[vfio-users] "No signal" on dual Nvidia setup

[Nicolas Roy-Renaud]

For the last few days, now, I've been trying to get a gpu passthrough to 
work on my computer, but I haven't been able to get the VM to output 
anything on my passthrough monitor at all since I started (I've had to 
either rely on a QXL adapteror just boot on the drive bare metal). 
Here's my situation :

I'm using 2 dedicated NVIDIA GPUs. One is a 970 GTX from Asus which I 
want to pass through (PCI:01:00.0; IOMMU group 1) and the other is an 
old OEM 210 GT which I'm going to be using to run the host (PCI:06:00.0; 
IOMMU group 14). Since the 970 is set at my primary GPU, it is 
responsible for displaying my bios and bootloader until linux boots, 
where I have its framebuffer disabled and vfio-pci latch onto it. The 
210 GT, however, is still managed by the nouveau driver. Note that from 
the moment linux starts up until I try running a VM attempting to access 
the passthrough, the framebuffer for the guest card remains untouched 
and keeps showing my bootloader (systemd-boot). It gets flushed as soon 
as I start my Windows VM, and from there on receives no signal.

My CPU (Xeon E3 1230v2) and motherboard (MSI Z77-G43) both seem to be 
compatible with IOMMU. I've looked into the GPU rom, which does appear 
to support EFI according to rom_parser, even if TechPowerUp says is 
shouldn't, and whether I try injecting one or using the embarked one 
doesn't change the final result (although I do get "Invalid ROM content" 
warnings if do the later). Trying to boot the guest with an extra QXL 
video adapter forces Windows to disable the guest card, and reenabling 
it causes an immediate blue screen (and nothing on the punitor plugged 
on the guest card). No error 43 there (yet), although I am using qemu 
2.5 with the cpu's hv_vendor_id blanked out. x-vga flat out refuses to 
work, as my guest GPU doesn't support it according to QEMU.

I'm not really sure where to go from there, so I thought I'd at least 
try my luck here before giving up. Actual logs will follow.