Re: [PATCH 2/6] x86: KVM: Enable AMX-FP16 CPUID and expose it to guest

Dave Hansen
2022-11-02T18:14:54Z

On 10/19/22 01:47, Jiaxi Chen wrote:
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 445626cb5779..9313240e3cdd 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -309,6 +309,7 @@
> #define X86_FEATURE_AVX_VNNI (12*32+ 4) /* AVX VNNI instructions */
> #define X86_FEATURE_AVX512_BF16 (12*32+ 5) /* AVX512 BFLOAT16 instructions */
> #define X86_FEATURE_CMPCCXADD (12*32+ 7) /* CMPccXADD instructions */
> +#define X86_FEATURE_AMX_FP16 (12*32+21) /* AMX fp16 Support */

Please zap these from /proc/cpuinfo by doing this:

#define X86_FEATURE_AMX_FP16 (12*32+21) /* "" AMX fp16 Support */

> /* AMD-defined CPU features, CPUID level 0x80000008 (EBX), word 13 */
> #define X86_FEATURE_CLZERO (13*32+ 0) /* CLZERO instruction */
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 3f745f6fdc43..d983ddb974ba 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -657,7 +657,7 @@ void kvm_set_cpu_caps(void)
> kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
>
> kvm_cpu_cap_mask(CPUID_7_1_EAX,
> - F(AVX_VNNI) | F(AVX512_BF16) | F(CMPCCXADD)
> + F(AVX_VNNI) | F(AVX512_BF16) | F(CMPCCXADD) | F(AMX_FP16)
> );
>
> kvm_cpu_cap_mask(CPUID_D_1_EAX,

KVM folks, is the idea that every feature that is enumerated to a guest
needs to be in one of these masks? Or is there something special about
the features in these masks?