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Date:   Thu, 8 Jun 2023 17:14:06 +0800
From:   James Liu <james.liu@hpe.com>
To:     "Rafael J. Wysocki" <rafael@kernel.org>,
        linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     mlangsdo@redhat.com, james.liu@hpe.com, craig.lamparter@hpe.com
Subject: Re: [PATCH v1] ACPI: reboot: Increase the delay to avoid racing
 after writing to ACPI RESET_REG on AMD Milan platforms.
Message-ID: <ZIGbBF+GxHAlTqGk@ILEIUN5Z4B.asiapacific.hpqcorp.net>
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On Wed, Jun 07, 2023 at 01:19:42PM +0200, Rafael J. Wysocki wrote:
> On Wed, Jun 7, 2023 at 5:44 AM James Liu <james.liu@hpe.com> wrote:
> >
> > For AMD Milan platforms, the delay of 15ms is insufficient to avoid racing
> > of reboot mechanisms. That said, the AMD Milan processors don't reboot
> > in 15ms after invoking acpi_reset().
> >
> > The proposed 50ms delay can effectively work around this issue.
> > This extended delay aligns better with ACPI v6.4 (i.e., sec. 4.8.4.6),
> > which indicates that ideally OSPM should execute spin loops on the CPUs
> > in the system following a write to this register.
> >
> > Signed-off-by: James Liu <james.liu@hpe.com>
> 
> Why do you want to affect everyone (including guest kernels running in
> virtual machines AFAICS) in order to address a problem specific to one
> platform?

I hoped to address this issue for any platform requiring a longer delay to
complete ACPI reset in time for any (maybe silicon-level) reasons. Some AMD Milan
platforms were the cases that we've found so far.

Except that, since ACPI spec indicates there should be a spin loop (long enough)
after the write instruction to Reset Register, I thought it should be no harms to
the other systems which well consider this spin loop when they claim to support
ACPI reboot.

Btw, I am just curious, why is the virtual machine mentioned here?

is the 50ms delay in acpi_reboot() for a guest OS on VM so long that some
unexpected behavior might happen?

> Wouldn't it be better to quirk that platform and document the quirk properly?

Yeah, it could be. Actually we considered this, and we will consider it again.

> > ---
> >  drivers/acpi/reboot.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/acpi/reboot.c b/drivers/acpi/reboot.c
> > index b79b7c99c237..002f7c7814a1 100644
> > --- a/drivers/acpi/reboot.c
> > +++ b/drivers/acpi/reboot.c
> > @@ -78,5 +78,5 @@ void acpi_reboot(void)
> >          * The 15ms delay has been found to be long enough for the system
> >          * to reboot on the affected platforms.
> >          */
> > -       mdelay(15);
> > +       mdelay(50);
> >  }
> > --
> > 2.40.1
> >