Date:   Tue, 24 Jan 2023 19:47:40 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Like Xu <like.xu.linux@gmail.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 5/8] KVM: x86/cpuid: Add X86_FEATURE_AMD_PMU_V2 as a
 KVM-only leaf entry
Message-ID: <Y9A13G5b1tuoIRUq@google.com>
References: <20221111102645.82001-1-likexu@tencent.com>
 <20221111102645.82001-6-likexu@tencent.com>
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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
--