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Re: [Qemu-discuss] Enabling multiple guest monitors

From: Michael Thayer
Subject: Re: [Qemu-discuss] Enabling multiple guest monitors
Date: Mon, 5 Dec 2016 16:26:50 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

Answering my own question.

03.12.2016 20:59, Michael Thayer wrote:
I was wondering if anyone could give me a hand getting Qemu working with
multiple monitors, preferably virtio-gpu, otherwise qxl.  I am trying to
get Fedora 25 to work better under VirtualBox, and want to compare with
how it works under Qemu.  Currently known problems on our side are that
Mutter is not sending dirty rectangle information when using software
rendering and that absolute input from the tablet emulation is not
translated to the right co-ordinates inside the guest (I think Mutter
expects one tablet per monitor, which we currently do not do, and that
any other Wayland compositor might potentially do this differently).  I
am particularly interested in the second issue.

I am currently using Qemu from git, with all default configuration
parameters except SDL ABI 2.0.  I am starting it with max_outputs=2.
sysfs in the guest says that the second output is present but not
connected.  So far I have not found a way to enable it; I am sure that I
will eventually do so between reading the Qemu source and looking at
code paths in gdb, but if someone has a hint for me it would be very

I was able to achieve this by commenting out the line
  dpy_gfx_replace_surface(g->scanout[i].con, NULL);
in hw/display/virtio-gpu.c and by resizing the second monitor once gnome-shell had started to make it notice it. Probably not quite the way it is supposed to work, but good enough for my needs.

If anyone is interested, Qemu is using VMMouse emulation by default, and GNOME Shell is clearly mapping the absolute positions sent to the entire virtual desktop, while with our device it is mapping all input to the first screen. And Qemu is getting screen updates.

Michael Thayer | VirtualBox engineer
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