linux-kernel - Re: [PATCH 01/10] x86: Add a comment about the "magic" behind shadow sti before mwait

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Message-ID: <CAJZ5v0ga+x5w87KktirdVLw4yMT97AEgsZeoQBKZHMWovxZd3Q@mail.gmail.com>
Date:   Fri, 11 Aug 2023 19:44:13 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: [PATCH 01/10] x86: Add a comment about the "magic" behind shadow
 sti before mwait

On Fri, Aug 11, 2023 at 7:00 PM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> Add a note to make sure we never miss and break the requirements behind
> it.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Acked-by: Rafael J. Wysocki <rafael@...nel.org>

> ---
>  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 778df05f8539..341ee4f1d91e 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();
> --
> 2.34.1
>