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

From: Dave Jones
Date: Wed Jun 13 2007 - 12:20:06 EST

On Wed, Jun 13, 2007 at 08:52:23AM +0200, Bodo Eggert wrote:
> 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.
> Wouldn't it be better to correct the MTRR, if possible? As far as I read
> here (LKML), the BIOS did not merge the entries

The size/alignment constraints of MTRRs (must be a power of 2)
means that the best-fit method of covering non power of 2 memory sizes
is the, well.. best fit. There's nothing that can be merged.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at