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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/03/17 13:19, Jan Beulich wrote:
>>>> On 02.03.17 at 13:56, <andrew.cooper3@xxxxxxxxxx> wrote:
>> 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.
> Well, is that (always) the case here already, or only after patch 7? My
> question after all is whether this works as intended at this point.

I am probably going to defer to Tim on the historical details here.

Most paths to creating a new shadow follow a walk of the guest
pagetables, and reserved guest entries fall out there without creating
shadows.

However, there are definitely paths to creating shadows which don't pass
through a guest walk (probably the adjacent prefetches), and those paths
need protection from creating bad shadows.

~Andrew

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