Messages in this thread | | | From | Jesse Barnes <> | Subject | Re: Intel's response Linux/MTRR/8GB Memory Support / Why doesn't the kernel realize the BIOS has problems and re-map appropriately? | Date | Mon, 4 Jun 2007 12:17:39 -0700 |
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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.
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