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    SubjectRe: [PATCH] trim memory not covered by WB MTRRs
    On 6/7/07, Jesse Barnes <> wrote:
    > 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.
    > Something similar could be done on i386 if needed, but the boot
    > ordering would be slightly different, since the MTRR code on i386
    > depends on the boot_cpu_data structure being setup.
    > This patch incorporates the feedback from Eric and Andi:
    > - use MAX_VAR_RANGES instead of NUM_VAR_RANGES
    > - move array declaration to header file as an extern
    > - add command line disable option "disable_mtrr_trim"
    > - don't run the trim code if the MTRR default type is cacheable
    > - don't run the trim code on non-Intel machines
    > Justin, feel free to test again if you have time and add your
    > "Tested-by" signoff.
    > Andi, as for large pages, do you think this is ok as is, or should
    > I trim a larger granularity? If so, what granularity?
    > Signed-off-by: Jesse Barnes <>
    > Thanks,
    > Jesse


    for AMD Rev F Opteron later CPU, BIOS will not set WB in MTRR for 4G above mem.

    This patch will get rid of those RAM.

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