I think CXL devices in general will become more and more common given that other players on the hardware side are now supporting CXL 1.1 / 2.0 protocols (Sapphire Rapids / 4th Gen EPYC[2]) and it's a way to scale memory past the limitations of a single machine and the number of memory slots / DIMM slots.

For now it's PCIe 5.0, but given that PCIe 6.0[2] will come around with PAM4 signalling, FEC and FLIT mode on CXL 3.0 spec it may be not be as bad or slow with this future configuration.

[1] https://www.amd.com/en/press-releases/2022-11-10-offering-un... [2] https://www.rambus.com/blogs/pcie-6/

Given that macOS has memory compression, it already has two "classes" of memory pages in a storage-hierarchy relationship, with active promotion/eviction. Just mount the CXL "external memory" to a different part of the address space, and default to writing compressed/cold pages there.