I had problem with my motherboard and VT-d.
Gigabyte GA-X58A-UDR3 v.1.0 with beta bios (8AUD3RUS) with VT-d enabled.
Cpu i7-920 and XEON E5620 both work.
If I tried to enable GPU passtrough this would pop-up:
internal error: process exited while connecting to monitor: 2015-09-06T16:32:53.970184Z qemu-system-x86_64: -device vfio-pci,host=03:00.0,bus=pcie.0,multifunction=on,x-vga=on: vfio: failed to set iommu for container: Operation not permitted
2015-09-06T16:32:53.970216Z qemu-system-x86_64: -device vfio-pci,host=03:00.0,bus=pcie.0,multifunction=on,x-vga=on: vfio: failed to setup container for group 17
2015-09-06T16:32:53.970225Z qemu-system-x86_64: -device vfio-pci,host=03:00.0,bus=pcie.0,multifunction=on,x-vga=on: vfio: failed to get group 17
2015-09-06T16:32:53.970236Z qemu-system-x86_64: -device vfio-pci,host=03:00.0,bus=pcie.0,multifunction=on,x-vga=on: Device initialization failed
2015-09-06T16:32:53.970247Z qemu-system-x86_64: -device vfio-pci,host=03:00.0,bus=pcie.0,multifunction=on,x-vga=on: Device 'vfio-pci' could not be initialized
I then searched the web and added bolded text to syslinux/syslinux.cfg
And I got it to work. Installed Win8.1 to test it.