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

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Date:	Tue, 12 Jun 2007 19:11:21 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jesse Barnes <jesse.barnes@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Justin Piszcz <jpiszcz@...idpixels.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs

Jesse Barnes <jesse.barnes@...el.com> writes:

> 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.

A quick update.  This patch is horribly incorrect on a socket F 
opteron/Athlon 64 with memory above 4GB.

In particular those cpus are capable of mapping all of memory
above 4GB as write back without using a single MTRR.

So examining MTRRs is insufficient.

Eric
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