From: Frederic Weisbecker Date: Wed, 15 Nov 2023 15:13:22 +0000 (-0500) Subject: x86: Add a comment about the "magic" behind shadow sti before mwait X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=7d09a052a3bdb62de9a86d43359d6c22eeaf105a;p=linux.git x86: Add a comment about the "magic" behind shadow sti before mwait Add a note to make sure we never miss and break the requirements behind it. Signed-off-by: Frederic Weisbecker Signed-off-by: Peter Zijlstra (Intel) Acked-by: Rafael J. Wysocki Link: https://lkml.kernel.org/r/20231115151325.6262-2-frederic@kernel.org --- diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h index 778df05f85391..341ee4f1d91e9 100644 --- a/arch/x86/include/asm/mwait.h +++ b/arch/x86/include/asm/mwait.h @@ -87,6 +87,15 @@ static __always_inline void __mwaitx(unsigned long eax, unsigned long ebx, :: "a" (eax), "b" (ebx), "c" (ecx)); } +/* + * Re-enable interrupts right upon calling mwait in such a way that + * no interrupt can fire _before_ the execution of mwait, ie: no + * instruction must be placed between "sti" and "mwait". + * + * This is necessary because if an interrupt queues a timer before + * executing mwait, it would otherwise go unnoticed and the next tick + * would not be reprogrammed accordingly before mwait ever wakes up. + */ static __always_inline void __sti_mwait(unsigned long eax, unsigned long ecx) { mds_idle_clear_cpu_buffers();