What would this sandboxing achieve? If you mean kernel internally using multiple protection rings, tricky to do in Linux, because the API in that either has to

What would this sandboxing achieve? If you mean kernel internally using multiple protection rings, tricky to do in Linux, because the API in that either has to be tight or it's still not too secure. E.g. if you just take RCU or softirqd into near userspace, as done in RT patchset, but then introduce context switching overhead, life is going to be relatively ok, but you've covered only relatively small parts of the drivers. And RT breaks drivers without even introducing more overhead. It would take a major overhaul.