Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

[H. Peter Anvin]

Good question...

Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
>On Wed, Aug 21, 2013 at 02:48:20PM +0100, David Vrabel wrote:
>> All,
>> 
>> 179ef71c (mm: save soft-dirty bits on swapped pages) introduces a new
>> PTE bit on x86 _PTE_SWP_SOFT_DIRTY which has the same value as
>_PTE_PSE
>> and _PTE_PAT.
>> 
>> With a Xen PV guest, the use of the _PTE_PAT will result in the page
>> having unexpected cachability which will introduce a range of subtle
>> performance and correctness issues.  Xen programs the entry 4 in the
>PAT
>> table with WC so a page that was previously WB will end up as WC.
>> 
>
>David, could you please explain, Xen keeps and analyze _PTE_PAT bit
>for ptes which are not present?

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