Re: [PATCH v4] acpi/processor: fix evaluating _PDC method when running as Xen dom0

From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Juergen Gross <jgross@suse.com>, Roger Pau Monne <roger.pau@citrix.com>
Cc: linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
	josef@oderland.se, Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Len Brown <lenb@kernel.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>,
	Alex Chiang <achiang@hp.com>,
	linux-acpi@vger.kernel.org
Subject: Re: [PATCH v4] acpi/processor: fix evaluating _PDC method when running as Xen dom0
Date: Mon, 20 Mar 2023 18:59:30 +0100	[thread overview]
Message-ID: <CAJZ5v0i-RGRQbkVFu0RUyhPrsbiJ1DpAM0rKU-xM4U3Yddvufg@mail.gmail.com> (raw)
In-Reply-To: <17ffe417-8ea0-df98-a8b7-8c370839134f@suse.com>

On Fri, Mar 17, 2023 at 1:42 PM Juergen Gross <jgross@suse.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@citrix.com>
>
> Reviewed-by: Juergen Gross <jgross@suse.com>

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