If you are pinned to a core, as non burstable instances should be, you are still pretty much vulnerable. Having noisy neighbours will pollute caches and make extracting data harder, but, eventually, everything will leak out.

I’ve been thinking what would happen if cores would be pinned to separate security domains - all kernel processes run on one set of cores and user processes on others. I imagine microkernel OSs could go that way much easier. If kernel and user space communicate only by messages and shared data, there’s no reason they’d need to even share an ISA.

Isn't the entire memory hierarchy (caches) part of the problem?

Yes. You’d need to split caches by security context too.

Maybe each security context needs to be in its own cloud account and own network. Long ago it was an axiom that if you share a call stack in a process you trust each other. Perhaps the current truth is if you share hardware you trust each other.

That should work with multiple services, but not in a user/kernel split as we have today.