Strangely, seabios supports booting from usb-uas with usb-ehci: https://drive.google.com/open?id=0B50PCxfm5KU1dUFTUG9HU0pEcXc
and usb-bot with nec-usb-xhci: https://drive.google.com/open?id=0B50PCxfm5KU1QnVzdTBIdEFzZW8 but not usb-uas with nec-usb-xhci: https://drive.google.com/open?id=0B50PCxfm5KU1Z0t1akNXaVZZLTQ This makes me wonder if it is just a missing feature of a bug. Btw, not sure if it's relevant, booting usb-uas is not at all supported in ovmf: https://github.com/tianocore/edk2/issues/85
