MTRR vs. PAT and Intel Graphics

[Bruce Perens]

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?

    Thanks

    Bruce