How to enable high DPI display scaling in KVM guest?

Question

I've just got brand new Lenovo P1 gen7 with 16" with decent 3840x2400 resolution.

Fedora 42 handles it pretty well: it let me set up scaling to whatever suits me, and that's all. However KVM guest does not seem to read the screen size properly. Vm's icons and texts are extremely tiny. It's just annoyng to set it up internally for every VM I'm runnig and change it every time I start it on other display. Is it possible to make qemu/KVM/libvirt somehow report the accual size of the window/screen in DPI or mm, so VM could decide if it should run on 1:1 or in scaling mode in more-less automatic manner?

Answer 1

There's two ways your virt-manager might be exchanging what is displayed with your VMs:

1. VNC
2. SPICE

VNC has several incompatible extensions, of which (I think) tigerVNC's extensions support sending over DPI information (but nothing makes use of that). However, VNC as implemented by Qemu does not support any of that.

SPICE to the best of my knowledge (and according to a search in its protocol definition) has no provisions for transporting display pitch.

So, nope, not possible.

What you can do is set the display resolution in the guest lower and let set the view to be scaled (under "View" menu when running the VM), which will not be great for text.

Now, that's not set in stone. A reasonable course of action would be to

1. Consult the SPICE protocol https://www.spice-space.org/spice-protocol.html
2. understand the "Display Channel definition" section, especially SPICE_MSG_DISPLAY_MODE, and add another message like SPICE_MSG_DISPLAY_PROPERTIES, that would allow for transport of dpi (and eventually things like maybe color profiles, preferred refresh rates).
3. implement that in the Spice guest agent and in the client library
4. ????
5. Profit!