Various issues when using multiple graphic outputs

[Gerd Hoffmann]

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

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