Source: https://www.reddit.com/r/jellyfin/comments/cig9kh/nvidia_quadro_p400_passthrough_on_proxmox/
Ok, finally got it working. I'll provide a writeup here of my steps. But these are for Proxmox with an Ubuntu 1810 container only. If you want to use a different setup, you will have to figure it out yourself.
- Install the Proxmox Linux Headers. The version should match your kernel
apt install pve-headers-$(uname -r)