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

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Message-ID: <20090508155031.4403f7e0@lxorguk.ukuu.org.uk>
Date:	Fri, 8 May 2009 15:50:31 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ian Campbell <Ian.Campbell@...citrix.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yinghai Lu <yinghai@...nel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Arjan van de Ven <arjan@...radead.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>
Subject: Re: [PATCH 06/16] xen: disable PAT

> Roughly speaking the kernel needs to cope with different values written
> to MSR_IA32_CR_PAT than the single hard coded value it would write
> itself if given the chance. Essentially _PAGE_CACHE_* need to become
> dynamic (or I guess we could change the native layout to match Xen's, I
> don't know why native differs from the initial state, I guess just to
> avoid using the PAT bit for some reason.

Because it is required for some processor variants due to errata. This is
also why the BIOS has them programmed as a mirror between 0-3 and 4-7.

I don't think changing Linux here is the right thing to do - if anything
Xen probably needs to change.

See Xeon errata P21.


Alan
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