Re: [Xen-devel] [PATCH 5/7] x86/shadow: Use the pagewalk reserved bits helpers

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.03.17 at 13:26, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 01/03/17 16:03, Jan Beulich wrote:
>>>>> On 27.02.17 at 15:03, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> The shadow logic should never create a shadow of a guest PTE which contains
>>> reserved bits from the guests point of view.  Such a shadowed entry might 
>>> not
>>> cause #PF[RSVD] when walked by hardware, thus won't behave architecturally
>>> from the guests point of view.
>> But are we already or-ing in the RSVD bit accordingly in such cases,
>> before handing the #PF to the guest? The patch here certainly
>> doesn't make any change towards that, afaics.
> 
> The purpose of this patch is to ensure we never create a shadow which
> risks causing hardware to generate #PF[RSVD] when running on the
> shadows, other than the one deliberate case (MMIO fastpath).

Right, but instead of answering my question this emphasizes the
need for an answer, as what you say basically means we'd never
(except for that one special case) see the RSVD bit set when
getting #PF handed by hardware, yet for forwarding to the guest
we need to set that bit then in such cases.

Jan


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