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A battle as to how Linux will handle future virtualization software from the likes of VMware and Xen has moved from a war of words to a war of indecision. The major parties involved - including Linux kernel maintainers - agree that a compromise over the virtualization interface must be reached, but no one seems to know exactly how to achieve this goal.

This particular virtualization war started last July when VMware introduced the Virtual Machine Interface (VMI) at the Linux Symposium - an event aimed at kernel developers and others touching the Linux operating system. VMware's engineering team hoped to give the Linux crowd, including competing Xen developers, an idea of how it planned to handle the latest and greatest form of server virtualization known as paravirtualization, which requires kernel level changes. VMware's pitch centered on a type of open interface that any server virtualization vendor - be it VMware, XenSource (the major backer of the open source Xen) or Microsoft - could plug into with their future products.

The idea of an open paravirtualization interface doesn't rankle the Xen crowd, but VMware's particular approach does. Some members of the Xen camp argue that VMI does not perform well enough to become the standard Linux interface. In addition, the most ardent open source backers question the long-term motives of a proprietary software vendor such as VMware and wonder if an "open" interface won't end up benefitting VMware the most down the road.

The last bit of thinking - some would say over-aggressive politicking - reflects how seriously folks take this issue. Xen, you see, has put forth a "Hypercall" paravirtualization interface of its own. The Xen backers once fully expected the Hypercall APIs to become part of the Linux kernel, and were caught off guard recently when kernel maintainer Andrew Morton was characterized in a media report as backing VMware's VMI interface over the Xen code.

So, which approach is better? Which one will make its way into the Linux kernel? And why should anyone care?

As it turns out, no camp - including Morton's Linux folks - can claim much certainty around the interface issue or how it will progress.

Morton, for example, said his preference for VMI has been "overstated".

"VMware have proposed an implementation that would allow, in theory, different kinds of hypervisors to run beneath the kernel," Morton said, in an interview with The Register. "It is, if you like, a hypervisor-neutral interface. The question remains if we want to have a hypervisor neutral interface. There might be the case that this generic interface isn't going to buy us anything, so maybe we will just merge with Xen.

"That decision has not yet been made. We need to step back and have a think about how we are going to do this."

Morton admits that he knows "very little" about the inner-workings of hypervisors - the core software layer used by some to manage virtual servers. With that in mind, he wants to see VMware and Xen developers hammer out the interface issue without involvement from him or Linus Torvalds.

"Any time Linus or I have to make a decision, the system has failed," he said, noting that the kernel crew is eager to see this particular issue resolved.

The major problem, however, is that VMware and Xen developers don't seem very close to having a meeting of the minds.

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