Comment 6 for bug 859137

I have just confirmed :

You do NOT want to enable the IOMMU mode on the Asus Croshair V Formula motherboard (AMD 990FX/SB950 chipset) if you want to have your usb working on Ubuntu.

In particular, if you enable IOMMU and if your keyboard and mouse are usb, then you will NOT be able to log on to Ubuntu.

If you use PS2 keyboard and PS2 mouse, though, then they will work, but you will have no usb functionality.

Among other things, this is a pity, because the BIOS of this motherboard explains: "IOMMU is supported on Linux based systems to convert 32 bit I/O to 64 bit MMIO".