[tip: x86/core] x86: Add a comment about the "magic" behind shadow sti before mwait

[tip-bot2 for Frederic Weisbecker]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     7d09a052a3bdb62de9a86d43359d6c22eeaf105a
Gitweb:        https://git.kernel.org/tip/7d09a052a3bdb62de9a86d43359d6c22eeaf105a
Author:        Frederic Weisbecker <frederic@xxxxxxxxxx>
AuthorDate:    Wed, 15 Nov 2023 10:13:22 -05:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 29 Nov 2023 15:44:01 +01:00

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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Rafael J. Wysocki <rafael@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20231115151325.6262-2-frederic@xxxxxxxxxx
---
 arch/x86/include/asm/mwait.h |  9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index 778df05..341ee4f 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();