linux-kernel - Re: [PATCH v4 4/6] x86: KVM: Advertise AVX-VNNI-INT8 CPUID to user space

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Message-ID: <76504a97-543a-c655-694c-4f3efb108673@linux.intel.com>
Date:   Mon, 21 Nov 2022 23:06:48 +0800
From:   Jiaxi Chen <jiaxi.chen@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.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, nathan@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/6] x86: KVM: Advertise AVX-VNNI-INT8 CPUID to user
 space



On 11/19/2022 1:17 AM, Sean Christopherson wrote:
> On Fri, Nov 18, 2022, Jiaxi Chen wrote:
>> AVX-VNNI-INT8 is a new set of instructions in the latest Intel platform
>> Sierra Forest, aims for the platform to have superior AI capabilities.
>> This instruction multiplies the individual bytes of two unsigned or
>> unsigned source operands, then adds and accumulates the results into the
>> destination dword element size operand.
>>
>> The bit definition:
>> CPUID.(EAX=7,ECX=1):EDX[bit 4]
>>
>> This CPUID is exposed to user space. Besides, there is no other VMX
>> control for this instruction.
>>
>> Signed-off-by: Jiaxi Chen <jiaxi.chen@...ux.intel.com>
>> ---
>>  arch/x86/kvm/cpuid.c         | 5 ++++-
>>  arch/x86/kvm/reverse_cpuid.h | 5 +++++
>>  2 files changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 5726afb2d14c..e2b8e5485474 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -660,6 +660,9 @@ void kvm_set_cpu_caps(void)
>>  		F(AVX_VNNI) | F(AVX512_BF16) | F(CMPCCXADD) | F(AMX_FP16) |
>>  		F(AVX_IFMA));
>>  
>> +	kvm_cpu_cap_init_scattered(CPUID_7_1_EDX,
> 
> Ah, this is going to be confusing and potentially error prone.  AVX_VNNI_INT8
> isn't actually scattered, i.e. kvm_cpu_cap_init_scattered() is poorly named.  And
> using SF() would be _really_ broken as boot_cpu_has() would consume garbage and
> potentially leak kernel state to userspace.
> 
> To address these issue and also document how to add KVM-only features, can you
> slot in the two attached patches at the begining of this series?
> 
Sure. Thanks for your kind contribution.
> Thanks!
> 
>> +		F(AVX_VNNI_INT8));
> 
> Terminators on a separate line please.
> 
>>  	kvm_cpu_cap_mask(CPUID_D_1_EAX,
>>  		F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | f_xfd
>>  	);

Got it. Thanks~
-- 
Regards,
Jiaxi