Firstly, on Windows 11 using ffmpeg 6.0, ffmpeg -i Downloads/FPS_test_1080p60_L4.2.mkv -c:v vp9_qsv -f null test just hangs. So for whatever reason a Windows guest cannot use VP9 encoding.
Secondly, when a Windows 11 guest is running a h264/h265 encode (Eg. ffmpeg -i Downloads/FPS_test_1080p60_L4.2.mkv -c:v h264_qsv -f null test) and a linux guest (sharing the same partitioned iGPU) starts a VP9 encode, the Window's encode locks up. Stopping the linux encode allows the windows encode to proceed as expected.
I haven't tried a linux-linux scenario.
Isn't this bad behavior? It seems like a linux guest is able to deny access to the quicksync encoder for any other Windows guest.