The virtues of putting virtualization on the chip

Brian Gammage pulls no punches when he assesses the importance of hardware-assisted virtualization: "This is the most significant architectural change we've seen in the x86 processor in 25 years," says Gammage, Gartner's lead analyst on PC virtualization. Even Intel and Advanced Micro Devices haven't explained adequately how significantly the technology affects server virtualization, he adds.

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Server virtualization is a watershed IT technology because it lets a single physical computer run multiple operating systems, vastly increasing rates of CPU use. But server virtualization also is a highly complex process, and many vendors over the years have been stymied in their attempts to create good virtual machine software. VMware, on the other hand, figured out how to build a binary translator that scans the issue of privilege-instructions processors to operating systems and rewrites the ones that can't be virtualized.

Essentially, VMware's early virtualization software tricked the operating system, Gammage says. Earlier processors contain four privilege levels, which create security boundaries -- they're like one-way doors, he says. A process running in Ring 1 had to ask Ring 0 for permission to access objects to which Ring 1 normally wouldn't have access. Under this setup, virtualization software "fools" an operating system into thinking it's running at Ring 0 -- the most privileged ring -- when it's really not.

Hardware-assisted virtualization changes all this by doubling the number of a processor's privilege levels. If the chip has a greater number of privilege levels, modifying the operating system becomes unnecessary, Gammage says.