Trying to passthrough the first GPU while using the second for host

Managed to disable the first GPU and reserve it for the guest while using the card from my 3rd pci slot (a titan x) for the host.

The problem is that I have no video output from the guest gpu when starting the VM.

dmesg | grep vfio shows something interesting:

[  947.207804] vfio_ecap_init: 0000:42:00.0 hiding ecap 0x1e@0x258
[  947.207826] vfio_ecap_init: 0000:42:00.0 hiding ecap 0x19@0x900
[  947.210200] vfio-pci 0000:42:00.0: BAR 3: can't reserve [mem 0x90000000-0x91ffffff 64bit pref]
[  950.413347] vfio-pci 0000:42:00.0: Invalid PCI ROM header signature: expecting 0xaa55, got 0xffff
[  950.413378] vfio-pci 0000:42:00.0: Invalid PCI ROM header signature: expecting 0xaa55, got 0xffff

lspci:

42:00.0 VGA compatible controller [0300]: NVIDIA Corporation Device [10de:1e07] (rev a1)
	Subsystem: NVIDIA Corporation Device [10de:1e07]
	Kernel driver in use: vfio-pci
	Kernel modules: nvidiafb, nouveau, nvidia_drm, nvidia
42:00.1 Audio device [0403]: NVIDIA Corporation Device [10de:10f7] (rev a1)
	Subsystem: NVIDIA Corporation Device [10de:1e07]
	Kernel driver in use: vfio-pci
	Kernel modules: snd_hda_intel
42:00.2 USB controller [0c03]: NVIDIA Corporation Device [10de:1ad6] (rev a1)
	Subsystem: NVIDIA Corporation Device [10de:1e07]
	Kernel driver in use: vfio-pci
42:00.3 Serial bus controller [0c80]: NVIDIA Corporation Device [10de:1ad7] (rev a1)
	Subsystem: NVIDIA Corporation Device [10de:1e07]
	Kernel driver in use: vfio-pci


0a:00.0 VGA compatible controller [0300]: NVIDIA Corporation GM200 [GeForce GTX TITAN X] [10de:17c2] (rev a1)
	Subsystem: ASUSTeK Computer Inc. GM200 [GeForce GTX TITAN X] [1043:8539]
	Kernel driver in use: nvidia
	Kernel modules: nvidiafb, nouveau, nvidia_drm, nvidia
0a:00.1 Audio device [0403]: NVIDIA Corporation GM200 High Definition Audio [10de:0fb0] (rev a1)
	Subsystem: ASUSTeK Computer Inc. GM200 High Definition Audio [1043:8539]
	Kernel driver in use: snd_hda_intel
	Kernel modules: snd_hda_intel

if you’re using libvirt, can we see your VM’s XML config file?
if not, what qemu commmand do you use to launch the VM?