linux-kernel - Re: [PATCH RFC] kvm: x86: Expose AVX512

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Date:   Thu, 20 Jun 2019 23:09:42 +0800
From:   "Liu, Jing2" <jing2.liu@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, jing2.liu@...el.com
Subject: Re: [PATCH RFC] kvm: x86: Expose AVX512_BF16 feature to guest

Hi Paolo,

On 6/20/2019 8:16 PM, Paolo Bonzini wrote:
> On 20/06/19 13:21, Jing Liu wrote:
>> +		for (i = 1; i <= times; i++) {
>> +			if (*nent >= maxnent)
>> +				goto out;
>> +			do_cpuid_1_ent(&entry[i], function, i);
>> +			entry[i].eax &= F(AVX512_BF16);
>> +			entry[i].ebx = 0;
>> +			entry[i].ecx = 0;
>> +			entry[i].edx = 0;
>> +			entry[i].flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> +			++*nent;
> 
> This woud be wrong for i > 1, so instead make this
> 
> 	if (entry->eax >= 1)
> 

I am confused about the @index parameter. @index seems not used for
every case except 0x07. Since the caller function only has @index=0, so
all other cases except 0x07 put cpuid info from subleaf=0 to max subleaf.

What do you think about @index in current function? Does it mean, we
need put cpuid from index to max subleaf to @entry[i]? If so, the logic
seems as follows,

if (index == 0) {
     // Put subleaf 0 into @entry
     // Put subleaf 1 into @entry[1]
} else if (index < entry->eax) {
     // Put subleaf 1 into @entry
} else {
     // Put all zero into @entry
}

But this seems not identical with other cases, for current caller
function. Or we can simply ignore @index in 0x07 and just put all possible
subleaf info back?

> and define F(AVX512_BF16) as a new constant kvm_cpuid_7_1_eax_features.
> 
Got it.

Thanks,
Jing

> Paolo
>