It could be shared memory all the way through, but the memory of the original VM should become read-only once a cloned VM starts reading from it. So then both VMs (the original VM and the new VM) should put their writes in a new CoW layer.

Using XFS with CoW has been the easiest way to enable this, but if there's a way that we can do this purely in-memory, that would be even faster.

That said, for hibernation we would still have to persist to disk, but timing is less important there.