This documentation is out-of-date. Since January 2019, an official documentation is available.
https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-3200007
virtio-gpu-documentation
Documentation for 3D commands on Virgl3D. Not an official documentation. THIS DOCUMENTATION IS OUTDATED.