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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.06.16 at 17:35, <andrew.cooper3@xxxxxxxxxx> wrote:
> 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>

Converting this back is easy to do, but I'll nevertheless wait for
Wei's opinion re 4.7 inclusion, as otherwise I'll eventually need to
rebase on top of yours anyway.

Jan


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