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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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564 lines
35 KiB
HTML
564 lines
35 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>>. <<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>
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<body id="gem5-syscall-emulation-multithreading" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect3">
|
||
<h4 id="gem5-syscall-emulation-multithreading"><a class="anchor" href="#gem5-syscall-emulation-multithreading"></a><a class="link" href="#gem5-syscall-emulation-multithreading">11.7.4. gem5 syscall emulation multithreading</a></h4>
|
||
<div class="paragraph">
|
||
<p>gem5 user mode multithreading has been particularly flaky compared <a href="qemu-user-mode-multithreading.html">to QEMU’s</a>, but work is being put into improving it.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>In gem5 syscall simulation, the <code>fork</code> syscall checks if there is a free CPU, and if there is a free one, the new threads runs on that CPU.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Otherwise, the <code>fork</code> call, and therefore higher level interfaces to <code>fork</code> such as <code>pthread_create</code> also fail and return a failure return status in the guest.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>For example, if we use just one CPU for <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/posix/pthread_self.c">userland/posix/pthread_self.c</a> which spawns one thread besides <code>main</code>:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --cpus 1 --emulator gem5 --userland userland/posix/pthread_self.c --cli-args 1</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>fails with this error message coming from the guest stderr:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>pthread_create: Resource temporarily unavailable</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>It works however if we add on extra CPU:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --cpus 2 --emulator gem5 --userland userland/posix/pthread_self.c --cli-args 1</pre>
|
||
</div>
|
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</div>
|
||
<div class="paragraph">
|
||
<p>Once threads exit, their CPU is freed and becomes available for new <code>fork</code> calls: For example, the following run spawns a thread, joins it, and then spawns again, and 2 CPUs are enough:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --cpus 2 --emulator gem5 --userland userland/posix/pthread_self.c --cli-args '1 2'</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>because at each point in time, only up to two threads are running.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>gem5 syscall emulation does show the expected number of cores when queried, e.g.:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --cpus 1 --userland userland/cpp/thread_hardware_concurrency.cpp --emulator gem5
|
||
./run --cpus 2 --userland userland/cpp/thread_hardware_concurrency.cpp --emulator gem5</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>outputs <code>1</code> and <code>2</code> respectively.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This can also be clearly by running <code>sched_getcpu</code>:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run \
|
||
--arch aarch64 \
|
||
--cli-args 4 \
|
||
--cpus 8 \
|
||
--emulator gem5 \
|
||
--userland userland/linux/sched_getcpu.c \
|
||
;</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>which necessarily produces an output containing the CPU numbers from 1 to 4 and no higher:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>1
|
||
3
|
||
4
|
||
2</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>TODO why does the <code>2</code> come at the end here? Would be good to do a detailed assembly run analysis.</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>← Previous: <a href="gem5-syscall-emulation-mode-syscall-tracing.html">gem5 syscall emulation mode syscall tracing</a> | ↑ Up: <a href="gem5-syscall-emulation-mode.html">gem5 syscall emulation mode</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="gem5-syscall-emulation-multiple-executables.html">gem5 syscall emulation multiple executables</a> →</p>
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