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linux-kernel-module-cheat/gem5-vs-qemu.html
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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;. EVERYTHING is built from source. &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-vs-qemu" class="article">
<div id="header">
<h1>Linux Kernel Module Cheat</h1>
</div>
<div id="content">
<div class="sect2">
<h3 id="gem5-vs-qemu"><a class="anchor" href="#gem5-vs-qemu"></a><a class="link" href="#gem5-vs-qemu">24.1. gem5 vs QEMU</a></h3>
<div class="ulist">
<ul>
<li>
<p>advantages of gem5:</p>
<div class="ulist">
<ul>
<li>
<p>simulates a generic more realistic <a href="gem5-cpu-types.html">optionally pipelined and out-of-order</a> CPU cycle by cycle, including a realistic DRAM memory access model with latencies, caches and page table manipulations. This allows us to:</p>
<div class="openblock">
<div class="content">
<div class="ulist">
<ul>
<li>
<p>do much more realistic performance benchmarking with it, which makes absolutely no sense in QEMU, which is purely functional</p>
</li>
<li>
<p>make certain functional observations that are not possible in QEMU, e.g.:</p>
<div class="ulist">
<ul>
<li>
<p>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.</p>
</li>
<li>
<p>spectre / meltdown:</p>
<div class="ulist">
<ul>
<li>
<p><a href="https://www.mail-archive.com/gem5-users@gem5.org/msg15319.html" class="bare">https://www.mail-archive.com/gem5-users@gem5.org/msg15319.html</a></p>
</li>
<li>
<p><a href="https://github.com/jlpresearch/gem5/tree/spectre-test" class="bare">https://github.com/jlpresearch/gem5/tree/spectre-test</a></p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="paragraph">
<p>It is not of course truly cycle accurate, as that:</p>
</div>
<div class="openblock">
<div class="content">
<div class="ulist">
<ul>
<li>
<p>would require exposing proprietary information of the CPU designs: <a href="https://stackoverflow.com/questions/17454955/can-you-check-performance-of-a-program-running-with-qemu-simulator/33580850#33580850" class="bare">https://stackoverflow.com/questions/17454955/can-you-check-performance-of-a-program-running-with-qemu-simulator/33580850#33580850</a></p>
</li>
<li>
<p>would make the simulation even slower TODO confirm, by how much</p>
</li>
</ul>
</div>
</div>
</div>
<div class="paragraph">
<p>but the approximation is reasonable.</p>
</div>
<div class="paragraph">
<p>It is used mostly for microarchitecture research purposes: when you are making a new chip technology, you don&#8217;t really need to specialize enormously to an existing microarchitecture, but rather develop something that will work with a wide range of future architectures.</p>
</div>
</li>
<li>
<p>runs are deterministic by default, unlike QEMU which has a special <a href="qemu-record-and-replay.html">QEMU record and replay</a> mode, that requires first playing the content once and then replaying</p>
</li>
<li>
<p>gem5 ARM at least appears to implement more low level CPU functionality than QEMU, e.g. QEMU only added EL2 in 2018: <a href="https://stackoverflow.com/questions/42824706/qemu-system-aarch64-entering-el1-when-emulating-a53-power-up" class="bare">https://stackoverflow.com/questions/42824706/qemu-system-aarch64-entering-el1-when-emulating-a53-power-up</a> See also: <a href="arm-exception-levels.html">Section 33.10.1, &#8220;ARM exception levels&#8221;</a></p>
</li>
<li>
<p>gem5 offers more advanced logging, even for non micro architectural things which QEMU models in some way, e.g. <a href="qemu-trace-memory-accesses.html">QEMU trace memory accesses</a>, because QEMU&#8217;s binary translation optimizations reduce visibility</p>
</li>
</ul>
</div>
</li>
<li>
<p>disadvantages of gem5:</p>
<div class="ulist">
<ul>
<li>
<p>slower than QEMU, see: <a href="benchmark-linux-kernel-boot.html">Section 35.2.1, &#8220;Benchmark Linux kernel boot&#8221;</a></p>
<div class="paragraph">
<p>This implies that the user base is much smaller, since no Android devs.</p>
</div>
<div class="paragraph">
<p>Instead, we have only chip makers, who keep everything that really works closed, and researchers, who can&#8217;t version track or document code properly &gt;:-) And this implies that:</p>
</div>
<div class="openblock">
<div class="content">
<div class="ulist">
<ul>
<li>
<p>the documentation is more scarce</p>
</li>
<li>
<p>it takes longer to support new hardware features</p>
</li>
</ul>
</div>
</div>
</div>
<div class="paragraph">
<p>Well, not that AOSP is that much better anyway.</p>
</div>
</li>
<li>
<p>not sure: gem5 has BSD license while QEMU has GPL</p>
<div class="paragraph">
<p>This suits chip makers that want to distribute forks with secret IP to their customers.</p>
</div>
<div class="paragraph">
<p>On the other hand, the chip makers tend to upstream less, and the project becomes more crappy in average :-)</p>
</div>
</li>
<li>
<p>gem5 is way more complex and harder to modify and maintain</p>
<div class="paragraph">
<p>The only hairy thing in QEMU is the binary code generation.</p>
</div>
<div class="paragraph">
<p>gem5 however has tended towards horrendous intensive <a href="gem5-code-generation.html">code generation</a> in order to support all its different hardware types</p>
</div>
<div class="paragraph">
<p>gem5 also has a complex Python interface which is also largely auto-generated, which greatly increases the maintenance complexity of the project: <a href="embedding-python-in-another-application.html">Embedding Python in another application</a>.</p>
</div>
<div class="paragraph">
<p>This is done so that reconfiguring platforms can be done quickly without recompiling, and it is amazing when it works, but the maintenance costs are also very high. For example, <a href="pybind11.html">pybind11</a> of several trivial <code>param_</code> files accounted for 50% of the build time at one point: <a href="pybind11-accounts-for-50-of-gem5-build-time.html">pybind11 accounts for 50% of gem5 build time</a>.</p>
</div>
<div class="paragraph">
<p>All of this also makes it hard to setup an IDE for developing gem5: <a href="gem5-eclipse-configuration.html">gem5 Eclipse configuration</a></p>
</div>
<div class="paragraph">
<p>The feelings of helplessness this brings are well summarized by the following CSDN article <a href="https://blog.csdn.net/maokelong95/article/details/85333905" class="bare">https://blog.csdn.net/maokelong95/article/details/85333905</a>:</p>
</div>
<div class="quoteblock">
<blockquote>
<div class="paragraph">
<p>Found DPRINTF based debugging unable to meet your needs?</p>
</div>
<div class="paragraph">
<p>Found GDB based debugging unfriendly to human beings?</p>
</div>
<div class="paragraph">
<p>Want to debug gem5 source with the help of modern IDEs like Eclipse?</p>
</div>
<div class="paragraph">
<p>Failed in getting help from GEM5 community?</p>
</div>
<div class="paragraph">
<p>Come on, dude! Here is the up-to-date tutorial for you!</p>
</div>
<div class="paragraph">
<p>Just be ready for THE ENDLESS NIGHTMARE gem5 will bring!</p>
</div>
</blockquote>
</div>
</li>
</ul>
</div>
</li>
</ul>
</div>
<div class="ulist">
<ul>
</ul>
</div>
</div>
<div class="paragraph nav-footer">
<p>↑ Up: <a href="gem5.html">gem5</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="gem5-run-benchmark.html">gem5 run benchmark</a> →</p>
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