Re: [Xen-devel] [PATCH v2 08/14] x86/cpuid: Handle leaf 0x80000008 in guest_cpuid()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.01.17 at 16:31, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 24/01/17 12:16, Jan Beulich wrote:
>>>>> On 23.01.17 at 15:39, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> AMD uses 24 bits in eax, although nothing thus far has ever exposed a 
> non-zero
>>> guest maxphysaddr to HVM guests.
>> I think exposing bits 16...23 should be limited to guests with nested
>> virt enabled, and should also be subject to bounds checking just
>> like is done for maxphysaddr.
> 
> I am not actually sure what the purpose of this field is.  It is zero
> when read by Xen, even on HAP-capable processors, and the AMD manuals
> make no reference to it.  (The CPUID section refers to the entirety of
> the SVM section, but the field isn't actually mentioned anywhere, even
> in the nested paging subsection.)

Very helpful indeed.

> I think it is intended to be filled in by the hypervisor for hap guests,
> but I can't think of a situation where it would be useful to tell a
> guest its HAP maxphysaddr differently to regular maxphysaddr.
> 
> Furthermore the field is explicitly documented as "if 0, refer to plain
> maxphysaddr".  At the moment, nothing ever fill in a value other than 0.

Considering all this, wouldn't we be better off not handing the
field through to guests (i.e. reducing the mask to 0x0000ffff) until
we understand its semantics?

Jan



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