[vfio-users] VM doesn't boot if I use GPU passthrough

[Ruben Felgenhauer]

Hi,

I am trying to pass my nVidia GTX 750ti to my QEMU guest.

Problem is: After the QEMU monitor pops up, nothing happens. The GPU's 
output is dead, and the vm won't be accessible via SSH anymore, so it's 
very likely that the VM isn't booting up at all. Also, there are no 
error messages from QEMU on the console whatsoever which makes debugging 
it especially hard.

This is how I start the vm with normal vga emulation:
qemu-system-x86_64 -hda vm.ovl -boot c -enable-kvm -m 1024 -cpu 
host,kvm=off -smp cores=4,threads=2 -redir tcp:5022::22
Everything runs fine in this case. To do the passthrough, I add this:
-device vfio-pci,host=01:00.0,multifunction=on,x-vga=on -vga none
This brings said problems with it. I also tried out multiple different 
combinations of -device's arguments or even adding a romfile for the 
GPU, but none of these steps changed anything at all.

Obviously, I am using a BIOS installation and I'm well-aware with this 
bug: https://bugzilla.kernel.org/show_bug.cgi?id=107561, but neither 
using less RAM (as you can see I am using 1GB now) nor switching to an 
older Kernel changed anything about the problem. I have tried Kernel 
4.1.0 and 4.3.0.

Host is Debian testing with QEMU 2.5.0.
I tried both Debian and Windows 7 as a guest, but both are showing 
exactly the same behaviour.
Mainboard is an ASUS Z87-PLUS. The 750ti is produced by ASUS aswell.

Any idea how I could get passthrough running?