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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Mon, 12 Jun 2023 18:57:01 +0200
Message-ID: <CAJZ5v0iR+CAOwip0zPV1uug-0duJXFS4YMxwNWDntUnK6a+oQQ@mail.gmail.com>
Subject: Re: [PATCH v1] ACPI: reboot: Increase the delay to avoid racing after
 writing to ACPI RESET_REG on AMD Milan platforms.
To:     James Liu <james.liu@hpe.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
        mlangsdo@redhat.com, craig.lamparter@hpe.com
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On Thu, Jun 8, 2023 at 11:14 AM James Liu <james.liu@hpe.com> wrote:
>
> 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.

Do you know about any other?

> 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?

The new delay would be over 3 times larger, so some users might be
surprised by it at least potentially.

> 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);
> > >  }
> > > --