It's more complicated than that these days. Rather than HLT, you call MWAIT with an argument that corresponds to the C state that you want to enter - the OS has

It's more complicated than that these days. Rather than HLT, you call MWAIT with an argument that corresponds to the C state that you want to enter - the OS has a better idea than the CPU of how long it's going to be asleep (basically what this article is about), so it can tell the CPU to enter a deeper state. The CPU may make an executive decision based on its own needs to enter a different state (potentially even a deeper one), but it's largely still up to the OS to choose rather than the CPU entering progressively deeper states.