Re: [Xen-devel] [PATCH v2 29/30] tools/libxc: Use featuresets rather than guesswork

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/02/16 08:55, Jan Beulich wrote:
>>>> On 05.02.16 at 14:42, <andrew.cooper3@xxxxxxxxxx> wrote:
>> @@ -467,11 +420,8 @@ static void xc_cpuid_config_xsave(xc_interface *xch,
>>          regs[1] = 512 + 64; /* FP/SSE + XSAVE.HEADER */
>>          break;
>>      case 1: /* leaf 1 */
>> -        regs[0] &= (XSAVEOPT | XSAVEC | XGETBV1 | XSAVES);
>> -        if ( !info->hvm )
>> -            regs[0] &= ~XSAVES;
>> -        regs[2] &= info->xfeature_mask;
>> -        regs[3] = 0;
>> +        regs[0] = info->featureset[featureword_of(X86_FEATURE_XSAVEOPT)];
>> +        regs[1] = regs[2] = regs[3] = 0;
>>          break;
> This change (to regs[2] handling) reminds me of an apparent issue
> in the earlier dependencies patch, which I realized only after having
> sent the reply, and then forgot to send another reply for: Shouldn't
> features requiring certain XSAVE states depend on that state's
> availability instead of just XSAVE? That would make the above use
> proper masking for both regs[2] and regs[3] (and also for regs[0]
> and regs[3] in the sub-leaf 0 case).

Have you looks at this patch in combination with the following one?

Strictly speaking, there is a difference between the xstate availability
and the features which use them.  However, the former are not level-able
on older hardware.  I have deliberately constructed the former from the
latter, to prevent the two becoming different, and unlike real hardware.

~Andrew

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