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

["Roger Pau Monné" <roger.pau@citrix.com>]

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?

Thanks, Roger.