Re: [Xen-devel] Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

[Cyrill Gorcunov <gorcunov@xxxxxxxxx>]

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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