Re: [Xen-devel] [PATCH 08/10] x86/cpuid: Handle leaf 0xb in guest_cpuid()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/02/17 09:16, Jan Beulich wrote:
>>>> On 20.02.17 at 12:00, <andrew.cooper3@xxxxxxxxxx> wrote:
>> Leaf 0xb is reserved by AMD, and uniformly hidden from guests by the 
>> toolstack
>> logic and hypervisor PV logic.
>>
>> The previous dynamic logic filled in the x2APIC ID for all HVM guests.  This
>> is modified to respect the entire leaf being reserved by AMD, but is altered
>> to include PV Intel guests, so they get more sensible values in their 
>> emulated
>> and faulted view of CPUID.
> Don't we expose x2APIC to HVM guests even on AMD systems? In
> which case we surely will want to also expose the x2APIC ID.

The x2apic feature bit is still listed as reserved in the latest AMD
SDM, and I haven't seen a piece of hardware which supports it.

Also, x2apic or not, this leaf is documented as fully reserved, so we
shouldn't be filling it in anyway.

>
>> @@ -959,6 +950,14 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
>>          }
>>          break;
>>  
>> +    case 0xb:
>> +        if ( p->x86_vendor == X86_VENDOR_INTEL )
>> +        {
>> +            /* Fix the x2APIC identifier. */
>> +            res->d = v->vcpu_id * 2;
>> +        }
>> +        break;
> Irrespective of the comment above, wouldn't the if() here better
> look at the x2APIC feature flag of the domain?

Perhaps.  There isn't a formal link called out, but looking at the
content, I am guessing that real hardware is consistent with their
support of x2apic and the availability of this leaf?

~Andrew

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