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

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Message-Id: <E1HyMiN-0000oB-TR@be1.lrz>
Date:	Wed, 13 Jun 2007 08:52:23 +0200
From:	Bodo Eggert <7eggert@....de>
To:	Jesse Barnes <jesse.barnes@...el.com>,
	linux-kernel@...r.kernel.org,
	stin Piszcz <jpiszcz@...idpixels.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs

Jesse Barnes <jesse.barnes@...el.com> 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, and this waste caused the
last part of the memory not to be covered. Off cause you can't DTRT for all
buggy MTRR setups, but if you're lucky, optionally merging the MTRR and
adding the rest of the memory may sometimes do the trick ...
-- 
Funny quotes:
10. Nothing is fool proof to a talented fool.

Friß, Spammer: v9Ttukbw@...7eggert.dyndns.org
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