Re: [Xen-devel] [PATCH 2/2] x86/HVM: don't calculate XSTATE area sizes in software

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/06/16 16:06, Jan Beulich wrote:
> @@ -3440,42 +3440,24 @@ void hvm_cpuid(unsigned int input, unsig
>              *eax = *ebx = *ecx = *edx = 0;
>              break;
>          }
> -        /* EBX value of main leaf 0 depends on enabled xsave features */
> -        if ( count == 0 && v->arch.xcr0 ) 
> -        {
> -            /* reset EBX to default value first */
> -            *ebx = XSTATE_AREA_MIN_SIZE; 
> -            for ( sub_leaf = 2; sub_leaf < 63; sub_leaf++ )
> -            {
> -                if ( !(v->arch.xcr0 & (1ULL << sub_leaf)) )
> -                    continue;
> -                domain_cpuid(d, input, sub_leaf, &_eax, &_ebx, &_ecx, 
> -                             &_edx);
> -                if ( (_eax + _ebx) > *ebx )
> -                    *ebx = _eax + _ebx;
> -            }
> -        }
> -
> -        if ( count == 1 )
> +        switch ( count )
>          {
> +        case 1:
>              *eax &= hvm_featureset[FEATURESET_Da1];
> -
> -            if ( *eax & cpufeat_mask(X86_FEATURE_XSAVES) )
> +            if ( !(*eax & cpufeat_mask(X86_FEATURE_XSAVES)) )
>              {
> -                uint64_t xfeatures = v->arch.xcr0 | v->arch.hvm_vcpu.msr_xss;
> -
> -                *ebx = XSTATE_AREA_MIN_SIZE;
> -                if ( xfeatures & ~XSTATE_FP_SSE )
> -                    for ( sub_leaf = 2; sub_leaf < 63; sub_leaf++ )
> -                        if ( xfeatures & (1ULL << sub_leaf) )
> -                        {
> -                            if ( test_bit(sub_leaf, &xstate_align) )
> -                                *ebx = ROUNDUP(*ebx, 64);
> -                            *ebx += xstate_sizes[sub_leaf];
> -                        }
> -            }
> -            else
>                  *ebx = *ecx = *edx = 0;
> +                break;
> +            }
> +            /* fall through */
> +        case 0:
> +            /*
> +             * Always read CPUID.0xD[ECX=0/1].EBX from hardware, rather than
> +             * domain policy.  It varies with enabled xstate, and the correct
> +             * xcr0/xss are in context.
> +             */
> +            cpuid_count(input, count, &dummy, ebx, &dummy, &dummy);
> +            break;

It would be helpful for my PKU bugfix if you could avoid collapsing this
into a fallthough, as the fallthough would need to be undone. 
Otherwise, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

~Andrew

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