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    SubjectRe: [PATCH v2 2/6] x86, mm, pat: Change reserve_memtype() to handle WT
    On Wed, Sep 10, 2014 at 1:30 PM, Toshi Kani <toshi.kani@hp.com> wrote:
    > On Wed, 2014-09-10 at 13:14 -0700, H. Peter Anvin wrote:
    >> On 09/10/2014 12:30 PM, Toshi Kani wrote:
    >> >
    >> > When WT is unavailable due to the PAT errata, it does not fail but gets
    >> > redirected to UC-. Similarly, when PAT is disabled, WT gets redirected
    >> > to UC- as well.
    >> >
    >>
    >> But on pre-PAT hardware you can still do WT.
    >
    > Yes, if we manipulates the bits directly, but such code is no longer
    > allowed for PAT systems. The PAT-based kernel interfaces won't work for
    > pre-PAT systems, and therefore requests are redirected to UC- on such
    > systems.
    >

    Right, the PWT bit. Forgot about that.

    I wonder whether it would make sense to do some followup patches to
    replace the current support for non-PAT machines with a "PAT" and
    corresponding reverse map that exactly matches the mapping when PAT is
    disabled. These patches are almost there.

    --Andy


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