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

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Message-ID: <46790DFA.5090503@aitel.hist.no>
Date:	Wed, 20 Jun 2007 13:22:34 +0200
From:	Helge Hafting <helge.hafting@...el.hist.no>
To:	Jesse Barnes <jesse.barnes@...el.com>
CC:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Justin Piszcz <jpiszcz@...idpixels.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs

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.
>   
I assume this cannot be fixed by the simple approach
of echoing some useful numbers into /proc/mtrr like
we used to do for video memory? (Before X did this
automatically?)

An extra bootscript seems better than loosing memory.


Helge Hafting
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