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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Date:	Mon, 04 Jun 2007 12:13:12 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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?

Jesse Barnes <jbarnes@...tuousgeek.org> writes:

> On Friday, June 1, 2007 2:19:43 Andi Kleen wrote:
>> 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 just checked, yes the MTRRs win for UC types.  But it sounds like the cases 
> we're talking about are actually situations where there's no MTRR coverage, 
> so the default type is used.  The manual doesn't specifically call out how 
> memory using the default type interacts with PAT, but it may well be that it 
> stays uncached if the default type is uncached.  Again that argues for fixing 
> the MTRR mapping problem in some way.

Last I looked PAT can only demote not promote the type of a page,
except for the specific exception of UC to WC.

Normally the default type is UC so putting a pat type of WB won't
help anything.  I may have missed some subtle detail but I remember
looking into this in some detail a while ago and coming to that
conclusion.

It is the BIOS's responsibility to mark all usable memory as WB,
using the MTRRs.  If it doesn't it is a BIOS bug.

Eric
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