Many thanks to oga@ for this. I have been using this patch for at least
a month now and not had any issues.
Relevant bits of dmesg:
pchb0 at pci0 dev 0 function 0 "NVIDIA nForce3 250 PCI Host" rev 0xa1
mmuagp0 at pchb0: 1 Miscellaneous Control unit(s) found
agp0 at mmuagp0: aperture at 0xf000
Most other OSen name this agp_amd64 or similar, no numbers in device
names here. this is a driver for the early amd64s that had an agp slot
which interfaces with the GART on the cpu.
This has been floating around for probably 3 years now, but never got
commited because it conflicted with the amd64
