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Re: Various issues when using multiple graphic outputs



On Fri, Sep 04, 2020 at 12:37:06PM +0200, Gerd Hoffmann 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.

IIRC, it Xorg can support multiple devices, but it is not zero-conf,
it needs a custom config file to be written, and has some functional
limitations compared to a multi-head layout. It has been at least 10+
years since I did something like this though :-)

> 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.

This is definitely the better approach to get zero-conf though.

> 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
> 

Regards,
Daniel
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