MTRR vs. PAT and Intel Graphics

[Jesse Barnes]

On Friday, June 1, 2007 7:49:59 Bruce Perens wrote:
> Keith and Co.,
>
> In the aftermath of my investigation of i965G performance dropping
> severely on my Gigabyte motherboard when a particular fan setting was
> configured, I found that this is ultimately attributable to the fact
> that Windows uses the more versatile Page Attribute Tables while Linux
> is still using MTRRs alone for the same purpose. And for this reason we
> are probably not going to get much help on the issue from BIOS and
> motherboard manufacturers. PCI Express systems seem to have made the
> situation worse in that memory-mapped I/O regions are in general less
> contiguous. nVidia, Jeff Hartmann, and others have been trying to get
> PAT into the kernel since at least 2004, the latest patch to do so
> appears to be a year old and didn't quite meet approval. More
> information is at http://technocrat.net/d/2007/6/1/20838
>
> Since this problem can severely effect Intel graphics performance on
> Linux systems while leaving Windows unscathed, I think it would be
> desirable for Intel to involve itself in this issue until PAT is
> successfully incorporated into mainstream kernels. Keith, can you help
> with this?

Hi Bruce, yes we (Intel) have people working on adding PAT support to the 
kernel.  I'm hoping it'll get upstream in the next release or two; it's 
definitely a good feature to have.

That said, it seems odd that a simple fan configuration setting would eat up 
four whole MTRRs.  What does /proc/mtrr look like with and without the fan 
setting enabled?  Linux could do a better job of verifying and reprogramming 
MTRRs in many cases than it does currently.

Jesse