Just terminology. Whenever a new higher-privileged entity is created, it is sometimes described as a negative ring.
There was a time when people thought if we could put the secure code in a lower ring, then with it we could protect the rest of the system. With virtualization, the hypervisor is in ring -1, which is technically not a ring, but rather a mode called VMX root operation, post-VMXON. This enables things like the blue pill attack, where the hypervisor is itself presented with a false image of the underlying physical hardware, by a malicious layer. You can find the same pattern in ARM TrustZone, where the secure code is repeatedly broken.
It's the same reason that the nominal thrust level on the Space Shuttle Main Engines is 104.5%[1].
No, not 100% (it was originally), 104.5%. Why? Because you don't go back and change all your rules and documentation following subsequent developments in the field, that causes unnecessary confusion and errors down the road.
Comments
Just terminology. Whenever a new higher-privileged entity is created, it is sometimes described as a negative ring.
There was a time when people thought if we could put the secure code in a lower ring, then with it we could protect the rest of the system. With virtualization, the hypervisor is in ring -1, which is technically not a ring, but rather a mode called VMX root operation, post-VMXON. This enables things like the blue pill attack, where the hypervisor is itself presented with a false image of the underlying physical hardware, by a malicious layer. You can find the same pattern in ARM TrustZone, where the secure code is repeatedly broken.
"if only we had ring -N"
Crazy complexity. Mind blown!
It's the same reason that the nominal thrust level on the Space Shuttle Main Engines is 104.5%[1].
No, not 100% (it was originally), 104.5%. Why? Because you don't go back and change all your rules and documentation following subsequent developments in the field, that causes unnecessary confusion and errors down the road.
[1]: https://en.wikipedia.org/wiki/RS-25#Engine_throttle/output
Likewise. Engineers do not idle well.