linux-kernel - Re: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT

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Message-ID: <4A05CD29.6050505@zytor.com>
Date:	Sat, 09 May 2009 11:36:25 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Keir Fraser <keir.fraser@...citrix.com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ian Campbell <Ian.Campbell@...citrix.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>, Linux@...or.com,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jan Beulich <jbeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>,
	Lu <yinghai@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT

Keir Fraser wrote:
> 
> My suggestion is that Xen changes its PAT[1] from WT to WC. AFAIK only
> suse's Linux port is currently using Xen's PAT support, and it does not
> actually use the WT cache attribute. Then Xen's PAT[0-3] would match what
> Linux expects, and Linux apparently does not risk using PAT[4-7] because of
> processor errata so their values are irrelevant. Linux can check that it is
> running on Xen with suitable PAT setup by reading MSR_IA32_CR_PAT and
> checking PAT[0-3], and only enable PAT usage in case of a match.
> 

There exists at least one processor erratum where the CPU will use
PAT[4-7] when the user requested PAT[0-3].  For those CPUs, it is unsafe
for *any* OS to have PAT[4-7] != PAT[0-3].

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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