The UAI feature is nice! Back when I was noodling on creative ways to use 64 bit addresses the idea of a 16 bit 'space id' was bandied about. A space ID in the

The UAI feature is nice! Back when I was noodling on creative ways to use 64 bit addresses the idea of a 16 bit 'space id' was bandied about. A space ID in the upper 16 bits would indicate the address space for a particular thing, be it a process, the kernel, a shared library, etc. And a new opcode to trampoline from one space to another for doing things like system calls or library calls, could provide a gateway for doing secure code analysis (basically no "side" entrances into a space) and it would be a much more robust pointer testing mechanism because any read or write with a pointer "outside" the space would be immediately flagged as an error. The use of segments was sort of like this on the 286. Ideally you'd have a fine grained page map so that you could avoid copies from one space to another (instead of copy_in()/copy_out() you would do map_in()/map_out()) to get access to physical memory, while preserving the semantic that when something was mapped out of your space you couldn't affect it.(zero copy network stacks have been known to have issues when a process left holding a pointer to a buffer modified it when it wasn't "owned" kinds of bugs) Could be a fun project for a RISC-V design. Not sure how many people are experimenting with various MMU systems but it is at least possible to do these kinds of things in nominally "real" hardware (it has always been possible to simulate of course).