linux-kernel - Re: [PATCH 1/3] acpi/processor: fix evaluating

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Date:   Fri, 2 Dec 2022 18:06:26 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Roger Pau Monné <roger.pau@...rix.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        jgross@...e.com, Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Len Brown <lenb@...nel.org>,
        Alex Chiang <achiang@...com>,
        Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
        linux-acpi@...r.kernel.org
Subject: Re: [PATCH 1/3] acpi/processor: fix evaluating _PDC method when
 running as Xen dom0

On Fri, Dec 2, 2022 at 5:37 PM Roger Pau Monné <roger.pau@...rix.com> wrote:
>
> On Fri, Dec 02, 2022 at 08:17:56AM -0800, Dave Hansen wrote:
> > On 12/2/22 04:24, Roger Pau Monné wrote:
> > > On the implementation side, is the proposed approach acceptable?
> > > Mostly asking because it adds Xen conditionals to otherwise generic
> > > ACPI code.
> >
> > That's a good Rafael question.

Sorry for joining late, but first off _PDC has been deprecated since
ACPI 3.0 (2004) and it is not even present in ACPI 6.5 any more.

It follows from your description that _PDC is still used in the field,
though, after 18 years of deprecation.  Who uses it, if I may know?

> > But, how do other places in the ACPI code handle things like this?
>
> Hm, I don't know of other places in the Xen case, the only resource
> in ACPI AML tables managed by Xen are Processor objects/devices AFAIK.
> The rest of devices are fully managed by the dom0 guest.
>
> I think such special handling is very specific to Xen, but maybe I'm
> wrong and there are similar existing cases in ACPI code already.
>
> We could add some kind of hook (iow: a function pointer in some struct
> that could be filled on a implementation basis?) but I didn't want
> overengineering this if adding a conditional was deemed OK.

What _PDC capabilities specifically do you need to pass to the
firmware for things to work correctly?

What platforms are affected?