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

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Date:   Wed, 2 Nov 2022 11:14:41 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jiaxi Chen <jiaxi.chen@...ux.intel.com>, kvm@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        seanjc@...gle.com, pbonzini@...hat.com, ndesaulniers@...gle.com,
        alexandre.belloni@...tlin.com, peterz@...radead.org,
        jpoimboe@...nel.org, chang.seok.bae@...el.com,
        pawan.kumar.gupta@...ux.intel.com, babu.moger@....com,
        jmattson@...gle.com, sandipan.das@....com, tony.luck@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, fenghua.yu@...el.com,
        keescook@...omium.org, jane.malalane@...rix.com, nathan@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] x86: KVM: Enable AMX-FP16 CPUID and expose it to
 guest

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?