Re: [PATCH RFC] kvm: x86: Expose AVX512_BF16 feature to guest

[Liu, Jing2]

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