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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT
From: "H. Peter Anvin" <hpa@xxxxxxxxx>
Date: Sat, 09 May 2009 11:36:25 -0700
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Jeremy Fitzhardinge <Jeremy.Fitzhardinge@xxxxxxxxxx>, Suresh Siddha <suresh.b.siddha@xxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, "Pallipadi, Venkatesh" <venkatesh.pallipadi@xxxxxxxxx>, Linux@xxxxxxxxx, Lu <yinghai@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>, Arjan van de Ven <arjan@xxxxxxxxxxxxx>
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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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