2024-04-23
Libvirt's virt-viewer and (guest) screen blanking
One of the things that I sometimes need to do with my libvirt-based virtual machines is connect to their 'graphical' consoles. There are a variety of ways to do this, but generally the most convenient way for me has been virt-viewer, followed by virt-manager. Virt-viewer is typically pretty great, but it has one little drawback that surfaces with some of my VMs, especially the Fedora ones that boot into graphics mode. The standard behavior for Fedora machines sitting idle in graphics mode, especially on the login screen, is that after a while they'll blank the screen, which winds up turning off video output.
On a physical machine, the way to un-blank the display is to send it some keyboard or mouse input. Unfortunately, once this happens, all virt-viewer will do is display a 'Connected to graphics server' message in an otherwise inactive display window. Typing, clicking the mouse buttons, or moving the mouse does nothing in my environment; virt-viewer seems to be unwilling to send keyboard input to a virtual machine with a powered-down display.
(Virt-viewer has a menu that will let you send various keystrokes to the virtual machine, but this menu is greyed out when the virt-viewer sees the screen as blanked.)
My traditional way to fix this was to briefly bring up the virtual machine's console in virt-manager, which would reliably unblank it. With the console display now active, I could switch back to virt-viewer. Recently I discovered a better way. One of the things that virsh can do is directly send keystrokes to a virtual machine guest with 'virsh send-key'. Sending any keys to the guest will cause it to un-blank the screen, which is the result I want.
(The process for sending keys is a little bit arcane, you'll want to consult either virtkeyname-linux or maybe virtkeycode-linux, both of which are part of the libvirt manual pages.)
What key or keys you want to send are up to you. Right now I'm sending an ESC, which feels harmless and is easy to remember. If I was clever I'd write an 'unblank' script that just took the virtual machine's name and did all of the necessary magic for it.
(And someday hopefully this will all be unnecessary because virt-viewer will learn how to do this itself. Possibly I'm missing something in virt-viewer that would fix this, or something in libvirt machine configuration that would disable screen blanking.)