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    Subject[PATCH RFC 0/3] x86: Full support of PAT
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    The x86 architecture offers via the PAT (Page Attribute Table) a way to
    specify different caching modes in page table entries. The PAT MSR contains
    8 entries each specifying one of 6 possible cache modes. A pte references one
    of those entries via 3 bits: _PAGE_PAT, _PAGE_PWT and _PAGE_PCD.

    The Linux kernel currently supports only 4 different cache modes. The PAT MSR
    is set up in a way that the setting of _PAGE_PAT in a pte doesn't matter: the
    top 4 entries in the PAT MSR are the same as the 4 lower entries.

    This results in the kernel not supporting e.g. write-through mode. Especially
    this cache mode would speed up drivers of video cards which now have to use
    uncached accesses.

    OTOH some old processors (Pentium) don't support PAT and the Xen hypervisor
    has been using a different PAT MSR configuration for some time now and can't
    change that as this setting is part of the ABI.

    This patch set abstracts the cache mode from the pte and introduces tables to
    translate between cache mode and pte bits (the default cache mode "write back"
    is hard-wired to PAT entry 0). The tables are statically initialized with
    values being compatible to old processors and current usage. As soon as the
    PAT MSR is changed (or - in case of Xen - is read at boot time) the tables are
    changed accordingly. Requests of mappings with special cache modes are always
    possible now, in case they are not supported there will be a fallback to a
    compatible but slower mode.

    [PATCH RFC 1/3] x86: Make page cache mode a real type
    [PATCH RFC 2/3] x86: Enable PAT to use cache mode translation tables
    [PATCH RFC 3/3] Support Xen pv-domains using PAT


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