Re: [PATCH 4/6] x86: KVM: Enable AVX-VNNI-INT8 CPUID and expose it to guest

Jiaxi Chen
2022-10-26T03:34:28Z

On 10/19/2022 11:09 PM, Sean Christopherson wrote:
> On Wed, Oct 19, 2022, Sean Christopherson wrote:
>> On Wed, Oct 19, 2022, Borislav Petkov wrote:
>>> On Wed, Oct 19, 2022 at 04:47:32PM +0800, Jiaxi Chen wrote:
>>>> AVX-VNNI-INT8 is a new set of instructions in the latest Intel platform
>>>> Sierra Forest. It multiplies the individual bytes of two unsigned or
>>>> unsigned source operands, then add and accumulate the results into the
>>>> destination dword element size operand. This instruction allows for the
>>>> platform to have superior AI capabilities.
>>>>
>>>> The bit definition:
>>>> CPUID.(EAX=7,ECX=1):EDX[bit 4]
>>>
>>> For this particular one, use scattered.c instead of adding a new leaf.
>>
>> Unless the kernel wants to use X86_FEATURE_AVX_VNNI_INT8, which seems unlikely,
>> there's no need to create a scattered entry. This can be handled in KVM by adding
>> a KVM-only leaf entry (which will be needed no matter what), plus a #define for
>> X86_FEATURE_AVX_VNNI_INT8 to direct it to the KVM entry.
>>
>> E.g.
>>
>> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
>> index a19d473d0184..25e7bfc61607 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_7_1_EDX,
>> NR_KVM_CPU_CAPS,
>>
>> NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
>> @@ -24,6 +25,16 @@ enum kvm_only_cpuid_leafs {
>> #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_AVX_VNNI_INT8 KVM_X86_FEATURE(CPUID_7_1_EDX, 4)
>> +
>> +/*
>> + * 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_AVX_VNNI_INT8 KVM_X86_FEATURE_AVX_VNNI_INT8
>
> Actually, there's no need for KVM_X86_FEATURE_AVX_VNNI_INT8 in this case, just
> #define X86_FEATURE_AVX_VNNI_INT8 directly. The KVM_ prefixed macro exists purely
> to redirect the non-KVM_ version, but that's unnecessary in this case.
>
> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> index a19d473d0184..38adafb03490 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_7_1_EDX,
> NR_KVM_CPU_CAPS,
>
> NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
> @@ -24,6 +25,13 @@ enum kvm_only_cpuid_leafs {
> #define KVM_X86_FEATURE_SGX1 KVM_X86_FEATURE(CPUID_12_EAX, 0)
> #define KVM_X86_FEATURE_SGX2 KVM_X86_FEATURE(CPUID_12_EAX, 1)
>
> +/*
> + * Omit the KVM_ prefix 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 handling in __feature_translate() is needed.
> + */
> +#define X86_FEATURE_AVX_VNNI_INT8 KVM_X86_FEATURE(CPUID_7_1_EDX, 4)
> +
> struct cpuid_reg {
> u32 function;
> u32 index;
> @@ -48,6 +56,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
> [CPUID_7_1_EAX] = { 7, 1, CPUID_EAX},
> [CPUID_12_EAX] = {0x00000012, 0, CPUID_EAX},
> [CPUID_8000_001F_EAX] = {0x8000001f, 0, CPUID_EAX},
> + [CPUID_7_1_EDX] = { 7, 1, CPUID_EDX},
> };

Use KVM-only leafs is better for edx in this case. Will follow this suggestion in v2.
Thanks Sean very much.
--
Regards,
Jiaxi