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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Wed, Aug 21, 2013 at 12:03 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> On Wed, Aug 21, 2013 at 08:50:26PM +0200, H. Peter Anvin wrote:
>> But is there a manifest bug or not?  What is the deal with Xen?
>>
>
> I personally don't see bug here because
>
>  - this swapped page soft dirty bit is set for non-present entries only,
>    never for present ones, just at moment we form swap pte entry
>
>  - i don't find any code which would test for this bit directly without
>    is_swap_pte call
>
> but the use of paw bit itself is confusing, so I'm working on patch which
> won't use it. Again, if someone knows where exactly access to pse bit when
> pte keeps swap entry may happen (for any purpose other than dirty page
> tracking) please share.

I doubt that there are cacheability issues here, since the swap type
already overlaps with PWT and PCD.

What happens if the page being swapped is a THP page?  (I have no idea
how, or even if, this works, but presumably PSE is important.)

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC

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