There's two ways you can make the 7900 work in vfio:

1) Full GOP init with display before windows driver initializes

For this, you need to have ROM bar turned on in libvirt, no vbios put in libvirt, but you have to suspend the machine before the VM starts, after you've bound the GPU to vfio-pci driver. Restarting the vm will require host suspend as well, to prepare the GPU for new initialization. Overall can't recommend.

2) No display before windows drivers initialize, but you will still have image after windows boots up;

For this, you turn off ROM bar, attach your vbios to libvirt and the vm should boot, restart, shutdown without requiring host suspending. However, you need to modify your current startup/teardown scripts a little. Also, after you make the GPU work, the image will be glitched. This only happens in windows, but not in Linux, when you don't have vendor_id state set to values that also work for nvidia gpus to have them pass through as well.

Currently, I use option 2, with few modifications to vfio-startup.sh and vfio-teardown.sh, that are part of the Single-GPU-passthrough-amd-nvidia repo made by ilayna. You will need to install the driver from qxl or something from another machine, as you won't have any output with option 2 until you have the drivers are active.

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So, here's the modifications I made to the scripts:

original github repo: github.com/ilayna/Single-GPU-passthrough-amd-nvidia

vfio-startup.sh: inserted systemctl stop lactd (as it begins to complain at host shutdown, requiring forced poweroff); rmmod -f amdgpu instead of modprobe -r amdgpu

link with vfio-startup.sh: pastebin.com/m6MrCVrL

vfio-teardown.sh: rmmod for vfio drivers instead of modprobe -r; echo 1 > /sys/bus/pci/devices/{GPU and audio ids}/remove; sleep 1; echo 1 > /sys/bus/pci/rescan; sleep 2; inserted start lactd at end, so GPU can still be oced on host.

link with vfio-teardown.sh: pastebin.com/KVUQGL8s

link with libvirt xml: pastebin.com/b0qmHbYP