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

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Message-ID: <13f4d8c0-4bf9-1822-f5de-8a45cfbb0199@linux.intel.com>
Date:   Thu, 3 Nov 2022 10:38:31 +0800
From:   Jiaxi Chen <jiaxi.chen@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.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, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 2/6] x86: KVM: Enable AMX-FP16 CPUID and expose it to
 guest



On 11/3/2022 2:21 AM, Dave Hansen wrote:
> On 11/2/22 11:16, Paolo Bonzini wrote:
>> 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.
> 
> Makes sense.
> 
> Intel folks, when you add these bits, can you please include information
> about the "vetting" that you performed?
> 
> For example, it would be handy to say:
> 
> 	AMX_FP16 is just a new instruction that operates on existing AMX
> 	tile registers.  It needs no additional enabling on top of the
> 	existing kernel AMX enabling.

Thanks for Dave's suggestion.  It makes the commit message much clear
and readable. Will modify it in v2.

-- 
Regards,
Jiaxi