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

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Date:	Thu, 7 Jun 2007 10:35:18 -0700
From:	Jesse Barnes <jesse.barnes@...el.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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

On Thursday, June 7, 2007 1:16 am Andi Kleen wrote:
> On Wed, Jun 06, 2007 at 12:29:23PM -0700, 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).
>
> In theory -- while not recommended -- a BIOS could also
> use a default fallback MTRR for cached and use explicit MTRRs to
> map the non existing ranges uncached. Would it make sense to handle
> this case?

Probably.  I could just check the default memory type and bail out if 
it's cacheable.

> Should also probably have some command line option
> to disable the check in case something bad happens with it.

Sure.

> Another thing that might be sense to investigate in relationship
> to this patch is large page mappings with MTRRs. iirc P4 and also K8
> splits pages internally with MTRR boundaries and might have some
> other bad side effects. Should we use this as hints to use 4K pages
> for the boundary areas?

Or I could trim to the nearest large page boundary...  We'd lose a 
little more memory but it would keep things simple.

Jesse
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