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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.11.18 at 15:19, <andrew.cooper3@xxxxxxxxxx> wrote:
> 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.

Well, of course, and Paul did say so. My point is that even when we
start using the feature, as long as we don't set A and/or D bits
ourselves, nothing is getting in the way of the merging code, as it
ceases to do anything once the domain starts running.

Jan



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