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Screen scaling and 4k support in libvirt with qemu/kvm

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  Screen scaling and 4k support in libvirt with qemu/kvm Rainer Dorsch <ml@bokomoko.de> - 2021-01-26 23:20 +0100
    Re: Screen scaling and 4k support in libvirt with qemu/kvm Linux-Fan <Ma_Sys.ma@web.de> - 2021-01-27 10:50 +0100
      Re: Screen scaling and 4k support in libvirt with qemu/kvm Linux-Fan <Ma_Sys.ma@web.de> - 2021-01-27 11:00 +0100
        Re: Screen scaling and 4k support in libvirt with qemu/kvm Rainer Dorsch <ml@bokomoko.de> - 2021-01-28 23:20 +0100

#231199 — Screen scaling and 4k support in libvirt with qemu/kvm

Hi,

with virtualbox, it is possible that the guest system rescales its screen if I 
change the window for the guest (and virtualbox guest tools are installed at 
least). Does anybody know if that is possible with virt-manager/libvirt/qemu/
kvm?

Also I noticed that I cannot scale my guest higher than 1920x1200. Is the max 
resolution configured somewhere or is this a fundamental limitation?

Both my host and guest are Debian 10.

Thanks
Rainer

-- 
Rainer Dorsch
http://bokomoko.de/

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#231204

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Rainer Dorsch writes:

> Hi,
>
> with virtualbox, it is possible that the guest system rescales its screen if  
> I change the window for the guest (and virtualbox guest tools are installed at
> least). Does anybody know if that is possible with virt-manager/libvirt/qemu/
> kvm?

It is certainly possible to set the guest resolution to exactly match the  
window size. I am not sure if it can be configured to _automatically_ adjust  
the guest resolution upon window size change though.

The protocol must support it, because I know that for Windows guest systems,  
the Windows resolution adjusts to the window size. Unlike with Linux guests,  
it does this in fixed steps of "common" monitor resolutions and thus does  
not usually match the exact window size but something smaller.

If you find out how to apply the sizes automatically on Linux, I'd be  
interested to learn how that works :) If no „solution” exists yet, it might  
be easy to script (although I have not bothered to do that so far).

My usual approach to bypass guest system screen resolution issues is to use  
ssh -X from the host to the guest and then run the program under the host's  
X11 session.

> Also I noticed that I cannot scale my guest higher than 1920x1200. Is the max
> resolution configured somewhere or is this a fundamental limitation?

This very much sounds like you are either using the VGA video adapter or do  
not have `spice-vdagent` running in the guest system?

Set Video "Model" to either QXL or virtio in virt-manager and then in the  
guest system install and run `spice-vdagent`. This will not apply the  
resolution immediately, but invoking `xrandr` in the guest it should be  
possible to see a lot larger list of virtual screen resolutions including  
one to match the host window size and including sizes above 1920x1200.

HTH
Linux-Fan

öö

[...]

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#231206

[Multipart message — attachments visible in raw view] — view raw

Linux-Fan writes:

> Rainer Dorsch writes:
>
>> Hi,
>>
>> with virtualbox, it is possible that the guest system rescales its screen if  
>> I change the window for the guest (and virtualbox guest tools are installed  
>> at
>> least). Does anybody know if that is possible with virt-manager/libvirt/qemu/
>> kvm?
>
> It is certainly possible to set the guest resolution to exactly match the  
> window size. I am not sure if it can be configured to _automatically_ adjust  
> the guest resolution upon window size change though.
>
> The protocol must support it, because I know that for Windows guest systems,  
> the Windows resolution adjusts to the window size. Unlike with Linux guests,  
> it does this in fixed steps of "common" monitor resolutions and thus does  
> not usually match the exact window size but something smaller.
>
> If you find out how to apply the sizes automatically on Linux, I'd be  
> interested to learn how that works :) If no „solution” exists yet, it might  
> be easy to script (although I have not bothered to do that so far).

Update: I found out how to enable automatic resizing:

 * Install `qemu-guest-agent` in the VM in addition to running
   `spice-vdagent`
 * Set Video model to VGA

This enables automatic resizing here although from "feeling" the graphics  
performance is worse than with QXL or virtio.

HTH
Linux-Fan

öö

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#231243

Am Mittwoch, 27. Januar 2021, 10:55:53 CET schrieb Linux-Fan:
> Linux-Fan writes:
> > Rainer Dorsch writes:
> >> Hi,
> >> 
> >> with virtualbox, it is possible that the guest system rescales its screen
> >> if I change the window for the guest (and virtualbox guest tools are
> >> installed at
> >> least). Does anybody know if that is possible with
> >> virt-manager/libvirt/qemu/ kvm?
> > 
> > It is certainly possible to set the guest resolution to exactly match the
> > window size. I am not sure if it can be configured to _automatically_
> > adjust the guest resolution upon window size change though.
> > 
> > The protocol must support it, because I know that for Windows guest
> > systems, the Windows resolution adjusts to the window size. Unlike with
> > Linux guests, it does this in fixed steps of "common" monitor resolutions
> > and thus does not usually match the exact window size but something
> > smaller.
> > 
> > If you find out how to apply the sizes automatically on Linux, I'd be
> > interested to learn how that works :) If no „solution” exists yet, it
> > might
> > be easy to script (although I have not bothered to do that so far).
> 
> Update: I found out how to enable automatic resizing:
> 
>  * Install `qemu-guest-agent` in the VM in addition to running
>    `spice-vdagent`
>  * Set Video model to VGA
> 
> This enables automatic resizing here although from "feeling" the graphics
> performance is worse than with QXL or virtio.

Many thanks for the quick response again.

I did not have spice-vdagent installed in the guest system. Installing spice-
vdagent and switching from QXL to virtio was sufficient here.

Interesting side effect:

On the host system, I get

rd@h370:~$ xrandr
Screen 0: minimum 320 x 200, current 3840 x 2160, maximum 8192 x 8192
HDMI-1 connected primary 3840x2160+0+0 (normal left inverted right x axis y 
axis) 609mm x 349mm
   3840x2160     30.00*   25.00    24.00    29.97    23.98  
   2560x1440     59.95  
   2048x1280     59.99  
   2048x1080     24.00  
   1920x1080     60.00    60.00    50.00    59.94    30.00    25.00    24.00    
29.97    23.98  
   1920x1080i    60.00    50.00    59.94    50.00  
   1600x1200     60.00  
   1600x900      60.00  
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x576i      50.00  
   720x480       60.00    59.94  
   720x480i      60.00    59.94  
   640x480       75.00    60.00    59.94  
   720x400       70.08  
HDMI-2 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
rd@h370:~$ 

After installing spice-vdagent on the guest, I get there

rd@debianVM:~$ xrandr 
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
Virtual-0 connected primary 1920x1080+0+0 0mm x 0mm
   3840x2160     60.00 +
   1920x1200     59.95  
   1920x1080     60.00* 
   1600x1200     59.95  
   1680x1050     60.00  
   1400x1050     60.00  
   1280x1024     59.95  
   1440x900      59.99  
   1280x960      59.99  
   1280x854      59.95  
   1280x800      59.96  
   1280x720      59.97  
   1152x768      59.95  
   1024x768      59.95  
   800x600       59.96  
   848x480       59.94  
   720x480       59.94  
   640x480       59.94  
Virtual-1 disconnected
Virtual-2 disconnected
Virtual-3 disconnected
rd@debianVM:~$


Note, the repetition rate (at least virtually) doubles in the guest at 4K.

Pretty cool!

Thanks again
Rainer


-- 
Rainer Dorsch
http://bokomoko.de/

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