linux-kernel - Re: [PATCH 3/4] ACPI: processor idle: Only flush cache on entering C3

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Date:   Wed, 8 Dec 2021 17:26:12 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH 3/4] ACPI: processor idle: Only flush cache on entering C3

On Mon, Dec 6, 2021 at 4:03 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Dec 06, 2021 at 03:29:51PM +0300, Kirill A. Shutemov wrote:
> > According to the ACPI spec v6.4, section 8.2, cache flushing required
> > on entering C3 power state.
> >
> > Avoid flushing cache on entering other power states.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  drivers/acpi/processor_idle.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> > index 76ef1bcc8848..01495aca850e 100644
> > --- a/drivers/acpi/processor_idle.c
> > +++ b/drivers/acpi/processor_idle.c
> > @@ -567,7 +567,8 @@ static int acpi_idle_play_dead(struct cpuidle_device *dev, int index)
> >  {
> >       struct acpi_processor_cx *cx = per_cpu(acpi_cstate[index], dev->cpu);
> >
> > -     ACPI_FLUSH_CPU_CACHE();
> > +     if (cx->type == ACPI_STATE_C3)
> > +             ACPI_FLUSH_CPU_CACHE();
> >
>
> acpi_idle_enter() already does this, acpi_idle_enter_s2idle() has it
> confused again,

No, they do the same thing: acpi_idle_enter_bm() if flags.bm_check is set.

> Also, I think acpi_idle_enter() does it too late; consider
> acpi_idle_enter_mb(). Either that or the BM crud needs more comments.

I think the latter.

Evidently, acpi_idle_play_dead(() doesn't support FFH and the BM
thing, so it is only necessary to flush the cache when using
ACPI_CSTATE_SYSTEMIO and when cx->type is C3.