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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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520 lines
34 KiB
HTML
520 lines
34 KiB
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<meta name="description" content="The perfect emulation setup to study and develop the <<linux-kernel>> v5.9.2, kernel modules, <<qemu-buildroot-setup,QEMU>>, <<gem5-buildroot-setup,gem5>> and x86_64, ARMv7 and ARMv8 <<userland-assembly,userland>> and <<baremetal-setup,baremetal>> assembly, <<c,ANSI C>>, <<cpp,C++>> and <<posix,POSIX>>. EVERYTHING is built from source. <<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 20.04 host.">
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<title>Linux Kernel Module Cheat</title>
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</head>
|
||
<body id="qemu-user-mode-multithreading" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect4">
|
||
<h5 id="qemu-user-mode-multithreading"><a class="anchor" href="#qemu-user-mode-multithreading"></a><a class="link" href="#qemu-user-mode-multithreading">24.3.1.1. QEMU user mode multithreading</a></h5>
|
||
<div class="paragraph">
|
||
<p><a href="user-mode-simulation.html">User mode simulation</a> QEMU v4.0.0 always shows the number of cores of the host, presumably because the thread switching uses host threads directly which would make that harder to implement.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>It does not seem possible to make the guest see a different number of cores than what the host has. Full system does have the <code>-smp</code> options, which controls this.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>E.g., all of of the following output the same as <code>nproc</code> on the host:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>nproc
|
||
./run --cpus 1 --userland userland/cpp/thread_hardware_concurrency.cpp
|
||
./run --cpus 2 --userland userland/cpp/thread_hardware_concurrency.cpp</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This random page suggests that QEMU splits one host thread thread per guest thread, and thus presumably delegates context switching to the host kernel: <a href="https://qemu.weilnetz.de/w64/2012/2012-12-04/qemu-tech.html#User-emulation-specific-details" class="bare">https://qemu.weilnetz.de/w64/2012/2012-12-04/qemu-tech.html#User-emulation-specific-details</a></p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>We can confirm that with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --userland userland/posix/pthread_count.c --cli-args 4
|
||
ps Haux | grep qemu | wc</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Remember <a href="qemu-user-mode-does-not-show-stdout-immediately.html">QEMU user mode does not show stdout immediately</a> though.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>At 369a47fc6e5c2f4a7f911c1c058b6088f8824463 + 1 QEMU appears to spawn 3 host threads plus one for every new guest thread created. Remember that <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/posix/pthread_count.c">userland/posix/pthread_count.c</a> spawns N + 1 total threads if you count the <code>main</code> thread.</p>
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<p>↑ Up: <a href="number-of-cores.html">Number of cores</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="gem5-arm-full-system-with-more-than-8-cores.html">gem5 ARM full system with more than 8 cores</a> →</p>
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