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    SubjectRe: [PATCH] trim memory not covered by WB MTRRs
    Date
    Jesse Barnes <jesse.barnes@intel.com> writes:

    > On some machines, buggy BIOSes don't properly setup WB MTRRs to
    > cover all available RAM, meaning the last few megs (or even gigs)
    > of memory will be marked uncached. Since Linux tends to allocate
    > from high memory addresses first, this causes the machine to be
    > unusably slow as soon as the kernel starts really using memory
    > (i.e. right around init time).
    >
    > This patch works around the problem by scanning the MTRRs at
    > boot and figuring out whether the current end_pfn value (setup
    > by early e820 code) goes beyond the highest WB MTRR range, and
    > if so, trimming it to match. A fairly obnoxious KERN_WARNING
    > is printed too, letting the user know that not all of their
    > memory is available due to a likely BIOS bug.

    A quick update. This patch is horribly incorrect on a socket F
    opteron/Athlon 64 with memory above 4GB.

    In particular those cpus are capable of mapping all of memory
    above 4GB as write back without using a single MTRR.

    So examining MTRRs is insufficient.

    Eric
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