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

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 19.05.11 at 18:08, "Huang2, Wei" <Wei.Huang2@xxxxxxx> wrote:
> That is right. Could you slip it into your patch and I can sign it off? Or I 
> can crank up one. Keir hasn't put your patch into tree yet.

I would specifically want this change to be separate from the one I did.

Jan

> Thanks,
> -Wei
> 
> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxxxx] 
> Sent: Thursday, May 19, 2011 11:06 AM
> To: Huang2, Wei
> Subject: Re: [Xen-devel] [PATCH] x86: clear CPUID output of leaf 0xd for Dom0 
> when xsave is disabled
> 
>>>> On 19.05.11 at 17:34, Wei Huang <wei.huang2@xxxxxxx> wrote:
>> I misunderstood your email then. Doesn't your patch already achieve this 
>> objective? We didn't check sub-leaf ID (ECX) in switch-case statement. 
>> So all sub-leaves will be cleaned out by your patch.
> 
> Exactly - all of them. However, with LWP currently supported for HVM
> guests only, the respective leaf should be cleared for Dom0 (while not
> clearing the others).
> 
> Jan
> 
>> -Wei
>> 
>> On 05/19/2011 01:47 AM, Jan Beulich wrote:
>>>>>> 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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