Re: [PATCH v4 12/12] KVM: x86/cpuid: Add AMD CPUID ExtPerfMonAndDbg leaf 0x80000022

Like Xu
2023-04-07T07:20:09Z

On 7/4/2023 9:50 am, Sean Christopherson wrote:
> On Tue, Feb 14, 2023, Like Xu wrote:
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index f4a4691b4f4e..2472fa8746c2 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -4916,6 +4916,12 @@ static __init void svm_set_cpu_caps(void)
>> } else {
>> /* AMD PMU PERFCTR_CORE CPUID */
>> kvm_cpu_cap_check_and_set(X86_FEATURE_PERFCTR_CORE);
>> + /*
>> + * KVM only supports AMD PerfMon V2, even if it supports V3+.
>
> Ha! A perfect example of why I strongly prefer that changelogs and comments avoid
> pronouns. The above "it" reads as:
>
> * KVM only supports AMD PerfMon V2, even if KVM supports V3+.
>
> which is clearly nonsensical.

I get your point. Thanks.

>
>
>> + * For PerfMon V3+, it's unsafe to expect V2 bit is set or cleared.
>
> If it's unsafe to assume anything v3+ implying v2 support, then it's definitely
> not safe to assume that KVM can blindly set v2 without future changes. I don't
> see any reason not to do
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index bd324962bb7e..1192f605ad47 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -756,6 +756,10 @@ void kvm_set_cpu_caps(void)
> F(NULL_SEL_CLR_BASE) | F(AUTOIBRS) | 0 /* PrefetchCtlMsr */
> );
>
> + kvm_cpu_cap_mask(CPUID_8000_0022_EAX,
> + F(PERFMON_V2)
> + );
> +
> /*
> * Synthesize "LFENCE is serializing" into the AMD-defined entry in
> * KVM's supported CPUID if the feature is reported as supported by the
>
>
> and then this code can be:
>
> if (kvm_pmu_cap.version != 2)
> kvm_cpu_cap_clear(X86_FEATURE_PERFMON_V2);
>
> Ah, but presumably the
>
> if (kvm_pmu_cap.num_counters_gp < AMD64_NUM_COUNTERS_CORE)
>
> path also needs to clear PERFMON_V2. I think I'd still vote to grab host CPUID
> and clear here (instead of setting).

Looks good to me.

>
> What is the relationship between PERFCTR_CORE and PERFMON_V2? E.g. if v2 depends
> on having PERFCTR_CORE, then we can do:

Yes, the PERFCTR_CORE bit will always be set if the v2 bit is set.

>
> if (enable_pmu) {
> if (kvm_pmu_cap.num_counters_gp < AMD64_NUM_COUNTERS_CORE)
> kvm_pmu_cap.num_counters_gp = AMD64_NUM_COUNTERS;
> else
> kvm_cpu_cap_check_and_set(X86_FEATURE_PERFCTR_CORE);
>
> if (kvm_pmu_cap.version != 2 ||
> !kvm_cpu_cap_has(X86_FEATURE_PERFCTR_CORE))
> kvm_cpu_cap_clear(X86_FEATURE_PERFMON_V2);