I'm sure the vast majority of it could be done from user code (CPU code & compute shaders) and it would be much more profitable for everyone if they'd expose the couple driver hooks they needed to achieve peek performance.
As it is now, it's super opaque and bundled (do you even know what kind of acceleration structure you're using ?) and seems to be going the exact opposite way that GPU programming has been going (more general & programmable).
Haven't tried it yet, but AMD has gone the open-source library way on this. I sure hope it works out well for them.
(also this seems like a nightmare for the wine people)