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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.08.13 at 16:12, 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?

No, the problem isn't with not-present PTEs (i.e. swap entries),
but with present ones - the same bit (7) is being used for both,
according to this comment:

/*
 * Tracking soft dirty bit when a page goes to a swap is tricky.
 * We need a bit which can be stored in pte _and_ not conflict
 * with swap entry format. On x86 bits 6 and 7 are *not* involved
 * into swap entry computation, but bit 6 is used for nonlinear
 * file mapping, so we borrow bit 7 for soft dirty tracking.
 */

Or are you telling me that the comment is misleading (at least me),
and this applies only to not-present PTEs? And even then - where
would the value of the original PAT bit be stored while swapped
out (or is it impossible - now and forever - for WC pages to get
swapped)?

Jan


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