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 <jesse.barnes@intel.com>]

On Monday, June 4, 2007 11:22 am Justin Piszcz wrote:
> 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.

That's a separate issue from the MTRR mapping though.  Regardless of the 
fact that the system needs some address space in its 8GB range reserved 
for I/O devices, the BIOS should properly setup the MTRRs to map all of 
*available* RAM.  So the person handling your bug report may have been 
confused into thinking that you were describing the former problem.

Jesse
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