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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Tue, Mar 21, 2023 at 3:02 PM Roger Pau Monné <roger.pau@citrix.com> wrote:
>
> On Tue, Mar 21, 2023 at 02:47:46PM +0100, Rafael J. Wysocki wrote:
> > On Thu, Mar 16, 2023 at 5:43 PM Roger Pau Monne <roger.pau@citrix.com> 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>
> > > ---
> > > Changes since v3:
> > >  - Protect xen_processor_present() definition with CONFIG_ACPI.
> > >
> > > Changes since v2:
> > >  - Extend and use the existing pcpu functionality.
> > >
> > > Changes since v1:
> > >  - Reword commit message.
> > > ---
> > >  arch/x86/include/asm/xen/hypervisor.h | 10 ++++++++++
> > >  drivers/acpi/processor_pdc.c          | 11 +++++++++++
> > >  drivers/xen/pcpu.c                    | 21 +++++++++++++++++++++
> > >  3 files changed, 42 insertions(+)
> > >
> > > diff --git a/arch/x86/include/asm/xen/hypervisor.h b/arch/x86/include/asm/xen/hypervisor.h
> > > index 5fc35f889cd1..990a1609677e 100644
> > > --- a/arch/x86/include/asm/xen/hypervisor.h
> > > +++ b/arch/x86/include/asm/xen/hypervisor.h
> > > @@ -63,4 +63,14 @@ void __init xen_pvh_init(struct boot_params *boot_params);
> > >  void __init mem_map_via_hcall(struct boot_params *boot_params_p);
> > >  #endif
> > >
> > > +#if defined(CONFIG_XEN_DOM0) && defined(CONFIG_ACPI)
> > > +bool __init xen_processor_present(uint32_t acpi_id);
> > > +#else
> > > +static inline bool xen_processor_present(uint32_t acpi_id)
> > > +{
> > > +       BUG();
> > > +       return false;
> > > +}
> > > +#endif
> > > +
> > >  #endif /* _ASM_X86_XEN_HYPERVISOR_H */
> > > diff --git a/drivers/acpi/processor_pdc.c b/drivers/acpi/processor_pdc.c
> > > index 8c3f82c9fff3..18fb04523f93 100644
> > > --- a/drivers/acpi/processor_pdc.c
> > > +++ b/drivers/acpi/processor_pdc.c
> > > @@ -14,6 +14,8 @@
> > >  #include <linux/acpi.h>
> > >  #include <acpi/processor.h>
> > >
> > > +#include <xen/xen.h>
> >
> > This along with the definition above is evidently insufficient for
> > xen_processor_present() to always be defined.  See
> > https://lore.kernel.org/linux-acpi/64198b60.bO+m9o5w+Hd8hcF3%25lkp@intel.com/T/#u
> > for example.
> >
> > I'm dropping the patch now, please fix and resend.
>
> Hello,
>
> Sorry.  I've sent a followup fix:
>
> https://lore.kernel.org/xen-devel/20230321112522.46806-1-roger.pau@citrix.com/T/#u
>
> Would you be fine with taking such followup, or would rather prefer
> for me to send the original fixed patch as v5?

Please fold the fix into the original patch and resend.