[vfio-users] Console output on the host stops after launching a VM with vfio GPU

[Alex Williamson]

On Tue, 9 May 2017 04:40:43 +0500
Roman Mamedov <rm at romanrm.net> wrote:

> Hello,
> 
> It feels like I'm missing just some small thing...
> 
> Got the vfio passthrough working quite easily with a modern GPU, but when I
> launch a VM with the GPU passed through, all console output on the host
> (which is booted to text console, no framebuffer) freezes: even though it's
> still shown on screen, it does not refresh. Keypresses seem to be accepted and
> stuff like NumLock or Alt-Sysrq-B works.
> 
> Host's primary boot video card is Matrox MGA 2164W [Millennium II]. Secondary
> GPU (passed through into a VM): NVIDIA GeForce GT 710 (10de:128b).
> 
> The guest VM can use the GPU including 3D absolutely fine, it's just that the
> host console is frozen.
> 
> I set up passthrough with "-vga none -device vfio-pci,host=07:00.0,x-vga=on
> -device vfio-pci,host=07:00.1". Thought maybe x-vga=on is to blame, but
> without that doesn't appear to work properly.
> 
> Any ideas how to fix this? 
> 

http://awilliam.github.io/presentations/KVM-Forum-2014/#/

On the "Issues:" slide, the line about drivers not participating
(vgacon)... that's the problem.  Watch the video, also see:

http://vfio.blogspot.com/2014/08/whats-deal-with-vga-arbitration.html

The host needs to be running a driver that participates in VGA
arbitration or else the first access to VGA space by the guest is going
to switch VGA routing to the assigned device and the driver running the
host graphics will never switch it back.  An even better option is to
use a UEFI capable graphics card with an OVMF guest and drop the x-vga
option.  Thanks,

Alex