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

From: Eric W. Biederman
Date: Mon Jun 04 2007 - 14:14:32 EST


Jesse Barnes <jbarnes@xxxxxxxxxxxxxxxx> 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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