linux-kernel - Re: [PATCH v4] acpi/processor: fix evaluating

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Message-ID: <CAJZ5v0i-RGRQbkVFu0RUyhPrsbiJ1DpAM0rKU-xM4U3Yddvufg@mail.gmail.com>
Date:   Mon, 20 Mar 2023 18:59:30 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Juergen Gross <jgross@...e.com>,
        Roger Pau Monne <roger.pau@...rix.com>
Cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        josef@...rland.se, 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>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
        Alex Chiang <achiang@...com>, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v4] acpi/processor: fix evaluating _PDC method when
 running as Xen dom0

On Fri, Mar 17, 2023 at 1:42 PM Juergen Gross <jgross@...e.com> wrote:
>
> On 16.03.23 17:42, Roger Pau Monne wrote:
> > In ACPI systems, the OS can direct power management, as opposed to the
> > firmware.  This OS-directed Power Management is called OSPM.  Part of
> > telling the firmware that the OS going to direct power management is
> > making ACPI "_PDC" (Processor Driver Capabilities) calls.  These _PDC
> > methods must be evaluated for every processor object.  If these _PDC
> > calls are not completed for every processor it can lead to
> > inconsistency and later failures in things like the CPU frequency
> > driver.
> >
> > In a Xen system, the dom0 kernel is responsible for system-wide power
> > management.  The dom0 kernel is in charge of OSPM.  However, the
> > number of CPUs available to dom0 can be different than the number of
> > CPUs physically present on the system.
> >
> > This leads to a problem: the dom0 kernel needs to evaluate _PDC for
> > all the processors, but it can't always see them.
> >
> > In dom0 kernels, ignore the existing ACPI method for determining if a
> > processor is physically present because it might not be accurate.
> > Instead, ask the hypervisor for this information.
> >
> > Fix this by introducing a custom function to use when running as Xen
> > dom0 in order to check whether a processor object matches a CPU that's
> > online.  Such checking is done using the existing information fetched
> > by the Xen pCPU subsystem, extending it to also store the ACPI ID.
> >
> > This ensures that _PDC method gets evaluated for all physically online
> > CPUs, regardless of the number of CPUs made available to dom0.
> >
> > Fixes: 5d554a7bb064 ('ACPI: processor: add internal processor_physically_present()')
> > Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
>
> Reviewed-by: Juergen Gross <jgross@...e.com>

Applied as 6.4 material under a slightly edited subject, thanks!