Re: [Xen-devel] [PATCHv2 2/3] x86/fpu: Add a per-domain field to set the width of FIP/FDP

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 23/02/16 15:24, Jan Beulich wrote:
>>>> On 23.02.16 at 12:05, <david.vrabel@xxxxxxxxxx> wrote:
>> @@ -292,17 +296,17 @@ void xsave(struct vcpu *v, uint64_t mask)
>>              asm volatile ( "fnstenv %0" : "=m" (fpu_env) );
>>              ptr->fpu_sse.fip.sel = fpu_env.fcs;
>>              ptr->fpu_sse.fdp.sel = fpu_env.fds;
>> -            word_size = 4;
>> +            fip_width = 4;
>>          }
>> +        else
>> +            fip_width = 8;
>>      }
>>      else
>>      {
>>          XSAVE("");
>> -        word_size = 4;
>>      }
>>  #undef XSAVE
>> -    if ( word_size >= 0 )
>> -        ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] = word_size;
>> +    ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] = fip_width;
>>  }
> 
> There's actually a pre-existing bug here that I think should get
> fixed at once: The 64-bit save path avoided to update
> ptr->fpu_sse.x[FPU_WORD_SIZE_OFFSET] when FPU state
> didn't get saved (by setting word_size to -1), which continues to be
> the case due to patch 1's changes. The 32-bit code path violated
> this even before your change. I.e. the last else visible above
> should get extended to return when !(mask & XSTATE_FP).

XSTATE_FP is always set in the mask. (see fpu_xsave()).

David

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