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

[Justin Piszcz <jpiszcz@lucidpixels.com>]

On Mon, 4 Jun 2007, Eric W. Biederman wrote:

> Jesse Barnes <jbarnes@virtuousgeek.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
>

According to Intel it is not a BIOS bug but rather the media controller
hub (MCH) uses memory for various purposes, outlined in their doc:

>From their response, it sounds like the kernel needs to setup the memory 
properly to deal with the MCH found in the 965 motherboards?

>From their e-mail:

Note before continuing: Debian* Linux Operating System is not an 
officially, validated, tested Operating System for the Intel(R) Desktop 
Board DG965WH 
(see http://downloadcenter.intel.com/Product_Filter.aspx?ProductID=2375); 
moreover, we do confirm that "on a system that has 8 GB of system memory 
installed, it is not possible to use all of the installed memory due to system 
address space being allocated for other system critical functions." [qtd. 
on page 43 of the Technical Product Specification (see 
http://download.intel.com/design/motherbd/wh/D5600801US.pdf)].  Thus, the
following suggestions are provided AS IS; we cannot guarantee the problem 
would be fixed afterwards:

Therefore, they are NOT going to fix their BIOS-- and I have already 
received an e-mail from one or two people who are experiencing this 
problem, I presume it will only get worse.

Justin.