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SubjectRe: [PATCH] trim memory not covered by WB MTRRs
Jesse Barnes <> 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.

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