Oh, regarding GPUs, AMD started to experiment on userland _hardware_ ring buffers... I don't know how they will handle their scarse VM id resources... the kernel may end up "only" mmaping event/command ring buffers and data dma/doorbells buffers with an "IRQ" event file descriptor.

We are talking a "near 0-driver"... but they will have to be very confident in their GPU robustness to do that, not to mention 3D pipeline programming from those userland _hardware_ buffers will have to be really simple and directly "ready" to work.