While exploring virt-manager and Linux Mint Cinnamon Edition, I kept getting the warning that software rendering was in use (instead of hardware acceleration.) The warning asks to open the Driver Manager, which then says no drivers are needed.
The open-source stack for doing accelerated graphics in this case is SPICE, but virt-manager had configured a different graphics stack by default for the “Ubuntu 20.04” OS setting I had used.
For the guest to use SPICE, it needed the
spice-vdagent package to be installed. After that, the guest was shut down, so that its hardware could be reconfigured.
First, the “video” element needed to be converted from
virtio driver, which includes a 3D acceleration option. Once the latter option’s checkbox was ticked, I clicked Apply to save the configuration.
Next, the “display” needed to be changed to support direct OpenGL rendering. I needed to change the Listen to
none, and to tick the OpenGL checkbox. (Doing this in the opposite order, an “invalid settings” message appeared, saying that OpenGL is not supported unless Listen is none.) This revealed an option to select a graphics device, but I only have one, so I left it alone. I then applied the display changes.
Finally, I booted the guest again. Everything seems to have worked, and the warning did not appear.