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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/03/17 12:51, Jan Beulich wrote:
>>>> 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.

This is intentional.

We hand #PF[RSVD] back to the guest based on walking the guest
pagetables, rather than what we find from hardware walking the shadows
we create.

~Andrew

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