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

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Date:   Tue, 6 Sep 2022 20:53:21 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sandipan Das <sandipan.das@....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 6/9/2022 1:36 am, Jim Mattson wrote:
> On Mon, Sep 5, 2022 at 5:45 AM Like Xu <like.xu.linux@...il.com> wrote:
>>
>> From: Sandipan Das <sandipan.das@....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@...cent.com>
>> Signed-off-by: Like Xu <likexu@...cent.com>
>> Signed-off-by: Sandipan Das <sandipan.das@....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:

	/* 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);
		}

		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
>>