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    SubjectRe: [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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