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

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Date:	Wed, 21 Aug 2013 16:22:44 +0200
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, Xen-devel@...ts.xen.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Jan Beulich <jbeulich@...e.com>
Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

Good question...

Cyrill Gorcunov <gorcunov@...il.com> 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?

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.
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