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

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Date:   Wed, 7 Sep 2022 11:22:54 +0530
From:   Sandipan Das <sandipan.das@....com>
To:     Jim Mattson <jmattson@...gle.com>,
        Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] KVM: x86/cpuid: Add AMD CPUID ExtPerfMonAndDbg leaf
 0x80000022

On 9/7/2022 9:41 AM, Jim Mattson wrote:
> On Tue, Sep 6, 2022 at 8:59 PM Like Xu <like.xu.linux@...il.com> wrote:
> [...]
>>>>>> 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.
> 
> I disagree. If CPUID.0H reports "AuthenticAMD," then AMD is the sole
> authority on behavior.
> 

I understand that official documentation is not out yet. However, for Zen 4
models, it is expected that both the PerfMonV2 bit of CPUID.80000022H EAX and
the PerfCtrExtCore bit of CPUID.80000001H ECX will be set.

>>>
>>>>          /* 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.
> 
> The APM says, "All implementations support the base set of four
> performance counter / event-select pairs." That is unequivocal.
> 

That is true. The same can be inferred from amd_core_pmu_init() in
arch/x86/events/amd/core.c. If PERFCTR_CORE is not detected, it assumes
that the four legacy counters are always available.

- Sandipan