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

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Date:	Wed, 6 Jun 2007 13:28:52 -0700
From:	Jesse Barnes <jesse.barnes@...el.com>
To:	Justin Piszcz <jpiszcz@...idpixels.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs

On Wednesday, June 6, 2007 1:26 pm Justin Piszcz wrote:
> On Wed, 6 Jun 2007, 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.
> >
> > Justin, can you please test and make sure this patch works for
> > you too?  It'll only work around the problem, but it's better
> > than having to do mem= by hand or waiting for a fix from your
> > BIOS vendor.
> >
> > Thanks,
> > Jesse
>
> Against what kernel version does this patch apply?

Um... git as of b4946ffb1860597b187d78d61ac6504177eb0ff8.  Sorry I 
should have updated before spinning the patch (will do now).

Jesse
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