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

Sean Christopherson
2023-01-24T19:47:54Z

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)
+
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
--