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[Xen-users] After secondary GPU passthrough, how to output Xorg to monitor ?

I have done a secondary GPU passthrough to a Linux domU, but I don't know how to output Xorg from the secondary GPU.

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 GPU:
`Kernel driver in use: cirrus`

Then, if I connect a VGA 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`

I can send the full verbose boot messages, for the scenarios above, if it helps.

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.

I've spent a long time looking for a solution. I can do everything else I want to, with Xen, except graphics... ! Thanks !

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