linux-kernel - Re: Intel's response Linux/MTRR/8GB Memory Support / Why doesn't the kernel realize the BIOS has problems and re-map appropriately?

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Message-Id: <200706011435.56440.jbarnes@virtuousgeek.org>
Date:	Fri, 1 Jun 2007 14:35:56 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Justin Piszcz <jpiszcz@...idpixels.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Intel's response Linux/MTRR/8GB Memory Support / Why doesn't the kernel realize the BIOS has problems and re-map appropriately?

On Friday, June 1, 2007 2:19:43 Andi Kleen wrote:
> On Fri, Jun 01, 2007 at 02:07:51PM -0700, Jesse Barnes wrote:
> > On Friday, June 1, 2007 2:14:17 Andi Kleen wrote:
> > > Jesse Barnes <jbarnes@...tuousgeek.org> writes:
> > > > (or we get proper PAT support, which I think would make this problem
> > > > go away as well).
> > >
> > > No it won't. If the basic MTRRs for memory are wrong just having PAT
> > > support in drivers (which already exist in a limited form already, just
> > > for UC only) won't change anything.
> >
> > No obviously just using PAT for drivers wouldn't help, I was thinking
> > more of having one PAT type be WB memory, and using it by default for
> > most PTEs
>
> Then the BIOS couldn't override it anymore in case it is needed somewhere.
> e.g. normally we just use normal 2MB direct mappings for the hole
> if there is memory beyond it and the hole doesn't need to be 2MB aligned.
> Just assuming UC for all reserved pages would be also pretty drastic
> and likely result in many 2MB pages being split and using a lot more
> TLB.
>
> > covering normal memory.  If that's not possible, then it seems sensible
> > to
>
> And normally the MTRRs win, don't they (if I remember the table correctly)
> So if the MTRR says UC and PAT disagrees it might not actually help

I didn't check that part of the spec, that might be true.  If so, then we 
really need some sort of MTRR fix no matter what.

Jesse
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