[David Dawes, from the XFree86 Project, asked me to foward this to the
appropriate NetBSD Mailing list, please try to include him
(dawes@xfree86.org) in the followups -- Matthieu]

For XFree86 4.0, we really need to be able to have the X server request
specific MTRR settings for different parts of a video card's physical
memory address space.  While turning WC on for the framebuffer is a big
performance boost, a more critical issue is being able to make sure that
WC is turned off for areas that are used for memory mapped I/O.  We've
found that some BIOSs enable WC in areas that our drivers want to use
for MMIO.  One example is the 0xb0000-0xbffff range.

So what I'm looking for is an interface that our X server can use to
request MTRR settings.  An interface for this is present in the Linux
2.2.x kernel (and we recently added code to use it), although it doesn't
currently allow changing the settings for the low 1MB of address space
(but I'm told that will be added at some point).