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

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Message-ID: <Ya4mTij+eQPq8dnu@hirez.programming.kicks-ass.net>
Date:   Mon, 6 Dec 2021 16:03:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     rafael@...nel.org, ak@...ux.intel.com, bp@...en8.de,
        dan.j.williams@...el.com, dave.hansen@...ux.intel.com,
        hpa@...or.com, knsathya@...nel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, rjw@...ysocki.net,
        sathyanarayanan.kuppuswamy@...ux.intel.com, tglx@...utronix.de,
        tony.luck@...el.com, x86@...nel.org
Subject: Re: [PATCH 3/4] ACPI: processor idle: Only flush cache on entering C3

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,

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