What Are Intel VT and AMD Pacifica?

Under the covers, virtualization software faces a difficult technical challenge. Modern OSs, such as Windows, Unix, and Linux, are all designed with a privileged low-level kernel and device drivers that talk directly to the hardware and a nonprivileged user space in which most OS services and all applications run. The CPU hardware is designed to prevent processes running in user space from directly accessing kernel-level processes or memory. In the Intel-designed x86 world, the CPU has four different privilege levels, with the most privileged called “Ring 0” and the least privileged called “Ring 3.” A process running at a given ring cannot affect components running at a lower-numbered ring except through carefully controlled entry and exit points. All standard OSs for the Intel architecture are written to run the kernel in Ring 0, and all nonprivileged processes run in Ring 3. (Rings 1 and 2 were never used, and support for them was eliminated in later x86 server models to provide support for the 64-bit extensions.)