With "GRUB_CMDLINE_LINUX="iommu=soft" set and IOMMU turned off in BIOS, viewport rendering is working fine. The only scenario when viewport rendering is not working is when you dont set "GRUB_CMDLINE_LINUX="iommu=soft" and HAVE IOMMU turned in BIOS ON.

When IOMMU is disabled in the BIOS my USB ports doesent work , including mouse and keyboard. I was not able to check if viewport work with IOMMU disabled. Now i have "GRUB_CMDLINE_LINUX="iommu=soft" and IOMMU enabled in bios and everything works well.

Hi,
I have similar or the same problem. I have two cards, GTX 660Ti(screen attached) and GTX 780Ti. View port is working only with 660Ti selected, any combination with 780Ti results in freeze. I have new Gigabyte GA-990FXA-UD5 motherboard and i have IOMMU setting on (same as Intel ® VTbut for AMD?). It is impossible to turn off IOMMU because all usb devices stop working with this option disabled (http://ubuntuforums.org/showthread.php?t=2143433&page=2). I need this option enabled for Xen also, so I would prefer to find workaround...