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

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Date:	Thu, 22 Aug 2013 08:54:43 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andy Lutomirski" <luto@...capital.net>
Cc:	"David Vrabel" <david.vrabel@...rix.com>,
	"Cyrill Gorcunov" <gorcunov@...il.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	<Xen-devel@...ts.xen.org>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"Pavel Emelyanov" <xemul@...allels.com>,
	"Ingo Molnar" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts
 with existing use

>>> On 21.08.13 at 19:28, Andy Lutomirski <luto@...capital.net> wrote:
> 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.

Xen's public headers have a comment explaining this, with the
main information being this table:

 *  The PAT MSR is as follow (it is a 64-bit value, each entry is 8 bits):
 *             PAT4                 PAT0
 *   +---+----+----+----+-----+----+----+
 *    WC | WC | WB | UC | UC- | WC | WB |  <= Linux
 *   +---+----+----+----+-----+----+----+
 *    WC | WT | WB | UC | UC- | WT | WB |  <= BIOS (default when machine boots)
 *   +---+----+----+----+-----+----+----+
 *    WC | WP | WC | UC | UC- | WT | WB |  <= Xen
 *   +---+----+----+----+-----+----+----+

i.e. Xen is retaining the BIOS (and legacy from non-PAT times)
meaning of the low four entries, putting WC and WP up into
the high half. The fact that the entry 6 is defined to be WC
is perhaps a mistake - it should really be considered reserved
for an eventual future memory type (just like entry 7). It also
seems like entry 6 is documented incorrectly here for Linux and
BIOS.

Jan

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