If you're on bare metal, then you have access to supervisor mode. I like the approach of hiding cpuid registers from user-space. It means, for example, the ope

If you're on bare metal, then you have access to supervisor mode. I like the approach of hiding cpuid registers from user-space. It means, for example, the operating system can easily control what hardware features user-space will use. That's useful for debugging, for process migration, for record and replay (I maintain rr), and to "discourage" user-space from using buggy or detrimental features. Simplifying the hardware features exposed to user-space makes evolution easier. Your OS kernel should be (needs to be) up to date, while you might have old user-space binaries that you want to keep running, so you can live with weaker compatibility guarantees for supervisor-mode-only features.