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 <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
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 778df05f8539181fb5531d0a0ea368549f665967..341ee4f1d91e9a53ee1f618ab91103f1a391dc64 100644 (file)
--- 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();