Re: [PATCH 4/4] KVM: x86/cpuid: Add AMD CPUID ExtPerfMonAndDbg leaf 0x80000022

[Jim Mattson]

On Tue, Sep 6, 2022 at 5:53 AM Like Xu <like.xu.linux@xxxxxxxxx> wrote:
>
> On 6/9/2022 1:36 am, Jim Mattson wrote:
> > On Mon, Sep 5, 2022 at 5:45 AM Like Xu <like.xu.linux@xxxxxxxxx> wrote:
> >>
> >> From: Sandipan Das <sandipan.das@xxxxxxx>
> >>
> >> CPUID leaf 0x80000022 i.e. ExtPerfMonAndDbg advertises some
> >> new performance monitoring features for AMD processors.
> >>
> >> Bit 0 of EAX indicates support for Performance Monitoring
> >> Version 2 (PerfMonV2) features. If found to be set during
> >> PMU initialization, the EBX bits of the same CPUID function
> >> can be used to determine the number of available PMCs for
> >> different PMU types.
> >>
> >> Expose the relevant bits via KVM_GET_SUPPORTED_CPUID so
> >> that guests can make use of the PerfMonV2 features.
> >>
> >> Co-developed-by: Like Xu <likexu@xxxxxxxxxxx>
> >> Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> >> Signed-off-by: Sandipan Das <sandipan.das@xxxxxxx>
> >> ---
> >>   arch/x86/include/asm/perf_event.h |  8 ++++++++
> >>   arch/x86/kvm/cpuid.c              | 21 ++++++++++++++++++++-
> >>   2 files changed, 28 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
> >> index f6fc8dd51ef4..c848f504e467 100644
> >> --- a/arch/x86/include/asm/perf_event.h
> >> +++ b/arch/x86/include/asm/perf_event.h
> >> @@ -214,6 +214,14 @@ union cpuid_0x80000022_ebx {
> >>          unsigned int            full;
> >>   };
> >>
> >> +union cpuid_0x80000022_eax {
> >> +       struct {
> >> +               /* Performance Monitoring Version 2 Supported */
> >> +               unsigned int    perfmon_v2:1;
> >> +       } split;
> >> +       unsigned int            full;
> >> +};
> >> +
> >>   struct x86_pmu_capability {
> >>          int             version;
> >>          int             num_counters_gp;
> >> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> >> index 75dcf7a72605..08a29ab096d2 100644
> >> --- a/arch/x86/kvm/cpuid.c
> >> +++ b/arch/x86/kvm/cpuid.c
> >> @@ -1094,7 +1094,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
> >>                  entry->edx = 0;
> >>                  break;
> >>          case 0x80000000:
> >> -               entry->eax = min(entry->eax, 0x80000021);
> >> +               entry->eax = min(entry->eax, 0x80000022);
> >>                  /*
> >>                   * Serializing LFENCE is reported in a multitude of ways, and
> >>                   * NullSegClearsBase is not reported in CPUID on Zen2; help
> >> @@ -1203,6 +1203,25 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
> >>                  if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
> >>                          entry->eax |= BIT(6);
> >>                  break;
> >> +       /* AMD Extended Performance Monitoring and Debug */
> >> +       case 0x80000022: {
> >> +               union cpuid_0x80000022_eax eax;
> >> +               union cpuid_0x80000022_ebx ebx;
> >> +
> >> +               entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
> >> +               if (!enable_pmu)
> >> +                       break;
> >> +
> >> +               if (kvm_pmu_cap.version > 1) {
> >> +                       /* AMD PerfMon is only supported up to V2 in the KVM. */
> >> +                       eax.split.perfmon_v2 = 1;
> >> +                       ebx.split.num_core_pmc = min(kvm_pmu_cap.num_counters_gp,
> >> +                                                    KVM_AMD_PMC_MAX_GENERIC);
> >
> > Note that the number of core PMCs has to be at least 6 if
> > guest_cpuid_has(vcpu, X86_FEATURE_PERFCTR_CORE). I suppose this leaf
> > could claim fewer, but the first 6 PMCs must work, per the v1 PMU
> > spec. That is, software that knows about PERFCTR_CORE, but not about
> > PMU v2, can rightfully expect 6 PMCs.
>
> I thought the NumCorePmc number would only make sense if
> CPUID.80000022.eax.perfmon_v2
> bit was present, but considering that the user space is perfectly fine with just
> configuring the
> NumCorePmc number without setting perfmon_v2 bit at all, so how about:

CPUID.80000022H might only make sense if X86_FEATURE_PERFCTR_CORE is
present. It's hard to know in the absence of documentation.

>         /* AMD Extended Performance Monitoring and Debug */
>         case 0x80000022: {
>                 union cpuid_0x80000022_eax eax;
>                 union cpuid_0x80000022_ebx ebx;
>                 bool perfctr_core;
>
>                 entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
>                 if (!enable_pmu)
>                         break;
>
>                 perfctr_core = kvm_cpu_cap_has(X86_FEATURE_PERFCTR_CORE);
>                 if (!perfctr_core)
>                         ebx.split.num_core_pmc = AMD64_NUM_COUNTERS;
>                 if (kvm_pmu_cap.version > 1) {
>                         /* AMD PerfMon is only supported up to V2 in the KVM. */
>                         eax.split.perfmon_v2 = 1;
>                         ebx.split.num_core_pmc = min(kvm_pmu_cap.num_counters_gp,
>                                                      KVM_AMD_PMC_MAX_GENERIC);
>                 }
>                 if (perfctr_core) {
>                         ebx.split.num_core_pmc = max(ebx.split.num_core_pmc,
>                                                      AMD64_NUM_COUNTERS_CORE);
>                 }

This still isn't quite right. All AMD CPUs must support a minimum of 4 PMCs.

>
>                 entry->eax = eax.full;
>                 entry->ebx = ebx.full;
>                 break;
>         }
>
> ?
>
> Once 0x80000022 appears, ebx.split.num_core_pmc will report only
> the real "Number of Core Performance Counters" regardless of perfmon_v2.
>
> >
> >
> >> +               }
> >> +               entry->eax = eax.full;
> >> +               entry->ebx = ebx.full;
> >> +               break;
> >> +       }
> >>          /*Add support for Centaur's CPUID instruction*/
> >>          case 0xC0000000:
> >>                  /*Just support up to 0xC0000004 now*/
> >> --
> >> 2.37.3
> >>