updated.

On the upstream Buildroot repo at :

....
./bench-all -B
....

Sample time on 2017.08: 11 minutes, 7 with full ccache hits. Breakdown: 47% GCC, 15% Linux kernel, 9% uclibc, 5% host-binutils. Conclusions:

* we have bloated our kernel build 3x with all those delicious features :-)
* GCC time increased 1.5x by our bloat, but its percentage of the total was greatly reduced, due to new packages being introduced.
+
`make graph-depends` shows that most new dependencies come from QEMU and GDB, which we can't get rid of anyways.

A quick look at the system monitor reveals that the build switches between times when:

* CPUs are at a max, memory is fine. So we must be CPU / memory speed bound. I bet that this happens during heavy compilation.
* CPUs are not at a max, and memory is fine. So we are likely disk bound. I bet that this happens during configuration steps.

This is consistent with the fact that ccache reduces the build time only partially, since ccache should only overcome the CPU bound compilation steps, but not the disk bound ones.

The instructions counts varied very little between the baseline and LKMC, so runtime overhead is not a big deal apparently.

Size:

* `bzImage`: 4.4M
* `rootfs.cpio`: 1.6M

Zipped: 4.9M, `rootfs.cpio` deflates 50%, `bzImage` almost nothing.

===== Benchmark gem5 build

How long it takes to build gem5 itself.

We will update this whenever the gem5 submoule is updated.

Sample results at gem5 2a9573f5942b5416fb0570cf5cb6cdecba733392: 10 to 12 minutes.

Get results with:

....
./bench-all --emulator gem5
tail -n+1 ../linux-kernel-module-cheat-regression/*/gem5-bench-build-*.txt
....

====== Benchmark gem5 single file change rebuild time

This is the critical development parameter, and is dominated by the link time of huge binaries.

In order to benchmark it better, make a comment only change to:

....
vim submodules/gem5/src/sim/main.cc
....

then rebuild with:

....
./build-gem5 --arch aarch64 --verbose
....

and then copy the link command to a separate Bash file. Then you can time and modify it easily.

Some approximate reference values on <<p51>>:

* `opt`
** unmodified: 10 seconds
** hack with `-fuse-ld=gold`: 6 seconds. Huge improvement!
* `debug`
** unmodified: 14 seconds. Why two times slower than unmodified?
** hack with `-fuse-ld=gold`: `internal error in read_cie, at ../../gold/ehframe.cc:919` on Ubuntu 18.04 all GCC. TODO report.
* `fast`
** `--force-lto`: 1 minute. Slower as expected, since more optimizations are done at link time. `--force-lto` is only used for `fast`, and it adds `-flto` to the build.

ramfs made no difference, the kernel must be caching files in memory very efficiently already.

Tested at: d4b3e064adeeace3c3e7d106801f95c14637c12f + 1.

=== Benchmark machines

==== P51

Lenovo ThinkPad link:https://www3.lenovo.com/gb/en/laptops/thinkpad/p-series/P51/p/22TP2WPWP51[P51 laptop]:

* 2500 USD in 2018 (high end)
* Intel Core i7-7820HQ Processor (8MB Cache, up to 3.90GHz) (4 cores 8 threads)
* 32GB(16+16) DDR4 2400MHz SODIMM
* 512GB SSD PCIe TLC OPAL2
* NVIDIA Quadro M1200 Mobile, latest Ubuntu supported proprietary driver
* Latest Ubuntu

=== Benchmark Internets

==== 38Mbps internet

2c12b21b304178a81c9912817b782ead0286d282:

* shallow clone of all submodules: 4 minutes.
* `make source`: 2 minutes

Google M-lab speed test: 36.4Mbps

=== Benchmark this repo bibliography

gem5:

* link:https://www.mail-archive.com/gem5-users@gem5.org/msg15262.html[] which parts of the gem5 code make it slow
* what are the minimum system requirements:
** https://stackoverflow.com/questions/47997565/gem5-system-requirements-for-decent-performance/48941793#48941793
** https://github.com/gem5/gem5/issues/25

== About this repo

=== Supported hosts

We tend to test this repo the most on the latest Ubuntu and on the latest link:https://askubuntu.com/questions/16366/whats-the-difference-between-a-long-term-support-release-and-a-normal-release[Ubuntu LTS].

For other Linux distros, everything will likely also just work if you install the analogous required packages for your distro.

Find out the packages that we install with:

....
./build --download-dependencies --dry-run <some-target> | less
....

and then just look for the `apt-get` commands shown on the log.

After installing the missing packages for your distro, do the build with:

....
./build --download-dependencies --no-apt <some-target>
....

which does everything as normal, except that it skips any `apt` commands.

Ports to new host systems are welcome and will be merged.

If something does not work however, <<docker>> should just work on any Linux distro.

Native Windows is unlikely feasible for Buildroot setups becuase Buildroot is a huge set of GNU Make scripts + host tools, just do everything from inside an Ubuntu in VirtualBox instance in that case.

Some setups of this repository are however very portable, notably setups under <<userland-setup>>, e.g. <<c>>.

=== Common build issues

[[put-source-uris-in-sources]]
==== You must put some 'source' URIs in your sources.list

If `./build --download-dependencies` fails with:

....
E: You must put some 'source' URIs in your sources.list
....

see this: https://askubuntu.com/questions/496549/error-you-must-put-some-source-uris-in-your-sources-list/857433#857433 I don't know how to automate this step. Why, Ubuntu, why.

==== Build from downloaded source zip files

It does not work if you just download the `.zip` with the sources for this repository from GitHub because we use link:.gitmodules[Git submodules], you must clone this repo.

`./build --download-dependencies` then fetches only the required submodules for you.

=== Run command after boot

If you just want to run a command after boot ends without thinking much about it, just use the `--eval-after` option, e.g.:

....
./run --eval-after 'echo hello'
....

This option passes the command to our init scripts through <<kernel-command-line-parameters>>, and uses a few clever tricks along the way to make it just work.

See <<init>> for the gory details.

=== Default command line arguments

It gets annoying to retype `--arch aarch64` for every single command, or to remember `--config` setups.

So simplify that, do:

....
cp config.py data/
....

and then edit the `data/config` file to your needs.

Source: link:config.py[]

You can also choose a different configuration file explicitly with:

....
./run --config data/config2.py
....

Almost all options names are automatically deduced from their command line `--help` name: just replace `-` with `_`.

More precisely, we use the `dest=` value of Python's link:https://docs.python.org/3/library/argparse.html[argparse module].

To get a list of all global options that you can use, try:

....
./getvar --type input
....

but note that this does not include script specific options.

=== Build the documentation

You don't need to depend on GitHub:

....
sudo apt install asciidoctor
./build-doc
xdg-open out/README.html
....

Source: link:build-doc[]

=== Clean the build

You did something crazy, and nothing seems to work anymore?

All our build outputs are stored under `out/`, so the coarsest and most effective thing you can do is:

....
rm -rf out
....

This implies a full rebuild for all archs however, so you might first want to explore finer grained cleans first.

All our individual `build-*` scripts have a `--clean` option to completely nuke their builds:

....
./build-gem5 --clean
./build-qemu --clean
./build-buildroot --clean
....

Verify with:

....
ls "$(./getvar qemu_build_dir)"
ls "$(./getvar gem5_build_dir)"
ls "$(./getvar buildroot_build_dir)"
....

Note that host tools like QEMU and gem5 store all archs in a single directory to factor out build objects, so cleaning one arch will clean all of them.

To only nuke only one Buildroot package, we can use the link:https://buildroot.org/downloads/manual/manual.html#pkg-build-steps[]`-dirclean`] Buildroot target:

....
./build-buildroot --no-all -- <package-name>-dirclean
....

e.g.:

....
./build-buildroot --no-all -- sample_package-dirclean
....

Verify with:

....
ls "$(./getvar buildroot_build_build_dir)"
....

=== ccache

link:https://en.wikipedia.org/wiki/Ccache[ccache] <<benchmark-builds,might>> save you a lot of re-build when you decide to <<clean-the-build>> or create a new <<build-variants,build variant>>.

We have ccache enabled for everything we build by default.

However, you likely want to add the following to your `.bashrc` to take better advantage of `ccache`:

....
export CCACHE_DIR=~/.ccache
export CCACHE_MAXSIZE="20G"
....

We cannot automate this because you have to decide:

* should I store my cache on my HD or SSD?
* how big is my build, and how many build configurations do I need to keep around at a time?

If you don't those variables it, the default is to use `~/.buildroot-ccache` with `5G`, which is a bit small for us.

To check if `ccache` is working, run this command while a build is running on another shell:

....
watch -n1 'make -C "$(./getvar buildroot_build_dir)" ccache-stats'
....

or if you have it installed on host and the environment variables exported simply with:

....
watch -n1 'ccache -s'
....

and then watch the miss or hit counts go up.

We have link:https://buildroot.org/downloads/manual/manual.html#ccache[enabled ccached] builds by default.

`BR2_CCACHE_USE_BASEDIR=n` is used for Buildroot, which means that:

* absolute paths are used and GDB can find source files
* but builds are not reused across separated LKMC directories

=== Rebuild Buildroot while running

It is not possible to rebuild the root filesystem while running QEMU because QEMU holds the file qcow2 file:

....
error while converting qcow2: Failed to get "write" lock
....

=== Simultaneous runs

When doing long simulations sweeping across multiple system parameters, it becomes fundamental to do multiple simulations in parallel.

This is specially true for gem5, which runs much slower than QEMU, and cannot use multiple host cores to speed up the simulation: link:https://github.com/cirosantilli-work/gem5-issues/issues/15[], so the only way to parallelize is to run multiple instances in parallel.

This also has a good synergy with <<build-variants>>.

First shell:

....
./run
....

Another shell:

....
./run --run-id 1
....

and now you have two QEMU instances running in parallel.

The default run id is `0`.

Our scripts solve two difficulties with simultaneous runs:

* port conflicts, e.g. GDB and link:gem5-shell[]
* output directory conflicts, e.g. traces and gem5 stats overwriting one another

Each run gets a separate output directory. For example:

....
./run --arch aarch64 --emulator gem5 --run-id 0 &>/dev/null &
./run --arch aarch64 --emulator gem5 --run-id 1 &>/dev/null &
....

produces two separate <<m5out-directory,`m5out` directories>>:

....
echo "$(./getvar --arch aarch64 --emulator gem5 --run-id 0 m5out_dir)"
echo "$(./getvar --arch aarch64 --emulator gem5 --run-id 1 m5out_dir)"
....

and the gem5 host executable stdout and stderr can be found at:

....
less "$(./getvar --arch aarch64 --emulator gem5 --run-id 0 termout_file)"
less "$(./getvar --arch aarch64 --emulator gem5 --run-id 1 termout_file)"
....

Each line is prepended with the timestamp in seconds since the start of the program when it appeared.

To have more semantic output directories names for later inspection, you can use a non numeric string for the run ID, and indicate the port offset explicitly:

....
./run --arch aarch64 --emulator gem5 --run-id some-experiment --port-offset 1
....

`--port-offset` defaults to the run ID when that is a number.

Like <<cpu-architecture>>, you will need to pass the `-n` option to anything that needs to know runtime information, e.g. <<gdb>>:

....
./run --run-id 1
./run-gdb --run-id 1
....

To run multiple gem5 checkouts, see: <<gem5-worktree>>.

Implementation note: we create multiple namespaces for two things:

* run output directory
* ports
** QEMU allows setting all ports explicitly.
+
If a port is not free, it just crashes.
+
We assign a contiguous port range for each run ID.
** gem5 automatically increments ports until it finds a free one.
+
gem5 60600f09c25255b3c8f72da7fb49100e2682093a does not seem to expose a way to set the terminal and VNC ports from `fs.py`, so we just let gem5 assign the ports itself, and use `-n` only to match what it assigned. Those ports both appear on <<config-ini>>.
+
The GDB port can be assigned on `gem5.opt --remote-gdb-port`, but it does not appear on `config.ini`.

=== Build variants

It often happens that you are comparing two versions of the build, a good and a bad one, and trying to figure out why the bad one is bad.

Our build variants system allows you to keep multiple built versions of all major components, so that you can easily switching between running one or the other.

==== Linux kernel build variants

If you want to keep two builds around, one for the latest Linux version, and the other for Linux `v4.16`:

....
# Build master.
./build-linux

# Build another branch.
git -C "$(./getvar linux_source_dir)" fetch --tags --unshallow
git -C "$(./getvar linux_source_dir)" checkout v4.16
./build-linux --linux-build-id v4.16

# Restore master.
git -C "$(./getvar linux_source_dir)" checkout -

# Run master.
./run

# Run another branch.
./run --linux-build-id v4.16
....

The `git fetch --unshallow` is needed the first time because `./build --download-dependencies` only does a shallow clone of the Linux kernel to save space and time, see also: https://stackoverflow.com/questions/6802145/how-to-convert-a-git-shallow-clone-to-a-full-clone

The `--linux-build-id` option should be passed to all scripts that support it, much like `--arch` for the <<cpu-architecture>>, e.g. to step debug:

.....
./run-gdb --linux-build-id v4.16
.....

To run both kernels simultaneously, one on each QEMU instance, see: <<simultaneous-runs>>.

==== QEMU build variants

Analogous to the <<linux-kernel-build-variants>> but with the `--qemu-build-id` option instead:

....
./build-qemu
git -C "$(./getvar qemu_source_dir)" checkout v2.12.0
./build-qemu --qemu-build-id v2.12.0
git -C "$(./getvar qemu_source_dir)" checkout -
./run
./run --qemu-build-id v2.12.0
....

==== gem5 build variants

Analogous to the <<linux-kernel-build-variants>> but with the `--gem5-build-id` option instead:

....
# Build master.
./build-gem5

# Build another branch.
git -C "$(./getvar gem5_source_dir)" checkout some-branch
./build-gem5 --gem5-build-id some-branch

# Restore master.
git -C "$(./getvar gem5_source_dir)" checkout -

# Run master.
./run --emulator gem5

# Run another branch.
git -C "$(./getvar gem5_source_dir)" checkout some-branch
./run --gem5-build-id some-branch --emulator gem5
....

Don't forget however that gem5 has Python scripts in its source code tree, and that those must match the source code of a given build.

Therefore, you can't forget to checkout to the sources to that of the corresponding build before running, unless you explicitly tell gem5 to use a non-default source tree with <<gem5-worktree>>. This becomes inevitable when you want to launch multiple simultaneous runs at different checkouts.

===== gem5 worktree

<<gem5-build-variants,`--gem5-build-id`>> goes a long way, but if you want to seamlessly switch between two gem5 tress without checking out multiple times, then `--gem5-worktree` is for you.

....
# Build gem5 at the revision in the gem5 submodule.
./build-gem5

# Create a branch at the same revision as the gem5 submodule.
./build-gem5 --gem5-worktree my-new-feature
cd "$(./getvar --gem5-worktree my-new-feature)"
vim create-bugs
git add .
git commit -m 'Created a bug'
cd -
./build-gem5 --gem5-worktree my-new-feature

# Run the submodule.
./run --emulator gem5 --run-id 0 &>/dev/null &

# Run the branch the need to check out anything.
# With --gem5-worktree, we can do both runs at the same time!
./run --emulator gem5 --gem5-worktree my-new-feature --run-id 1 &>/dev/null &
....

`--gem5-worktree <worktree-id>` automatically creates:

* a link:https://git-scm.com/docs/git-worktree[Git worktree] of gem5 if one didn't exit yet for `<worktree-id>`
* a separate build directory, exactly like `--gem5-build-id my-new-feature` would

We promise that the scripts sill never touch that worktree again once it has been created: it is now up to you to manage the code manually.

`--gem5-worktree` is required if you want to do multiple simultaneous runs of different gem5 versions, because each gem5 build needs to use the matching Python scripts inside the source tree.

The difference between `--gem5-build-id` and `--gem5-worktree` is that `--gem5-build-id` specifies only the gem5 build output directory, while `--gem5-worktree` specifies the source input directory.

Each Git worktree needs a branch name, and we append the `wt/` prefix to the `--gem5-worktree` value, where `wt` stands for `WorkTree`. This is done to allow us to checkout to a test `some-branch` branch under `submodules/gem5` and still use `--gem5-worktree some-branch`, without conflict for the worktree branch, which can only be checked out once.

===== gem5 private source trees

Suppose that you are working on a private fork of gem5, but you want to use this repository to develop it as well.

Simply adding your private repository as a remote to `submodules/gem5` is dangerous, as you might forget and push your private work by mistake one day.

Even removing remotes is not safe enough, since `git submodule update` and other submodule commands can restore the old public remote.

Instead, we provide the following safer process.

First do a separate private clone of you private repository outside of this repository:

....
git clone https://my.private.repo.com/my-fork/gem5.git gem5-internal
gem5_internal="$(pwd)/gem5-internal"
....

Next, when you want to build with the private repository, use the `--gem5-build-dir` and  `--gem5-source-dir` argument to override our default gem5 source and build locations:

....
cd linux-kernel-module-cheat
./build-gem5 \
  --gem5-build-dir "${gem5_internal}/build" \
  --gem5-source-dir "$gem5_internal" \
;
./run-gem5 \
  --gem5-build-dir "${gem5_internal}/build" \
  --gem5-source-dir "$gem5_internal" \
;
....

With this setup, both your private gem5 source and build are safely kept outside of this public repository.

===== gem5 debug build

The `gem5.debug` executable has optimizations turned off unlike the default `gem5.opt`, and provides a much better <<debug-the-emulator,debug experience>>:

....
./build-gem5 --arch aarch64 --gem5-build-type debug
./run --arch aarch64 --debug-vm --emulator gem5 --gem5-build-type debug
....

The build outputs are automatically stored in a different directory from other build types such as `.opt` build, which prevents `.debug` files from overwriting `.opt` ones.

Therefore, `--gem5-build-id` is not required.

The price to pay for debuggability is high however: a Linux kernel boot was about 14 times slower than opt at 71e927e63bda6507d5a528f22c78d65099bdf36f between the commands:

....
./run --arch aarch64 --eval 'm5 exit' --emulator gem5 --linux-build-id v4.16
./run --arch aarch64 --eval 'm5 exit' --emulator gem5 --linux-build-id v4.16 --gem5-build-type debug
....

so you will likely only use this when it is unavoidable.

==== Buildroot build variants

Allows you to have multiple versions of the GCC toolchain or root filesystem.

Analogous to the <<linux-kernel-build-variants>> but with the `--build-id` option instead:

....
./build-buildroot
git -C "$(./getvar buildroot_source_dir)" checkout 2018.05
./build-buildroot --buildroot-build-id 2018.05
git -C "$(./getvar buildroot_source_dir)" checkout -
./run
./run --buildroot-build-id 2018.05
....

=== Directory structure

==== lkmc directory

link:lkmc/[] contains sources and headers that are shared across kernel modules, userland and baremetal examples.

We chose this awkward name so that our includes will have an `lkmc/` prefix.

Another option would have been to name it as `includes/lkmc`, but that would make paths longer, and we might want to store source code in that directory as well in the future.

===== Userland objects vs header-only

When factoring out functionality across userland examples, there are two main options:

* use header-only implementations
* use separate C files and link to separate objects.

The downsides of the header-only implementation are:

* slower compilation time, especially for C++
* cannot call C implementations from assembly files

The advantages of header-only implementations are:

* easier to use, just `#include` and you are done, no need to modify build metadata.

As a result, we are currently using the following rule:

* if something is only going to be used from C and not assembly, define it in a header which is easier to use
+
The slower compilation should be OK as long as split functionality amongst different headers and only include the required ones.
+
Also we don't have a choice in the case of C++ template, which must stay in headers.
* if the functionality will be called from assembly, then we don't have a choice, and must add it to a separate source file and link against it.

==== buildroot_packages directory

Source: link:buildroot_packages/[]

Every directory inside it is a Buildroot package.

Those packages get automatically added to Buildroot's `BR2_EXTERNAL`, so all you need to do is to turn them on during build, e.g.:

....
./build-buildroot --config 'BR2_PACKAGE_SAMPLE_PACKAGE=y'
....

then test it out with:

....
./run --eval-after './sample_package.out'
....

and you should see:

....
hello sample_package
....

Source: link:buildroot_packages/sample_package/sample_package.c[]

You can force a rebuild with:

....
./build-buildroot --config 'BR2_PACKAGE_SAMPLE_PACKAGE=y' -- sample_package-reconfigure
....

Buildroot packages are convenient, but in general, if a package if very important to you, but not really mergeable back to Buildroot, you might want to just use a custom build script for it, and point it to the Buildroot toolchain, and then use `BR2_ROOTFS_OVERLAY`, much like we do for <<userland-setup>>.

A custom build script can give you more flexibility: e.g. the package can be made work with other root filesystems more easily, have better <<9p>> support, and rebuild faster as it evades some Buildroot boilerplate.

===== kernel_modules package

Source: link:buildroot_packages/kernel_modules/[]

An example of how to use kernel modules in Buildroot.

Usage:

....
./build-buildroot \
  --build-linux \
  --config 'BR2_PACKAGE_KERNEL_MODULES=y' \
  --no-overlay \
  -- \
  kernel_modules-reconfigure \
;
....

Then test one of the modules with:

....
./run --buildroot-linux --eval-after 'modprobe buildroot_hello'
....

Source: link:buildroot_packages/kernel_modules/buildroot_hello.c[]

As you have just seen, this sets up everything so that <<modprobe>> can conrrectly find the module.

`./build-buildroot --build-linux` and `./run --buildroot-linux` are needed because the Buildroot kernel modules must use the Buildroot Linux kernel at build and run time.

The `--no-overlay` is required otherwise our `modules.order` generated by `./build-linux` and installed with `BR2_ROOTFS_OVERLAY` overwrites the Buildroot generated one.

Implementattion described at: https://stackoverflow.com/questions/40307328/how-to-add-a-linux-kernel-driver-module-as-a-buildroot-package/43874273#43874273

==== patches directory

===== patches/global

Has the following structure:

....
package-name/00001-do-something.patch
....

The patches are then applied to the corresponding packages before build.

Uses `BR2_GLOBAL_PATCH_DIR`.

===== patches/manual

Patches in this directory are never applied automatically: it is up to users to manually apply them before usage following the instructions in this documentation.

These are typically patches that don't contain fundamental functionality, so we don't feel like forking the target repos.

==== rootfs_overlay

We use this directory for:

* customized configuration files
* userland module test scripts that don't need to be compiled.
+
C files for example need compilation, and must go through the regular package system, e.g. through link:kernel_modules/user[].

This directory is copied into the target filesystem by:

....
./copy-overlay
./build-buildroot
....

Source: link:copy-overlay[]

Build Buildroot is required for the same reason as described at: <<your-first-kernel-module-hack>>.

However, since the link:rootfs_overlay[] directory does not require compilation, unlike say <<your-first-kernel-module-hack,kernel modules>>, we also make it <<9p>> available to the guest directly even without `./copy-overlay` at:

....
ls /mnt/9p/rootfs_overlay
....

This way you can just hack away the scripts and try them out immediately without any further operations.

==== lkmc.c

The files:

* link:lkmc.c[]
* link:lkmc.h[]

contain common C function helpers that can be used both in userland and baremetal. Oh, the infinite <<about-the-baremetal-setup,joys of Newlib>>.

Those files also contain arch specific helpers under ifdefs like:

....
#if defined(__aarch64__)
....

We try to keep as much as possible in those files. It bloats builds a little, but just makes everything simpler to understand.

==== rand_check.out

Print out several parameters that normally change randomly from boot to boot:

....
./run --eval-after './linux/rand_check.out;./linux/poweroff.out'
....

Source: link:userland/linux/rand_check.c[]

This can be used to check the determinism of:

* <<norandmaps>>
* <<qemu-record-and-replay>>

==== lkmc_home

`lkmc_home` refers to the target base directory in which we put all our custom built stuff, such as <<userland-setup,userland executables>> and <<your-first-kernel-module-hack,kernel modules>>.

The current value can be found with:

....
./getvar guest_lkmc_home
....

In the past, we used to dump everything into the root filesystem, but as the userland structure got more complex with subfolders, we decided that the risk of conflicting with important root files was becoming too great.

To save you from typing that path every time, we have made our most common commands `cd` into that directory by default for you, e.g.:

* interactive shells `cd` there through <<busybox-shell-initrc-files>>
* `--eval` and `--eval-after` through <<replace-init>> and <<init-busybox>>

Whenever a relative path is used inside a guest sample command, e.g. `insmod hello.ko` or `./hello.out`, it means that the path lives in `lkmc_home` unless stated otherwise.

=== Test this repo

==== Automated tests

Run almost all tests:

....
./build-test --size 3 && \
./test --size 3
echo $?
....

should output 0.

Sources:

* link:build-test[]
* link:test[]

The link:test[] script runs several different types of tests, which can also be run separately as explained at:

* link:test-boot[]
* <<test-userland-in-full-system>>
* <<user-mode-tests>>
* <<baremetal-tests>>
* <<test-gdb>>
* <<gem5-unit-tests>>

link:test[] does not all possible tests, because there are too many possible variations and that would take forever. The rationale is the same as for `./build all` and is explained in `./build --help`.

===== Test arch and emulator selection

You can select multiple archs and emulators of interest, as for an other command, with:

....
./test-user-mode \
  --arch x86_64 \
  --arch aarch64 \
  --emulator gem5 \
  --emulator qemu \
;
....

You can also test all supported archs and emulators with:

....
./test-user-mode \
  --all-archs \
  --all-emulators \
;
....

This command would run the test four times, using `x86_64` and `aarch64` with both gem5 and QEMU.

Without those flags, it defaults to just running the default arch and emulator once: `x86_64` and `qemu`.

===== Quit on fail

By default, tests stop running as soon as the first failure happens.

You can prevent this with the `--no-quit-on-fail option, e.g.:

....
./test-user-mode --no-quit-on-fail
....

You can then see which tests failed on the test summary report at the end.

===== Test userland in full system

TODO: we really need a mechanism to automatically generate the test list automatically e.g. based on <<path-properties>>, currently there are many tests missing, and we have to add everything manually which is very annoying.

We could just generate it on the fly on the host, and forward it to guest through CLI arguments.

Run all userland tests from inside full system simulation (i.e. not <<user-mode-simulation>>):

....
./test-userland-full-system
....

This includes, in particular, userland programs that test the kernel modules, which cannot be tested in user mode simulation.

Basically just boots and runs: link:rootfs_overlay/lkmc/test_all.sh[]

Failure is detected by looking for the <<magic-failure-string>>

Most userland programs that don't rely on kernel modules can also be tested in user mode simulation as explained at: <<user-mode-tests>>.

===== Test GDB

We have some link:https://github.com/pexpect/pexpect[pexpect] automated tests for the baremetal programs!

....
./build --all-archs test-gdb && \
./test-gdb --all-archs --all-emulators
....

Sources:

* link:build-test-gdb[]
* link:test-gdb[]

If a test fails, re-run the test commands manually and use `--verbose` to understand what happened:

....
./run --arch arm --background --baremetal baremetal/add.c --gdb-wait &
./run-gdb --arch arm --baremetal baremetal/add.c --verbose -- main
....

and possibly repeat the GDB steps manually with the usual:

....
./run-gdb --arch arm --baremetal baremetal/add.c --no-continue --verbose
....

To debug GDB problems on gem5, you might want to enable the following <<gem5-tracing,tracing>> options:

....
./run \
  --arch arm \
  --baremetal baremetal/add.c \
  --gdb-wait \
  --trace GDBRecv,GDBSend \
  --trace-stdout \
;
....

===== Magic failure string

We do not know of any way to set the emulator exit status in QEMU arm full system.

For other arch / emulator combinations, we know how to do it:

* aarch64: aarch64 semihosting supports exit status
* gem5: <<m5-fail>> works on all archs
* user mode: QEMU forwards exit status, gem5 we do some log parsing: <<gem5-syscall-emulation-exit-status>>

Since we can't do it for QEMU arm, the only reliable solution is to just parse the guest serial output for a magic failure string to check if tests failed.

Our run scripts parse the serial output looking for a line line containing only exactly the magic regular expression:

....
lkmc_exit_status_(\d+)
....

and then exit with the given regular expression, e.g.:

....
./run --arch aarch64 baremetal/return2.c
echo $?
....

should output:

....
2
....

This magic output string is notably generated by:

* link:rootfs_overlay/lkmc/test_fail.sh[], which is used by <<test-userland-in-full-system>>
* the `exit()` baremetal function when `status != 1`.
+
Unfortunately the only way we found to set this up was with `on_exit`: link:https://github.com/cirosantilli/linux-kernel-module-cheat/issues/59[].
+
Trying to patch `_exit` directly fails since at that point some de-initialization has already happened which prevents the print.
+
So setup this `on_exit` automatically from all our <<baremetal-bootloaders>>, so it just works automatically for the examples that use the bootloaders: https://stackoverflow.com/questions/44097610/pass-parameter-to-atexit/49659697#49659697
+
The following examples end up testing that our setup is working:
+
* link:baremetal/assert_fail.c[]
* link:baremetal/lkmc_assert_fail.c[]
* link:baremetal/return1.c[]
* link:baremetal/return2.c[]
* link:baremetal/exit0.c[]
* link:baremetal/exit1.c[]
* link:baremetal/arch/arm/return1.S[]
* link:baremetal/arch/aarch64/return1.S[]

Beware that on Linux kernel simulations, you cannot even echo that string from userland, since userland stdout shows up on the serial.

====== baremetal assert

TODO: implement enough syscalls for it, so we can get the error line:

....
cd baremetal
ln -s ../lkmc/assert_fail.c
cd ..
./build --arch aarch64
....

fails with:

....
/path/to/linux-kernel-module-cheat/out/crosstool-ng/build/default/install/aarch64/lib/gcc/aarch64-unknown-elf/8.1.0/../../../../aarch64-unknown-elf/lib/libg.a(lib_a-signalr.o): In function `_kill_r':
/path/to/linux-kernel-module-cheat/out/crosstool-ng/build/default/build/aarch64-unknown-elf/src/newlib/newlib/libc/reent/signalr.c:53: undefined reference to `_kill'
/path/to/linux-kernel-module-cheat/out/crosstool-ng/build/default/build/aarch64-unknown-elf/src/newlib/newlib/libc/reent/signalr.c:53:(.text+0x20): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `_kill'
/path/to/linux-kernel-module-cheat/out/crosstool-ng/build/default/install/aarch64/lib/gcc/aarch64-unknown-elf/8.1.0/../../../../aarch64-unknown-elf/lib/libg.a(lib_a-signalr.o): In function `_getpid_r':
/path/to/linux-kernel-module-cheat/out/crosstool-ng/build/default/build/aarch64-unknown-elf/src/newlib/newlib/libc/reent/signalr.c:83: undefined reference to `_getpid'
/path/to/linux-kernel-module-cheat/out/crosstool-ng/build/default/build/aarch64-unknown-elf/src/newlib/newlib/libc/reent/signalr.c:83:(.text+0x44): relocation truncated to fit: R_AARCH64_JUMP26 against undefined symbol `_getpid'
....

at 406ee82cf33a6e3df0067b219b0414c59d7018b3 + 1.

==== Non-automated tests

===== Test GDB Linux kernel

For the Linux kernel, do the following manual tests for now.

Shell 1:

....
./run --gdb-wait
....

Shell 2:

....
./run-gdb start_kernel
....

Should break GDB at `start_kernel`.

Then proceed to do the following tests:

* `./count.sh` and `break __x64_sys_write`
* `insmod timer.ko` and `break lkmc_timer_callback`

===== Test the Internet

You should also test that the Internet works:

....
./run --arch x86_64 --kernel-cli '- lkmc_eval="ifup -a;wget -S google.com;poweroff;"'
....

===== CLI script tests

`build-userland` and `test-user-mode` have a wide variety of target selection modes, and it was hard to keep them all working without some tests:

* link:test-build-userland[]
* link:test-test-user-mode[]

=== Bisection

When updating the Linux kernel, QEMU and gem5, things sometimes break.

However, for many types of crashes, it is trivial to bisect down to the offending commit, in particular because we can make QEMU and gem5 exit with status 1 on kernel panic: <<exit-emulator-on-panic>>.

For example, when updating from QEMU `v2.12.0` to `v3.0.0-rc3`, the Linux kernel boot started to panic for `arm`.

We then bisected it as explained at: https://stackoverflow.com/questions/4713088/how-to-use-git-bisect/22592593#22592593 with the link:qemu-bisect-boot[] script:

....
root_dir="$(pwd)"
cd "$(./getvar qemu_source_dir)"
git bisect start

# Check that our test script fails on v3.0.0-rc3 as expected, and mark it as bad.
"${root_dir}/qemu-bisect-boot"
# Should output 1.
echo #?
git bisect bad

# Same for the good end.
git checkout v2.12.0
"${root_dir}/qemu-bisect-boot"
# Should output 0.
echo #?
git bisect good

# This leaves us at the offending commit.
git bisect run ../bisect-qemu-linux-boot

# Clean up after the bisection.
git bisect reset
git submodule update
"${root_dir}/build-qemu" --clean --qemu-build-id bisect
....

TODO broken, fix: An example of Linux kernel commit bisection on gem5 boots can be found at: link:bisect-linux-boot-gem5[].

[[path-properties]]
=== path_properties

In order to build and run each userland and <<baremetal-setup,baremetal>> example properly, we need per-file metadata such as compiler flags and required number of cores.

This data is stored is stored in link:path_properties.py[] at `path_properties_tuples`.

Maybe we should embed it magically into source files directories to make it easier to see? But one big Python dict was easier to implement so we started like this. And it allows factoring chunks out easily.

The format is as follows:

....
'path_component': (
    {'property': value},
    {
        'child_path_component':
        {
            {'child_property': },
            {}
        }
    }
)
....

and as a shortcut, paths that don't have any children can be written directly as:

.....
'path_component': {'property': value}
.....

Properties of parent directories apply to all children.

Lists coming from parent directories are extended instead of overwritten by children, this is especially useful for C compiler flags.

=== Update a forked submodule

This is a template update procedure for submodules for which we have some patches on on top of mainline.

This example is based on the Linux kernel, for which we used to have patches, but have since moved to mainline:

....
# Last point before out patches.
last_mainline_revision=v4.15
next_mainline_revision=v4.16
cd "$(./getvar linux_source_dir)"

# Create a branch before the rebase in case things go wrong.
git checkout -b "lkmc-${last_mainline_revision}"
git remote set-url origin git@github.com:cirosantilli/linux.git
git push
git checkout master

git remote add up git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
git fetch up
git rebase --onto "$next_mainline_revision" "$last_mainline_revision"

# And update the README to show off.
git commit -m "linux: update to ${next_mainline_revision}"
....

=== Release

==== Release procedure

Ensure that the <<automated-tests>> are passing on a clean build:

....
mv out out.bak
./build-test --size 3 && ./test --size 3
....

The clean build is necessary as it generates clean images since <<remove-buildroot-packages,it is not possible to remove Buildroot packages>>

Run all tests in <<non-automated-tests>> just QEMU x86_64 and QEMU aarch64.

TODO: not working currently, so skipped: Ensure that the <<benchmark-this-repo,benchmarks>> look fine:

....
./bench-all -A
....

Create a release candidate and upload it:

....
git tag -a -m '' v3.0-rc1
git push --follow-tags
./release-zip --all-archs
# export LKMC_GITHUB_TOKEN=<your-token>
./release-upload
....

Now let's do an out-of-box testing for the release candidate:

....
cd ..
git clone https://github.com/cirosantilli/linux-kernel-module-cheat linux-kernel-module-cheat-release
cd linux-kernel-module-cheat-release
....

Test <<prebuilt>>.

Clean up, and re-start from scratch:

....
cd ..
rm -rf linux-kernel-module-cheat-release
git clone https://github.com/cirosantilli/linux-kernel-module-cheat linux-kernel-module-cheat-release
cd linux-kernel-module-cheat-release
....

Go through all the other <<getting-started>> sections in order.

Once everything looks fine, publish the release with:

....
git tag -a  v3.0
# Describe the release int the tag message.
git push --follow-tags
./release-zip --all-archs
# export LKMC_GITHUB_TOKEN=<your-token>
./release-upload
....

==== release-zip

Create a zip containing all files required for <<prebuilt>>:

....
./build --all-archs release && ./release-zip --all-archs
....

Source: link:release-zip[]

This generates a zip file:

....
echo "$(./getvar release_zip_file)"
....

which you can then upload somewhere.

==== release-upload

After:

* running <<release-zip>>
* creating and pushing a tag to GitHub

you can upload the release to GitHub automatically with:

....
# export LKMC_GITHUB_TOKEN=<your-token>
./release-upload
....

Source: link:release-upload[]

The HEAD of the local repository must be on top of a tag that has been pushed for this to work.

Create `LKMC_GITHUB_TOKEN` under: https://github.com/settings/tokens/new and save it to your `.bashrc`.

The implementation of this script is described at:

* https://stackoverflow.com/questions/5207269/how-to-release-a-build-artifact-asset-on-github-with-a-script/52354732#52354732
* https://stackoverflow.com/questions/38153418/can-someone-give-a-python-requests-example-of-uploading-a-release-asset-in-githu/52354681#52354681

=== Design rationale

==== Design goals

This project was created to help me understand, modify and test low level system components by using system simulators.

System simulators are cool compared to real hardware because they are:

* free
* make experiments highly reproducible
* give full visibility to the system: you can inspect any byte in memory, or the state of any hardware register. The laws of physics sometimes get in the way when doing that for real hardware.

The current components we focus the most on are:

* <<linux-kernel>> and Linux kernel modules
* full systems emulators, currently <<qemu-buildroot-setup,qemu>> and <<gem5-buildroot-setup,gem5>>
* <<buildroot>>. We use and therefore document, a large part of its feature set.

The following components are not covered, but they would also benefit from this setup, and it shouldn't be hard to add them:

* C standard libraries
* compilers. Project idea: add a new instruction to x86, then hack up GCC to actually use it, and make a C program that generates it.

The design goals are to provide setups that are:

* highly automated: "just works"
* thoroughly documented: you know what "just works" means
* can be fully built from source: to give visibility and allow modifications
* can also use <<prebuilt, prebuilt binaries>> as much as possible: in case you are lazy or unable to build from source

We aim to make a documentation that contains a very high runnable example / theory bullshit ratio.

Having at least one example per section is ideal, and it should be the very first thing in the section if possible.

==== Setup trade-offs

The trade-offs between the different <<getting-started,setups>> are basically a balance between:

* speed ans size: how long and how much disk space do the build and run take?
* visibility: can you GDB step debug everything and read source code?
* modifiability: can you modify the source code and rebuild a modified version?
* portability: does it work on a Windows host? Could it ever?
* accuracy: how accurate does the simulation represent real hardware?
* compatibility: how likely is is that all the components will work well together: emulator, compiler, kernel, standard library, ...
* guest software availability: how wide is your choice of easily installed guest software packages? See also: <<linux-distro-choice>>

==== Resource tradeoff guidelines

Choosing which features go into our default builds means making tradeoffs, here are our guidelines:

* keep the root filesystem as tiny as possible to make <<prebuilt>> small: only add BusyBox to have a small interactive system.
+
It is easy to add new packages once you have the toolchain, and if you don't there are infinitely many packages to cover and we can't cover them all.
* enable every feature possible on the toolchain (GCC, Binutils), because changes imply Buildroot rebuilds
* runtime is sacred. Faster systems are:
+
--
** easier to understand
** run faster, which is specially for <<gem5>> which is slow
--
+
Runtime basically just comes down to how we configure the Linux kernel, since in the root filesystem all that matters is `init=`, and that is easy to control.
+
One possibility we could play with is to build loadable modules instead of built-in modules to reduce runtime, but make it easier to get started with the modules.

In order to learn how to measure some of those aspects, see: <<benchmark-this-repo>>

==== Linux distro choice

We haven't found the ultimate distro yet, here is a summary table of trade-offs that we care about:

[options="header"]
|===
|Distro |Packages in single Git tree |Git tracked docs |Cross build without QEMU |Prebuilt downloads |Number of packages

|Buildroot 2018.05
|y
|y
|y
|n
|2k (1)

|Ubuntu 18.04
|n
|n
|n
|y
|50k (3)

|Yocto 2.5 (8)
|?
|y (5)
|?
|y (6)
|400 (7)

|Alpine Linux 3.8.0
|y
|n (1)
|?
|y
|2000 (4)

|===

* (1): Wiki... https://wiki.alpinelinux.org/wiki/Main_Page
* (2): `ls packages | wc`
* (3): https://askubuntu.com/questions/120630/how-many-packages-are-in-the-main-repository
* (4): `ls main community non-free | wc`
* (5): yes, but on a separate Git tree... https://git.yoctoproject.org/cgit/cgit.cgi/yocto-docs/
* (6): yes, but the initial Poky build / download still took 5 hours on <<38mbps-internet>>, and QEMU failed to boot at the end... https://bugzilla.yoctoproject.org/show_bug.cgi?id=12953
* (7): `ls recipes-* | wc`
* (8): Poky reference system: http://git.yoctoproject.org/cgit/cgit.cgi/poky

Other interesting possibilities that I haven't evaluated well:

* NixOS https://nixos.org/ Seems to support full build from source well. Not much cross compilation information however.
* Gentoo https://en.wikipedia.org/wiki/Gentoo_Linux Seems to support full build from source well.

=== Soft topics

==== Fairy tale

____
Once upon a time, there was a boy called Linus.

Linus made a super fun toy, and since he was not very humble, decided to call it Linux.

Linux was an awesome toy, but it had one big problem: it was very difficult to learn how to play with it!

As a result, only some weird kids who were very bored ended up playing with Linux, and everyone thought those kids were very cool, in their own weird way.

One day, a mysterious new kid called Ciro tried to play with Linux, and like many before him, got very frustrated, and gave up.

A few years later, Ciro had grown up a bit, and by chance came across a very cool toy made by the boy Petazzoni and his gang: it was called Buildroot.

Ciro noticed that if you used Buildroot together with Linux, and Linux suddenly became very fun to play with!

So Ciro decided to explain to as many kids as possible how to use Buildroot to play with Linux.

And so everyone was happy. Except some of the old weird kernel hackers who wanted to keep their mystique, but so be it.

THE END
____

==== Should you waste your life with systems programming?

Being the hardcore person who fully understands an important complex system such as a computer, it does have a nice ring to it doesn't it?

But before you dedicate your life to this nonsense, do consider the following points:

* almost all contributions to the kernel are done by large companies, and if you are not an employee in one of them, you are likely not going to be able to do much.
+
This can be inferred by the fact that the `devices/` directory is by far the largest in the kernel.
+
The kernel is of course just an interface to hardware, and the hardware developers start developing their kernel stuff even before specs are publicly released, both to help with hardware development and to have things working when the announcement is made.
+
Furthermore, I believe that there are in-tree devices which have never been properly publicly documented. Linus is of course fine with this, since code == documentation for him, but it is not as easy for mere mortals.
+
There are some less hardware bound higher level layers in the kernel which might not require being in a hardware company, and a few people must be living off it.
+
But of course, those are heavily motivated by the underlying hardware characteristics, and it is very likely that most of the people working there were previously at a hardware company.
+
In that sense, therefore, the kernel is not as open as one might want to believe.
* it is impossible to become rich with this knowledge.
+
This is partly implied by the fact that you need to be in a big company to make useful low level things, and therefore you will only be a tiny cog in the engine.
+
The key problem is that the entry cost of hardware design is just too insanely high for startups in general.
* Is learning this the most useful thing that you think can do for society?
+
Or are you just learning it for job security and having a nice sounding title?
+
I'm not a huge fan of the person, but I think Jobs said it right: https://www.youtube.com/watch?v=FF-tKLISfPE
+
First determine the useful goal, and then backtrack down to the most efficient thing you can do to reach it.
* there are two things that sadden me compared to physics-based engineering:
+
--
** you will never become eternally famous. All tech disappears sooner or later, while laws of nature, at least as useful approximations, stay unchanged.
** every problem that you face is caused by imperfections introduced by other humans.
+
It is much easier to accept limitations of physics, and even natural selection in biology, which is are produced by a sentient being (?).
--
+
Physics-based engineering, just like low level hardware, is of course completely closed source however, since wrestling against the laws of physics is about the most expensive thing humans can do.

Are you fine with those points, and ready to continue wasting your life?

Good. In that case, read on, and let's have some fun together ;-)

=== Bibliography

Runnable stuff:

* https://lwn.net/Kernel/LDD3/ the best book, but outdated. Updated source: https://github.com/martinezjavier/ldd3 But examples non-minimal and take too much brain power to understand.
* https://github.com/satoru-takeuchi/elkdat manual build process without Buildroot, very few and simple kernel modules. But it seem ktest + QEMU working, which is awesome. `./test` there patches ktest config dynamically based on CLI! Maybe we should just steal it since GPL licensed.
* https://github.com/tinyclub/linux-lab Buildroot based, no kernel modules?
* https://github.com/agelastic/eudyptula
* https://github.com/linux-kernel-labs Yocto based, source inside a kernel fork subdir: https://github.com/linux-kernel-labs/linux/tree/f08b9e4238dfc612a9d019e3705bd906930057fc/tools/labs which the author would like to upstream https://www.reddit.com/r/programming/comments/79w2q9/linux_device_driver_labs_the_linux_kernel/dp6of43/
* Android AOSP: https://stackoverflow.com/questions/1809774/how-to-compile-the-android-aosp-kernel-and-test-it-with-the-android-emulator/48310014#48310014 AOSP is basically a uber bloated Buildroot (2 hours build vs 30 minutes), Android is Linux based, and QEMU is the emulator backend. These instructions might work for debugging the kernel: https://github.com/Fuzion24/AndroidKernelExploitationPlayground
* https://github.com/s-matyukevich/raspberry-pi-os Does both an OS from scratch, and annotates the corresponding kernel source code. For RPI3, no QEMU support: https://github.com/s-matyukevich/raspberry-pi-os/issues/8
* https://github.com/pw4ever/linux-kernel-hacking-helper as of bd9952127e7eda643cbb6cb4c51ad7b5b224f438, Bash, Arch Linux rootfs
* https://github.com/MichielDerhaeg/build-linux untested. Manually builds musl and BusyBox, no Buildroot. Seems to use host packaged toolchain and tested on x86_64 only. Might contain a minimized kernel config.

Theory:

* http://nairobi-embedded.org you will fall here a lot when you start popping the hard QEMU Google queries. They have covered everything we do here basically, but with a more manual approach, while this repo automates everything.
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I couldn't find the markup source code for the tutorials, and as a result when the domain went down in May 2018, you have to use http://web.archive.org/ to see the pages...
* https://balau82.wordpress.com awesome low level resource
* https://rwmj.wordpress.com/ awesome red hatter
* https://lwn.net
* http://www.makelinux.net
* https://notes.shichao.io/lkd/

Awesome lists:

* https://github.com/gurugio/lowlevelprogramming-university
* https://github.com/uhub/awesome-c