Re: [PATCH v3 5/8] KVM: x86/cpuid: Add X86_FEATURE_AMD_PMU_V2 as a KVM-only leaf entry

Like Xu
2023-02-06T11:47:26Z

On 25/1/2023 3:47 am, Sean Christopherson wrote:
> On Fri, Nov 11, 2022, Like Xu wrote:
>> From: Like Xu <likexu@tencent.com>
>>
>> Alias X86_FEATURE_AMD_PMU_V2 for feature AMD_PMU_V2 in KVM-only leafs that
>> aren't scattered by cpufeatures.h so that it can be used in KVM, e.g. to
>> query guest CPUID. As a bonus, no translation is needed for these features
>> in __feature_translate().
>>
>> Suggested-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Like Xu <likexu@tencent.com>
>> ---
>> arch/x86/kvm/reverse_cpuid.h | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
>> index a19d473d0184..7cfedb3e47c0 100644
>> --- a/arch/x86/kvm/reverse_cpuid.h
>> +++ b/arch/x86/kvm/reverse_cpuid.h
>> @@ -13,6 +13,7 @@
>> */
>> enum kvm_only_cpuid_leafs {
>> CPUID_12_EAX = NCAPINTS,
>> + CPUID_8000_0022_EAX,
>> NR_KVM_CPU_CAPS,
>>
>> NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
>> @@ -23,7 +24,15 @@ enum kvm_only_cpuid_leafs {
>> /* Intel-defined SGX sub-features, CPUID level 0x12 (EAX). */
>> #define KVM_X86_FEATURE_SGX1 KVM_X86_FEATURE(CPUID_12_EAX, 0)
>> #define KVM_X86_FEATURE_SGX2 KVM_X86_FEATURE(CPUID_12_EAX, 1)
>> +#define KVM_X86_FEATURE_AMD_PMU_V2 KVM_X86_FEATURE(CPUID_8000_0022_EAX, 0)
>>
>> +/*
>> + * Alias X86_FEATURE_* to the KVM variant for features in KVM-only leafs that
>> + * aren't scattered by cpufeatures.h so that X86_FEATURE_* can be used in KVM,
>> + * e.g. to query guest CPUID. As a bonus, no translation is needed for these
>> + * features in __feature_translate().
>> + */
>> +#define X86_FEATURE_AMD_PMU_V2 KVM_X86_FEATURE_AMD_PMU_V2
>
> I gave you bad input earlier, for purely KVM-defined flags there's no need for an
> intermediate KVM_X86_FEATURE_AMD_PMU_V2, this could simply be:
>
> #define X86_FEATURE_AMD_PMU_V2 KVM_X86_FEATURE(CPUID_8000_0022_EAX, 0)
>
> That's a moot point though because, after much searching because I had a very hard
> time believing the kernel wouldn't want to know about this flag, I found commit
>
> d6d0c7f681fd ("x86/cpufeatures: Add PerfMonV2 feature bit")
>
> from nearly a year ago. I.e. to avoid confusiong, this needs to be a scattered
> flag, not a purely KVM flag.
>
> ---
> arch/x86/kvm/reverse_cpuid.h | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> index 4945456fd646..333e28b0a13c 100644
> --- a/arch/x86/kvm/reverse_cpuid.h
> +++ b/arch/x86/kvm/reverse_cpuid.h
> @@ -15,6 +15,7 @@ enum kvm_only_cpuid_leafs {
> CPUID_12_EAX = NCAPINTS,
> CPUID_7_1_EDX,
> CPUID_8000_0007_EDX,
> + CPUID_8000_0022_EAX,
> NR_KVM_CPU_CAPS,
>
> NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
> @@ -47,6 +48,9 @@ enum kvm_only_cpuid_leafs {
> /* CPUID level 0x80000007 (EDX). */
> #define KVM_X86_FEATURE_CONSTANT_TSC KVM_X86_FEATURE(CPUID_8000_0007_EDX, 8)
>
> +/* CPUID level 0x80000022 (EAX) */
> +#define KVM_X86_FEATURE_PERFMON_V2 KVM_X86_FEATURE(CPUID_8000_0022_EAX, 0)

I'm very confused and is this the usage you want to see:

kvm_cpu_cap_set(KVM_X86_FEATURE_PERFMON_V2)
kvm_cpu_cap_has(KVM_X86_FEATURE_PERFMON_V2)
guest_cpuid_has(vcpu, X86_FEATURE_PERFMON_V2)

? then what about X86_FEATURE_PERFMON_V2 ?

> +
> struct cpuid_reg {
> u32 function;
> u32 index;
> @@ -73,6 +77,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
> [CPUID_8000_001F_EAX] = {0x8000001f, 0, CPUID_EAX},
> [CPUID_7_1_EDX] = { 7, 1, CPUID_EDX},
> [CPUID_8000_0007_EDX] = {0x80000007, 0, CPUID_EDX},
> + [CPUID_8000_0022_EAX] = {0x80000022, 0, CPUID_EAX},
> };
>
> /*
> @@ -107,6 +112,8 @@ static __always_inline u32 __feature_translate(int x86_feature)
> return KVM_X86_FEATURE_SGX_EDECCSSA;
> else if (x86_feature == X86_FEATURE_CONSTANT_TSC)
> return KVM_X86_FEATURE_CONSTANT_TSC;
> + else if (x86_feature == X86_FEATURE_PERFMON_V2)
> + return KVM_X86_FEATURE_PERFMON_V2;
>
> return x86_feature;
> }
>
> base-commit: 5f3f3cc1279cd5cd52d301b97844bd3ce40c8020