Pop!_os 21.04 black/no login screen when a screen is connected to 2nd gpu (pci pass-through)


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Dec 8, 2021
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i have a Radeon RX6600 for the main system and an GTX 1080 for PCI Pass-Through for a Windows kvm.
Everything works fine*. I can play Steam VR with the 1080 on my Windows kvm (better than on my main system with Proton)
BUT If a screen or VR headset is connected to the 1080 at boot i still see POST & EFI (Radeon output) but when the login screen should appear it stays black. Doesn't show on the 1080 either. I can confirm that the system is running at this point, because i can ssh into it.

I haven't installed any AMD GPU drivers since they ship with the kernel and i've not installed NVIDIA drivers since i shouldn't need them for the main system.

My Setup:
  • MB: Asrock x399 Taichi (UEFI 3.90)
  • CPU: AMD Threadripper 2950x (16c, 32threads)
  • RAM: 32GB (4x8GB)
  • SSD1: M.2 NVMe Samsung 970 EVO 500GB
  • SSD2: M.2 MVMe Samsung 970 EVO 500GB
  • SSD3: M.2 NVMe Samsung 970 EVO Plus 1TB
  • GPU1: Sapphire RX 6600 Pulse (not XT variant)
  • GPU2: MSI GTX 1080 (for PCI pass-through)
  • 8K LG OLED TV as monitor
System partition with "btrfs balance" for a "RAID1" to 2nd 500GB M.2. root and home are btrfs sub-volumes.
The root partitions are encrypted with luks (partition > lunks > LVM > btrfs) and automatically decrypted at boot with a keyfile on an usb drive.

I've made sure that both GPUs are in separate IOMMU groups and that the 1080 (alongside a PCIe USB3 controller) is isolated from the system:

 "user": {
    "kernel_options": [
      "[email protected]",

10de:1b80 = Nvodia GPU
10de:10f0 = Nvidia Audio
1b73:1100 = USB3 Controller Card

title Pop!_OS
linux /EFI/Pop_OS-.../vmlinuz.efi
initrd /EFI/Pop_OS-.../initrd.img
options root=UUID=... ro quiet loglevel=0 systemd.show_status=false splash [email protected] amd_iommu=on amd_iommu=pt vfio_pci.ids=10de:1b80,10de:10f0,1b73:1100 kvm.ignore_msrs=1

Though i'm not sure if this works? It should since i can pass it through to the kvm?

The closed i've come to solve this issue was by editing "/etc/gdm3/custom.conf" and commenting out "WaylandEnable=false". With the result that i had an login screen, but ended in a login loop (login -> immediately back to login screen).

So i hope someone has some advice for me :)

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