I’ve layered gnome-boxes on stock Fedora Silverblue 35. In the properties for a given guest box, 3D acceleration is not available.
After also layering virt-manager, qemu-device-display-virtio-vga, and qemu-ui-opengl, 3D acceleration for the guest (under QEMU/KVM User session connection) can be enabled with virt-manager (Video Virtio > 3D acceleration, Display Spice > OpenGL), and this works, but running the guest using gnome-boxes reverts the virt-manager selections and 3D acceleration is again not available to the guest.
Is there a way to make 3D acceleration for guests available using gnome-boxes?
Find out where the config files are from virt-manager and incorporate them in the config of gnome-boxes.
Both apps are back-ends for libvirt and use separate config paths. That’s why you not have the same config while change one of them.
I can confirm this, sort of. Tinkering with gnome-boxes from Nightly GNOME Apps, the ‘3D Acceleration’ switch mentioned here does not appear, however the app includes an ‘Edit Configuration’ button and from there it’s possible to copy the Video virtio & Display spice settings from virt-manager’s XML view. 3D Acceleration then works but the app still regularly reverts the config requiring manual editing again.
According to the libvirt faq, editing the on-disk XML files can be problematic, so the files I see at, e.g., ~/.config/libvirt/qemu/ubuntu20.04.xml (for layered gnome-boxes) or ~/.var/app/org.gnome.BoxesDevel/config/libvirt/qemu/ubuntu20.04.xml (for flatpak gnome-boxes), are probably just dumps, because editing them has no effect.
This is probably the bottom of the rabbit hole, and I’ll just wait to see if this feature is ironed out for GNOME 42’s gnome-boxes.