Re: [PATCH v2] acpi/processor: fix evaluating _PDC method when running as Xen dom0

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

On Wed, Mar 15, 2023 at 01:38:18PM +0100, Juergen Gross wrote:
> On 15.03.23 13:10, 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.
> > 
> > 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 v1:
> >   - Reword commit message.
> > ---
> >   arch/x86/include/asm/xen/hypervisor.h | 10 ++++++++++
> >   arch/x86/xen/enlighten.c              | 27 +++++++++++++++++++++++++++
> >   drivers/acpi/processor_pdc.c          | 11 +++++++++++
> >   3 files changed, 48 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/xen/hypervisor.h b/arch/x86/include/asm/xen/hypervisor.h
> > index 5fc35f889cd1..f14e39bce2cb 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
> > +#ifdef CONFIG_XEN_DOM0
> > +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/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> > index b8db2148c07d..d4c44361a26c 100644
> > --- a/arch/x86/xen/enlighten.c
> > +++ b/arch/x86/xen/enlighten.c
> > @@ -346,3 +346,30 @@ void xen_arch_unregister_cpu(int num)
> >   }
> >   EXPORT_SYMBOL(xen_arch_unregister_cpu);
> >   #endif
> > +
> > +#ifdef CONFIG_XEN_DOM0
> > +bool __init xen_processor_present(uint32_t acpi_id)
> > +{
> > +	unsigned int i, maxid;
> > +	struct xen_platform_op op = {
> > +		.cmd = XENPF_get_cpuinfo,
> > +		.interface_version = XENPF_INTERFACE_VERSION,
> > +	};
> > +	int ret = HYPERVISOR_platform_op(&op);
> > +
> > +	if (ret)
> > +		return false;
> > +
> > +	maxid = op.u.pcpu_info.max_present;
> > +	for (i = 0; i <= maxid; i++) {
> > +		op.u.pcpu_info.xen_cpuid = i;
> > +		ret = HYPERVISOR_platform_op(&op);
> > +		if (ret)
> > +			continue;
> > +		if (op.u.pcpu_info.acpi_id == acpi_id)
> > +			return op.u.pcpu_info.flags & XEN_PCPU_FLAGS_ONLINE;
> > +	}
> > +
> > +	return false;
> > +}
> > +#endif
> 
> Did you consider not to do the hypercall again and again, but to reuse the
> pcpu list from drivers/xen/pcpu.c instead? You'd need to store the acpi_id
> in this list, of course.

Oh, didn't know this was available.  Seems to be initialized before
the _PDC evaluation, so I can give it a try.

Thanks, Roger.