The virtues of putting virtualisation on the chip

Virtualisation at the chip level can do away with complex code to modify the operating system, as processes are handled by the hardware.


Brian Gammage pulls no punches when he assesses the importance of hardware-assisted virtualisation: "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 virtualisation.

Even Intel and Advanced Micro Devices haven't explained adequately how significantly the technology affects server virtualisation, he adds.

Server virtualisation is a watershed IT technology because it lets a single physical computer run multiple operating systems, vastly increasing rates of CPU use. But server virtualisation 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 virtualised.

Essentially, VMware's early virtualisation 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, virtualisation software "fools" an operating system into thinking it's running at Ring 0 - the most privileged ring - when it's really not.

Hardware-assisted virtualisation 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.

Embedded routines

Supporting virtualisation at the chip level greatly reduces the amount of virtual-machine code needed, in part because routine operations now are handled in hardware. As XenSource CTO Simon Crosby says, "The more features in the hardware, the more code we can throw away."

Indeed, hardware-assisted virtualisation gives second-tier virtualisation vendors an opportunity to catch up to VMware, which established a clear lead over competitors by developing workarounds to many hardware limitations, says Frank Gillett, an analyst with Forrester Research.

XenSource, which sells products related to the Xen open source hypervisor, is among VMware competitors benefiting from hardware-assisted virtualisation, as is Virtual Iron.

Previously, Virtual Iron supported Linux because it could be rewritten to its purposes, says Alex Vasilevsky, the company's CTO. Now, without the need for modifications, it can support Windows, too, he says.

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