virtio_balloon(4): how to tell if it's doing anything?

I'm running 14.0-STABLE (GENERIC kernel) in a virtualbox VM. It looks as if this driver is built in to the GENERIC kernel.

I can command virtualbox on the hosting system to inflate the memory balloon, but I can't see if it is taking effect. sysctl -a doesn't seem to emit any virtio balloon information.

How can I tell how much memory virtio_balloon is reserving?