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

[Like Xu <like.xu.linux@gmail.com>]

On 7/9/2022 4:08 am, Jim Mattson wrote:
> On Tue, Sep 6, 2022 at 5:53 AM Like Xu <like.xu.linux@gmail.com> 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@gmail.com> wrote:
>>>>
>>>> From: Sandipan Das <sandipan.das@amd.com>
>>>>
>>>> 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@tencent.com>
>>>> Signed-off-by: Like Xu <likexu@tencent.com>
>>>> Signed-off-by: Sandipan Das <sandipan.das@amd.com>
>>>> ---
>>>>    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.

Whenever this happens, we may always leave the definition of behavior to the 
hypervisor.

> 
>>          /* 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.

K7 at least. I could not confirm that all antique AMD CPUs have 4 counters w/o 
perfctr_core.

> 
>>
>>                  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
>>>>