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

From: Juergen Gross <jgross@suse.com>
To: Roger Pau Monne <roger.pau@citrix.com>, linux-kernel@vger.kernel.org
Cc: 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: Fri, 17 Mar 2023 13:42:41 +0100	[thread overview]
Message-ID: <17ffe417-8ea0-df98-a8b7-8c370839134f@suse.com> (raw)
In-Reply-To: <20230316164257.42590-1-roger.pau@citrix.com>


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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>


Juergen


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