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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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652 lines
40 KiB
HTML
652 lines
40 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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|
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</head>
|
||
<body id="arm-wfe-and-sev-instructions" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect4">
|
||
<h5 id="arm-wfe-and-sev-instructions"><a class="anchor" href="#arm-wfe-and-sev-instructions"></a><a class="link" href="#arm-wfe-and-sev-instructions">33.10.3.1. ARM WFE and SEV instructions</a></h5>
|
||
<div class="paragraph">
|
||
<p>The WFE and SEV instructions are just hints: a compliant implementation can treat them as NOPs.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Concrete examples of the instruction can be seen at:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/arch/aarch64/nostartfiles/wfe.S">userland/arch/aarch64/nostartfiles/wfe.S</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/arch/aarch64/freestanding/linux/wfe.S">userland/arch/aarch64/freestanding/linux/wfe.S</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/arch/aarch64/freestanding/linux/sevl_wfe.S">userland/arch/aarch64/freestanding/linux/sevl_wfe.S</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/arch/aarch64/freestanding/linux/wfe_wfe.S">userland/arch/aarch64/freestanding/linux/wfe_wfe.S</a>: run WFE twice, because gem5 390a74f59934b85d91489f8a563450d8321b602d does not sleep on the first, see also: <a href="gem5-arm-wfe.html">gem5 ARM WFE</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/baremetal/arch/aarch64/no_bootloader/wfe_loop.S">baremetal/arch/aarch64/no_bootloader/wfe_loop.S</a>, see: <a href="gem5-simulate-limit-reached.html">gem5 simulate() limit reached</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/arch/aarch64/inline_asm/wfe_sev.cpp">userland/arch/aarch64/inline_asm/wfe_sev.cpp</a>: one Linux thread runs WFE and the other runs SEV to wake it up</p>
|
||
</li>
|
||
<li>
|
||
<p><a href="arm-baremetal-multicore.html">ARM baremetal multicore</a> shows baremetal examples where WFE sleeps and another thread wakes it up:</p>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/baremetal/arch/arm/multicore.c">baremetal/arch/arm/multicore.c</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/baremetal/arch/aarch64/multicore.c">baremetal/arch/aarch64/multicore.c</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/baremetal/arch/arm/no_bootloader/multicore_asm.S">baremetal/arch/arm/no_bootloader/multicore_asm.S</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>However, likely no implementation likely does (TODO confirm), since:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p>WFE is intended to put the core in a low power mode</p>
|
||
</li>
|
||
<li>
|
||
<p>SEV wakes up cores from a low power mode</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>and power consumption is key in ARM applications.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Quotes for the above <a href="armarm8-db.html">ARMv8 architecture reference manual db</a> G1.18.1 "Wait For Event and Send Event":</p>
|
||
</div>
|
||
<div class="quoteblock">
|
||
<blockquote>
|
||
<div class="paragraph">
|
||
<p>The following events are WFE wake-up events:</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>\[…​]</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p>An event caused by the clearing of the global monitor associated with the PE</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
</blockquote>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>and <a href="armarm8-db.html">ARMv8 architecture reference manual db</a> E2.9.6 "Use of WFE and SEV instructions by spin-locks":</p>
|
||
</div>
|
||
<div class="quoteblock">
|
||
<blockquote>
|
||
<div class="paragraph">
|
||
<p>ARMv8 provides Wait For Event, Send Event, and Send Event Local instructions, WFE, SEV, SEVL, that can assist with reducing power consumption and bus contention caused by PEs repeatedly attempting to obtain a spin-lock. These instructions can be used at the application level, but a complete understanding of what they do depends on a system level understanding of exceptions. They are described in Wait For Event and Send Event on page G1-5308. However, in ARMv8, when the global monitor for a PE changes from Exclusive Access state to Open Access state, an event is generated.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Note This is equivalent to issuing an SEVL instruction on the PE for which the monitor state has changed. It removes the need for spinlock code to include an SEV instruction after clearing a spinlock.</p>
|
||
</div>
|
||
</blockquote>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The recommended ARMv8 spinlock implementation is shown at <a href="http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dht0008a/ch01s03s02.html" class="bare">http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dht0008a/ch01s03s02.html</a> where <code>WAIT_FOR_UPDATE</code> is as explained in that section a macro that expands to WFE. TODO SEV is used explicitly in those examples via SIGNAL_UPDATE, where is the example that shows how SEV can be eliminated due to implicit monitor signals?</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>In QEMU 3.0.0, SEV is a NOPs, and WFE might be, but I’m not sure, see: <a href="https://github.com/qemu/qemu/blob/v3.0.0/target/arm/translate-a64.c#L1423" class="bare">https://github.com/qemu/qemu/blob/v3.0.0/target/arm/translate-a64.c#L1423</a></p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre> 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;</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>TODO: what does the WFE code do? How can it not be a NOP if SEV is a NOP? <a href="https://github.com/qemu/qemu/blob/v3.0.0/target/arm/translate.c#L4609" class="bare">https://github.com/qemu/qemu/blob/v3.0.0/target/arm/translate.c#L4609</a> might explain why, but it is Chinese to me (I only understand 30% ;-)):</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre> * 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.
|
||
*/</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>For gem5 however, if we comment out the SVE instruction, then it actually exits with <code>simulate() limit reached</code>, so the CPU truly never wakes up, which is a more realistic behaviour, since gem5 is more focused on simulating a realistic microarchitecture and power consumption.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The following Raspberry Pi bibliography helped us get this sample up and running:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://github.com/bztsrc/raspi3-tutorial/tree/a3f069b794aeebef633dbe1af3610784d55a0efa/02_multicorec" class="bare">https://github.com/bztsrc/raspi3-tutorial/tree/a3f069b794aeebef633dbe1af3610784d55a0efa/02_multicorec</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/dwelch67/raspberrypi/tree/a09771a1d5a0b53d8e7a461948dc226c5467aeec/multi00" class="bare">https://github.com/dwelch67/raspberrypi/tree/a09771a1d5a0b53d8e7a461948dc226c5467aeec/multi00</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/LdB-ECM/Raspberry-Pi/blob/3b628a2c113b3997ffdb408db03093b2953e4961/Multicore/SmartStart64.S" class="bare">https://github.com/LdB-ECM/Raspberry-Pi/blob/3b628a2c113b3997ffdb408db03093b2953e4961/Multicore/SmartStart64.S</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/LdB-ECM/Raspberry-Pi/blob/3b628a2c113b3997ffdb408db03093b2953e4961/Multicore/SmartStart32.S" class="bare">https://github.com/LdB-ECM/Raspberry-Pi/blob/3b628a2c113b3997ffdb408db03093b2953e4961/Multicore/SmartStart32.S</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>For how userland spinlocks and mutexes are implemented see <a href="userland-mutex-implementation.html">Userland mutex implementation</a>.</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="arm-wfe-global-monitor-events.html">ARM WFE global monitor events</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="wfe-from-userland.html">WFE from userland</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="armv8-spinlock-pattern.html">ARMv8 spinlock pattern</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="gem5-arm-wfe.html">gem5 ARM WFE</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="arm-yield-instruction.html">ARM YIELD instruction</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>↑ Up: <a href="arm-baremetal-multicore.html">ARM baremetal multicore</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="arm-wfe-global-monitor-events.html">ARM WFE global monitor events</a> →</p>
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