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    SubjectRe: [Xen-devel] Re: [PATCH 06/16] xen: disable PAT
    On 08/05/2009 15:50, "Alan Cox" <> 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

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      Last update: 2009-05-09 10:17    [W:0.021 / U:0.276 seconds]
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