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 19:16:37 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        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, 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 11/2/22 19:14, Dave Hansen wrote:
>>   
>>   	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?

Yes, all features are vetted manually to see whether they require new 
MSRs and the like.  Therefore, anything that userspace can set in the 
guest's CPUID must be in the list.

Paolo