liberatehongkong
/
linux-kernel-module-cheat


			
				
					
						
						
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<meta name="description" content="The perfect emulation setup to study and develop the &lt;&lt;linux-kernel&gt;&gt; v5.9.2, kernel modules, &lt;&lt;qemu-buildroot-setup,QEMU&gt;&gt;, &lt;&lt;gem5-buildroot-setup,gem5&gt;&gt; and x86_64, ARMv7 and ARMv8 &lt;&lt;userland-assembly,userland&gt;&gt; and &lt;&lt;baremetal-setup,baremetal&gt;&gt; assembly, &lt;&lt;c,ANSI C&gt;&gt;, &lt;&lt;cpp,C++&gt;&gt; and &lt;&lt;posix,POSIX&gt;&gt;. &lt;&lt;gdb&gt;&gt; and &lt;&lt;kgdb&gt;&gt; just work. Powered by &lt;&lt;about-the-qemu-buildroot-setup,Buildroot&gt;&gt; and &lt;&lt;about-the-baremetal-setup,crosstool-NG&gt;&gt;. Highly automated. Thoroughly documented. Automated &lt;&lt;test-this-repo,tests&gt;&gt;. "Tested" in an Ubuntu 20.04 host.">
<title>Linux Kernel Module Cheat</title>
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</head>
<body id="gem5-ruby-build" class="article">
<div id="header">
<h1>Linux Kernel Module Cheat</h1>
</div>
<div id="content">
<div class="sect3">
<h4 id="gem5-ruby-build"><a class="anchor" href="#gem5-ruby-build"></a><a class="link" href="#gem5-ruby-build">24.16.6. gem5 Ruby build</a></h4>
<div class="paragraph">
<p>gem5 has two types of memory system:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>the classic memory system, which is used by default, its caches are covered at: <a href="gem5-event-queue-timingsimplecpu-syscall-emulation-freestanding-example-analysis-with-caches.html">gem5 event queue TimingSimpleCPU syscall emulation freestanding example analysis with caches</a></p>
</li>
<li>
<p>the Ruby memory system</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>The Ruby memory system includes the SLICC domain specific language to describe memory systems: <a href="http://gem5.org/Ruby" class="bare">http://gem5.org/Ruby</a> SLICC transpiles to C++ auto-generated files under <code>build/&lt;isa&gt;/mem/ruby/protocol/</code>.</p>
</div>
<div class="paragraph">
<p>Ruby seems to have usage outside of gem5, but the naming overload with the <a href="https://en.wikipedia.org/wiki/Ruby_(programming_language)">Ruby programming language</a>, which also has <a href="https://thoughtbot.com/blog/writing-a-domain-specific-language-in-ruby">domain specific languages</a> as a concept, makes it impossible to google anything about it!</p>
</div>
<div class="paragraph">
<p>Since it is not the default, Ruby is generally less stable that the classic memory model. However, because it allows describing a wide variety of important <a href="cache-coherence.html">cache coherence protocols</a>, while the classic system only describes a single protocol, Ruby is very importanonly describes a single protocol, Ruby is a very important feature of gem5.</p>
</div>
<div class="paragraph">
<p>Ruby support must be enabled at compile time with the <code>scons PROTOCOL=</code> flag, which compiles support for the desired memory system type.</p>
</div>
<div class="paragraph">
<p>Note however that most ISAs already implicitly set <code>PROTOCOL</code> via the <code>build_opts/</code> directory, e.g. <code>build_opts/ARM</code> contains:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>PROTOCOL = 'MOESI_CMP_directory'</pre>
</div>
</div>
<div class="paragraph">
<p>and therefore ARM already compiles <code>MOESI_CMP_directory</code> by default.</p>
</div>
<div class="paragraph">
<p>Then, with <code>fs.py</code> and <code>se.py</code>, you can choose to use either the classic or the ruby system type selected at build time with <code>PROTOCOL=</code> at runtime by passing the <code>--ruby</code> option:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>if <code>--ruby</code> is given, use the ruby memory system that was compiled into gem5. Caches are always present when Ruby is used, since the main goal of Ruby is to specify the cache coherence protocol, and it therefore hardcodes cache hierarchies.</p>
</li>
<li>
<p>otherwise, use the classic memory system. Caches may be optional for certain CPU types and are enabled with <code>--caches</code>.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Note that the <code>--ruby</code> option has some crazy side effects besides enabling Ruby, e.g. it <a href="https://github.com/gem5/gem5/blob/9fc9c67b4242c03f165951775be5cd0812f2a705/configs/ruby/Ruby.py#L61">sets the default <code>--cpu-type</code> to <code>TimingSimpleCPU</code> instead of the otherwise default <code>AtomicSimpleCPU</code></a>. TODO: I have been told that this is because <a href="gem5-functional-vs-atomic-vs-timing-memory-requests.html">sends the packet atomically,atomic requests do not work with Ruby, only timing</a>.</p>
</div>
<div class="paragraph">
<p>It is not possible to build more than one Ruby system into a single build, and this is a major pain point for testing Ruby: <a href="https://gem5.atlassian.net/browse/GEM5-467" class="bare">https://gem5.atlassian.net/browse/GEM5-467</a></p>
</div>
<div class="paragraph">
<p>For example, to use a two level <a href="mesi-cache-coherence-protocol.html">MESI cache coherence protocol</a> we can do:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>./build-gem5 --arch aarch64 --gem5-build-id ruby -- PROTOCOL=MESI_Two_Level
./run --arch aarch64 --emulator -gem5 --gem5-build-id ruby -- --ruby</pre>
</div>
</div>
<div class="paragraph">
<p>and during build we see a humongous line of type:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>[   SLICC] src/mem/protocol/MESI_Two_Level.slicc -&gt; ARM/mem/protocol/AccessPermission.cc, ARM/mem/protocol/AccessPermission.hh, ...</pre>
</div>
</div>
<div class="paragraph">
<p>which shows that dozens of C++ files are being generated from Ruby SLICC.</p>
</div>
<div class="paragraph">
<p>The relevant Ruby source files live in the source tree under:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>src/mem/protocol/MESI_Two_Level*</pre>
</div>
</div>
<div class="paragraph">
<p>We already pass the <code>SLICC_HTML</code> flag by default to the build, which generates an HTML summary of each memory protocol under (TODO broken: <a href="https://gem5.atlassian.net/browse/GEM5-357" class="bare">https://gem5.atlassian.net/browse/GEM5-357</a>):</p>
</div>
<div class="literalblock">
<div class="content">
<pre>xdg-open "$(./getvar --arch aarch64 --gem5-build-id ruby gem5_build_build_dir)/ARM/mem/protocol/html/index.html"</pre>
</div>
</div>
<div class="paragraph">
<p>A minimized ruby config which was not merged upstream can be found for study at: <a href="https://gem5-review.googlesource.com/c/public/gem5/+/13599/1" class="bare">https://gem5-review.googlesource.com/c/public/gem5/+/13599/1</a></p>
</div>
<div class="paragraph">
<p>One easy way to see that Ruby is being used without understanding it in detail is to <a href="gem5-tracing.html">enable some logging</a>:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>./run \
  --arch aarch64 \
  --emulator gem5 \
  --gem5-worktree master \
  --userland userland/arch/aarch64/freestanding/linux/hello.S \
  --static \
  --trace ExecAll,FmtFlag,Ruby,XBar \
  -- \
  --ruby \
;
cat "$(./getvar --arch aarch64 --emulator gem5 trace_txt_file)"</pre>
</div>
</div>
<div class="paragraph">
<p>Then:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>when the <code>--ruby</code> flag is given, we see a gazillion Ruby related messages prefixed e.g. by <code>RubyPort:</code>.</p>
<div class="paragraph">
<p>We also observe from <code>ExecEnable</code> lines that instruction timing is not simple anymore, so the memory system must have latencies</p>
</div>
</li>
<li>
<p>without <code>--ruby</code>, we instead see <code>XBar</code> (Coherent Crossbar) related messages such as <code>CoherentXBar:</code>, which I believe is the more precise name for the memory model that the classic memory system uses: <a href="gem5-crossbar-interconnect.html">gem5 crossbar interconnect</a>.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Certain features may not work in Ruby. For example, <a href="gem5-checkpoint.html">gem5 checkpoint</a> creation is only possible in Ruby protocols that support flush, which is the case for <code>PROTOCOL=MOESI_hammer</code> but not <code>PROTOCOL=MESI_Three_Level</code>: <a href="https://www.mail-archive.com/gem5-users@gem5.org/msg17418.html" class="bare">https://www.mail-archive.com/gem5-users@gem5.org/msg17418.html</a></p>
</div>
<div class="paragraph">
<p>Tested in gem5 d7d9bc240615625141cd6feddbadd392457e49eb.</p>
</div>
<div class="ulist">
<ul>
<li>
<p><a href="gem5-ruby-mi-example-protocol.html">gem5 Ruby MI_example protocol</a></p>
</li>
<li>
<p><a href="gem5-crossbar-interconnect.html">gem5 crossbar interconnect</a></p>
</li>
</ul>
</div>
</div>
<div class="paragraph nav-footer">
<p>← Previous: <a href="gem5-sanitation-build.html">gem5 sanitation build</a> | ↑ Up: <a href="gem5-build-options.html">gem5 build options</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="gem5-ruby-mi-example-protocol.html">gem5 Ruby MI_example protocol</a> →</p>
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