Various issues when using multiple graphic outputs

Christian Ehrhardt christian.ehrhardt at
Fri Sep 4 10:41:24 UTC 2020

On Fri, Sep 4, 2020 at 12:37 PM Gerd Hoffmann <kraxel at> wrote:
> On Fri, Sep 04, 2020 at 12:05:08PM +0200, Christian Ehrhardt wrote:
> > Hi,
> > I've had continuous issues with this and wanted to reach out
> > if that is a common issue everyone has or just me lacking a little
> > detail on my setup.
> > - QXL with multiple heads
> >   - works fine (so multi display in general seems fine for the
> >     involved components)
> Yep.
> > - VGA + QXL, QXL + Virtio, Virtio + QXL:
> >   - Guest only detects primary display
> >   - Can't convince X in the guest to use the second device as well
> >   - lspci lists both devices just fine
> >   - dmesg shows the kernel is initializing them, e.g. drm for virtio
> As far I know xorg can't really deal with multiple display devices.
> So typically you have a single device with multiple connectors, so
> you can hook up multiple monitors.
> In the virtual world qxl and virtio support multiple heads, so pick
> one of these two and enable as many heads as you need.

Well my initial target before I realized that it seemed to affect all
combinations in different ways was to get a mediated device to work well.
So If the answer is "just use only one" then I need to find a way to convince
libvirt to -not- re-add a graphics device when the mdev of the vgpu is

> Also make sure
> spice-agent is active in the guest, otherwise operating the mouse is a

I think it was active but surely will give it a second look to be sure
- thank you!

> Dunno how things are with wayland.
> > - 2*QXL
> >   - X has issues to initialize on user login
> >   - no error in the log, just hung
> The windows guest driver doesn't support qxl devices with multiple
> outputs and expects one qxl device per head instead.  So use this
> one for windows guests.
> HTH,
>   Gerd

Christian Ehrhardt
Staff Engineer, Ubuntu Server
Canonical Ltd

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