linux-kernel - Re: [PATCH v3 5/8] KVM: x86/cpuid: Add X86_FEATURE_AMD_PMU

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Message-ID: <4a7fb3b6-f9eb-6cde-7120-7d256d9d288e@gmail.com>
Date:   Mon, 6 Feb 2023 19:47:13 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 5/8] KVM: x86/cpuid: Add X86_FEATURE_AMD_PMU_V2 as a
 KVM-only leaf entry

On 25/1/2023 3:47 am, Sean Christopherson wrote:
> On Fri, Nov 11, 2022, Like Xu wrote:
>> From: Like Xu <likexu@...cent.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@...gle.com>
>> Signed-off-by: Like Xu <likexu@...cent.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