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Date: Sat, 09 May 2009 09:04:08 +0100 From: Keir Fraser <keir.fraser@...citrix.com> To: Alan Cox <alan@...rguk.ukuu.org.uk>, Ian Campbell <Ian.Campbell@...citrix.com> CC: Jeremy Fitzhardinge <jeremy@...p.org>, the arch/x86 maintainers <x86@...nel.org>, Suresh Siddha <suresh.b.siddha@...el.com>, <Linux@...r.kernel.org>, "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>, "H. Peter Anvin" <hpa@...or.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 On 08/05/2009 15:50, "Alan Cox" <alan@...rguk.ukuu.org.uk> wrote: >> 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. 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. -- Keir -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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