Re: [PATCH] trim memory not covered by WB MTRRs

From: Andrew Morton
Date: Fri Jun 08 2007 - 17:15:54 EST

On Wed, 6 Jun 2007 12:29:23 -0700
Jesse Barnes <jesse.barnes@xxxxxxxxx> 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.

i386 allmodconfig:

arch/i386/kernel/cpu/mtrr/main.c: In function 'mtrr_trim_uncached_memory':
arch/i386/kernel/cpu/mtrr/main.c:655: error: 'end_pfn' undeclared (first use in this function)
arch/i386/kernel/cpu/mtrr/main.c:655: error: (Each undeclared identifier is reported only once
arch/i386/kernel/cpu/mtrr/main.c:655: error: for each function it appears in.)

I'll poke some ifdefs in there.
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