Xen: Graphical output on domU after secondary GPU passthrough

I have done a secondary GPU passthrough to a Linux domU, but I don't know how to output Xorg from the secondary GPU. The dom0 is using an Intel integrated graphics card. I am passing through an AMD 6450 to domU, as it there were two mentions of it here:
https://wiki.xenproject.org/wiki/Xen_VGA_Passthrough_Tested_Adapters Short list of what I did: 1. Enabled VT-d in motherboard BIOS 2. Added intel_iommu=on to dom0 GRUB 3. Blacklisted modules of GPU and it's audio subsystem 4. modprobe xen-pciback 5. xl pci-assignable-add _______ for both GPU and it's audio subsystem 6. Added the two PCI IDs to the Xen configuration file: pci = [ '_____', '_____' ] 7. Booted and installed Linux in the domU When I boot, after the above steps, I get one error message: libxl: error: libxl_pci.c:1151:libxl__device_pci_reset: The kernel doesn't support reset from sysfs for PCI device 0000:01.00.0 Despite the above error, when I run lspci -nnk on the domU, it shows the GPU and audio sub-device, including these lines: Kernel driver in use: radeon Kernel driver in use: snd_hda_intel Also, of course, a line for the primary domU virtual GPU: Kernel driver in use: cirrus Then, if I connect a VGA or DVI cable from the passed through GPU to the monitor, the monitor stays on (green power button) with a dark grey screen. Whereas usually, when the monitor detects no input, it just goes straight to standby (orange power button). So there is some kind of GPU output. If I startx and run xrandr through VNC, there doesn't seem to be any sign of the physical monitor, it seems there is only the virtualized VNC/cirrus monitor. If I add gfx_passthru=1 to the configuration file, it always fails at boot. The first error message that appears when I add gfx_passthru=1 is: libxl: error: libxl_dm.c:1302:libxl__build_device_model_args_new: unable to detect required gfx_passthru_kind Before I ever used virtualisation, to switch output from primary/secondary graphics card to the other, I would usually just change the settings in the BIOS, and Linux would follow. So Xorg + domU is very different territory for me. Does anyone know how I can output Xorg through the secondary GPU in the domU, after this point ? Various GPU passthrough guides online only cover Windows. **UPDATE** After StrongBad's response, I have moved from an Arch domU to a Debian 8.8 from the Debian wiki domU, as fglrx is not in the official Arch repos anymore. However, unlike the Arch domU, the Debian domU has no mysterious dark grey monitor output with always-on green power button, when the passed through GPU is connected to the monitor. Nonetheless, I did the following: 1. Installed fglrx 2. Ran sudo aticonfig --initial 3. Added the four lines to /etc/xorg.conf from the Debian wiki However, when I run startx from a VNC TTY, it just crashes. The first error message in Xorg.0.log states: (EE) fglrx(0): Failed to open CMMQS connection