- xHCI support for virtualization described in chapter 8 of the spec: https://www.intel.com/content/dam/www/public/us/en/documents/technical-specifications/extensible-host-controler-interface-usb-xhci.pdf
- Initial (ancient, assignment only) implementation for USB para-virtualization on Xen by Fujitsu http://www-archive.xenproject.org/files/xensummit_tokyo/26_NoboruIwamatsu-en.pdf
- New (more mature but still incomplete, with sharing) implementation of USB pararvirtualization on Xen by OpenXT https://github.com/OpenXT/pv-linux-drivers/tree/master/openxt-vusb
- ACRN also has an implementation for USB virtualization: https://projectacrn.github.io/latest/developer-guides/hld/usb-virt-hld.html
See also minutes from February 12, February 19 and April 23.