CXL 2: Pooling, sharing, and I/O-memory resources

The kernel has had at least some support for memory disappearing underneath it for a long time -- ECC RAM can say "whoops, this RAM suddenly doesn't work anymore, sorry!" and the kernel will try to recover gracefully. (E.g. it might kill the process that owns that memory but the rest of the system will keep going.)

I have no idea how well or poorly this will extend to CXL failure modes, but it's at least precedented.