Originally Posted by meralias
Just to add a little more to this:
Currently running on ArchLinux,
Motherboard: Sabertooth 990FX (bios: 0901)
CPU: Phenom II X4 965
With both IOMMU and the 64Mb option enabled, the xorg driver fails to load, giving the same error as mentioned before. If I pass "iommu=pt" as a boot option, Xorg starts up correctly.
"iommu=pt" allows my system to boot too. But I'm not sure the IOMMU is being used fully when this parameter is specified. See the notes on this patchset http://thread.gmane.org/gmane.linux.kernel/885604