MIDI has never been userland in multitasking environments. It's a real-time, low-latency interface between some number of running processes and some number of h

MIDI has never been userland in multitasking environments. It's a real-time, low-latency interface between some number of running processes and some number of hardware ports: the timing glitches and weird buffer behaviors you'll get from mixing together different hardware are totally kernel's wheelhouse. If you meant General MIDI synthesis, that's a different spec and related only in that the MIDI message encoding is reused to describe sequences.