My initial GPU-attaching attempts were “just attach the Renoir iGPU device”, and I could observe that:
- starting sys-gpu-gui, even with iGPU not attached, even manually after starting with
qubes.skip_autostart, would make the system unusable (I’m being voluntarily vague here) - with sys-gpu-gui freshly installed, even with iGPU not attached, and with no apparent autostart flag set, it gets nevertheless autostarted. Could not tell precisely why, but it seems to have something to do with sys-usb: whenever I start this one, sys-gui-gpu will attempt to start (though
qvm-prefs sys-usbdoes not reveal to sys-gui-gpu).
Trying now to avoid possibly unsupported configuration by attaching the whole set of functions on the same device (which includes all USB controllers, which I detached from sys-usb for a first test)…
- sys-gpu-gui this time does not get autostarted
But then How to enable the (new) GUI VM? - #5 by marmarek talking about rollback does tell we have to unset autostart and default_guivm… so let’s set that and see… I see the system booting, but suddenly reseting, supposedly when trying to start sys-gui-gpu.
Naturally once you have set default_guivm to sys-gui-gpu, if you need to start with autostart disabled and need to start any app in a vm, you need to either default_guivm or the guivm pref for that vm back to dom0. Failing that, you can see the vm consuming CPU cycles… but no window on your screen.
Now I’m pretty sure I recently read in a post some xen and linux boot parameters to avoid the reboot and properly capture the dying words of the system, from @marmarek probably, but can’t find it, and can’t find the info in any “troubleshooting” page either – and just removing rhgb in the hope a cam will get anything useful is not much more than wishful thinking. Sure we could have the info in such a doc 
Edit: despite the difficulty, my cam can still capture part of a message from amdgpu claiming “Hotplug removal is not supported”, followed by a “finishing device” line.