Re: [PATCH for-4.6] p2m/ept: Set the A bit only if PML is enabled

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 16/09/2015 09:47, Ross Lagerwall wrote:
> Since commit 191b3f3344ee ("p2m/ept: enable PML in p2m-ept for
> log-dirty"), the A and D bits of EPT paging entries are set
> unconditionally, regardless of whether PML is enabled or not. This
> causes a regression in Xen 4.6 on some processors due to Intel Errata
> AVR41 -- HVM guests get severe memory corruption when the A bit is
> set. The errata affects the Atom C2000 family (Avaton).

^ Due to incorrect TLB flushing on mov to cr3.

>
> Instead, only set the bits if PML is enabled.

(You have missed a SoB)

While this certainly does fix the issue, I am not certain if it is the
correct fix; It relies on no affected systems actually supporting PML.

The root issue is that ept a/d bits may not be used, even just as
software-defined bits on these systems, and calling this out should be
as specific quirk against Avoton systems, rather than being related to PML.

~Andrew