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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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. The code is also of dubious
> benefit and was the subject XSA-275.
> 
> This patch removes the code, freeing up the remaining PTE 'ignored' bits
> for other potential use and shortening the source by 170 lines.

No word at all about the performance implications? Do you have
any plans to re-introduce properly working page recombining
code? I realize VT-d doesn't have any either (the maintainers at
some point in the distant past had promised to implement it, but
I guess that's long been forgotten), but anyway...

Jan



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