On 08/27/2015 09:54 PM, Ingo Molnar wrote:
>
> * Dave Hansen <d...@sr71.net> wrote:
>
>> +static int xfeature_is_supervisor(int xfeature_nr)
>> +{
>> + /*
>> + * We currently do not suport supervisor states, but if
>> + * we did, we could find out like this.
>> + *
>> + * SDM says: If state component i is a user state component,
>> + * ECX[0] return 0; if state component i is a supervisor
>> + * state component, ECX[0] returns 1.
>> + u32 eax, ebx, ecx, edx;
>> + cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx);
>> + return !!(ecx & 1);
>> + */
>> + return 0;
>> +}
>
> So if this CPUID is documented to work, why not use it to sanity check things?
>
> I.e. do something like:
>
> u32 eax, ebx, ecx, edx;
>
> cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx);
>
> /* Linux doesn't support supervisor states (yet): */
> WARN_ON_ONCE(ecx & 1);
>
> return 0;
>
> That would give us a gentle way to double check our assumptions here.
Actually, the newest state that you will see in the wild is for
Processor Trace, and it _is_ a supervisor state. However, we don't use
it in Linux for our Processor Trace support, and Andi says we probably
never will.
So we probably shouldn't warn on it.
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