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Re: [Xen-devel] [PATCH] amd-iommu: remove page merging code

> -----Original Message-----
> From: Andrew Cooper
> Sent: 27 November 2018 14:19
> To: Jan Beulich <JBeulich@xxxxxxxx>; Paul Durrant
> <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>; Brian Woods
> <brian.woods@xxxxxxx>; Suravee Suthikulpanit
> <suravee.suthikulpanit@xxxxxxx>
> Subject: Re: [Xen-devel] [PATCH] amd-iommu: remove page merging code
> On 27/11/2018 13:06, Jan Beulich wrote:
> >>>> On 26.11.18 at 18:30, <paul.durrant@xxxxxxxxxx> wrote:
> >> The page merging logic makes use of bits 1-8 and bit 63 of a PTE, which
> used
> >> to be specified as ignored. However, bits 5 and 6 are now specified as
> >> 'accessed' and 'dirty' bits and their use only remains safe as long as
> >> the DTE 'Host Access Dirty' bits remain clear.
> > Upon second thought - is this actually true with the XSA-275
> > changes in place? As long as the domain is not running yet,
> > how would A and/or D bits get set?
> DTE.HAD is an IOMMU control which Xen doesn't currently enable, so this
> isn't an issue in current usage.

Yes, that was the point of my comment but I'll clarify that 'remain clear' 
means that Xen does not set them.


> ~Andrew
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