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

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Message-ID: <CALCETrWZqGUC4+43VT54gFd8qAf+0o4Ekyq8dKVqYpuZOCR9qg@mail.gmail.com>
Date:	Wed, 21 Aug 2013 10:28:38 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, 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

On Wed, Aug 21, 2013 at 6:48 AM, David Vrabel <david.vrabel@...rix.com> 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.
>

Kind of off topic, but do you have a summary of how Xen uses the high
PAT bits?  I'm the one who wants WT, and if there's already precedent
for using the high PAT bits, it'll be helpful.

(Also, Xen is a little weird, and I'd rather not break it again by
accident like I did with the vsyscall changes a couple years ago.)

--Andy
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