dmytrokyrychuk / spice-autorandr

Automatically adjust the client window resolution in Linux KVM guests using the SPICE driver

spice-autorandr

The problem

Since some time, the spice client (or the X driver? not shure) does no longer automatically resize the window if the virt-viewer window is resized. Instead, it is up to the desktop environment to receive the RandR notification and act upon it. Unfortunately, not all desktop environments have this implemented yet (I know only of GNOME, maybe plasma/KDE has, too).

The solution

First I created a small script, basically doing (pseudo code, but you get the idea)

xev --root --event randr | \
while read e; do
    ignore_line e && continue
    xrandr --output Virtual-0 --auto
done

with the ignore_line function (not shown) containing some magic, that avoids calling xrandr too often.

Since a bash script seemed slightly unwieldy for this, I implemented similar functionality in C.

Building

You need (at least) the following devel-packages installed:

• xrandr
• x11
• xproto
• xorg-macros

On an openSUSE system, zypper install 'pkgconfig(xrandr)' 'pkgconfig(x11)' 'pkgconfig(xproto)' 'pkgconfig(xorg-macros)' should install all that's needed. gcc and make are of course also needed.

Building should be easy:

./configure
make

If you are building directly from a git checkout, prepend these with autoreconf -is.

Technical details

This is the actual xrandr output of a VM (some resolutions omitted):

tux@factory-vm:~> xrandr
Screen 0: minimum 320 x 200, current 1024 x 680, maximum 8192 x 8192
Virtual-0 connected 1024x680+0+0 0mm x 0mm
   1024x680      59.91*+
   1280x1024     59.89  
   1280x960      59.94  
   1280x800      59.81  
   1280x720      59.86  
   1024x768      59.92  
   800x600       59.86  
   640x480       59.38  
Virtual-1 disconnected
Virtual-2 disconnected
Virtual-3 disconnected

Now I'm resizing the virt-viewer window:

tux@factory-vm:~> xrandr
Screen 0: minimum 320 x 200, current 1024 x 680, maximum 8192 x 8192
Virtual-0 connected 1024x680+0+0 0mm x 0mm
   1024x582      59.94 +
   1280x1024     59.89
   1280x960      59.94
   1280x800      59.81
   1280x720      59.86
   1024x768      59.92
   800x600       59.86
   640x480       59.38
Virtual-1 disconnected
Virtual-2 disconnected
Virtual-3 disconnected
  1024x680 (0x28c) 56.250MHz -HSync +VSync
        h: width  1024 start 1072 end 1176 total 1328 skew    0 clock  42.36KHz
        v: height  680 start  683 end  693 total  707           clock  59.91Hz

The interesting line is the

1024x582      59.94 +

The + telling us, that it's the "preferred" mode, whatever this means. If you now run ```xrandr --output Virtual-0 --auto``, this will do what we want, resizing the screen to 1024x582.

I then found, that the preferred mode is always the first mode in the list. This allows to use xrandr -s 0 instead of specifying the output.

xrandr -s uses legacy XRandR code (XRandR version less than 1.2) which in turn makes this much easier to code, this is why I went for this route :-)