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

On 24/01/17 15:50, Jan Beulich wrote:
>>>> 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?

Yes.  (I'd already started doing this.)


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