Hypervisor Mode
Recent CPUs from Intel and AMD offer x86 virtualization instructions for a hypervisor to control Ring 0 hardware access. Although they are mutually incompatible, both Intel VT-x (codenamed "Vanderpool") and AMD-V (codenamed "Pacifica") create a new "Ring -1" so that a guest operating system can run Ring 0 operations natively without affecting other guests or the host OS.
"To assist virtualization, VT and Pacifica insert a new privilege level beneath Ring 0. Both add nine new machine code instructions that only work at "Ring -1," intended to be used by the hypervisor. "
Read more about this topic: Ring (computer Security)
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—Ralph Waldo Emerson (1803–1882)