Re: [Xen-devel] [PATCH] x86: clear CPUID output of leaf 0xd for Dom0 when xsave is disabled

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 18.05.11 at 23:01, Wei Huang <wei.huang2@xxxxxxx> wrote:
> I tested cpuid on a real hardware. If software wants, reading all 
> sub-leaves are allowed even on hardware which doesn't support XSAVE. The 
> instruction just returns 0. So I don't think we need to zap output for 
> sub-leaves > 1.

"Returning zero" is what "zapping" means to me. Returning non-zero
possibly mis-guiding OSes (just as is the case with the xsaveopt
feature flag in Linux) is what I want to avoid.

Jan

>     0x0000000c 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 
> edx=0x00000000
>     0x0000000d 0x00: eax=0x00000003 ebx=0x00000240 ecx=0x00000240 
> edx=0x00000000
>     0x0000000d 0x02: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 
> edx=0x00000000
>     0x0000000d 0x3e: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 
> edx=0x00000000
> 
> On 05/18/2011 04:53 AM, Jan Beulich wrote:
>> Linux starting with 2.6.36 uses the XSAVEOPT instruction and has
>> certain code paths that look only at the feature bit reported through
>> CPUID leaf 0xd sub-leaf 1 (i.e. without qualifying the check with one
>> evaluating leaf 4 output). Consequently the hypervisor ought to mimic
>> actual hardware in clearing leaf 0xd output when not supporting xsave.
>>
>> (Note that this is only a minimal fix. It may be necessary, e.g. for
>> LWP, to also adjust sub-leaf 0's bit masks and perhaps zap output of
>> sub-leaves>  1 when the respective bit in sub-leaf 0 is getting
>> cleared.)
>>
>> Signed-off-by: Jan Beulich<jbeulich@xxxxxxxxxx>
>>
>> --- a/xen/arch/x86/traps.c
>> +++ b/xen/arch/x86/traps.c
>> @@ -836,6 +836,10 @@ static void pv_cpuid(struct cpu_user_reg
>>           __clear_bit(X86_FEATURE_NODEID_MSR % 32,&c);
>>           __clear_bit(X86_FEATURE_TOPOEXT % 32,&c);
>>           break;
>> +    case 0xd: /* XSAVE */
>> +        if ( xsave_enabled(current) )
>> +            break;
>> +        /* fall through */
>>       case 5: /* MONITOR/MWAIT */
>>       case 0xa: /* Architectural Performance Monitor Features */
>>       case 0x8000000a: /* SVM revision and features */
>>
>>
>>




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