Linux brings VMware into the fold

Linux programmers are moving toward a change that would put virtualisation software from VMware on a more even footing with open source rival Xen.

Xen was expected to be built tightly into the Linux kernel at the heart of the open source operating system. But Andrew Morton, a key deputy to Linux leader Linus Torvalds, is advocating an interface in the Linux kernel that would let it work with any virtualisation foundation.

Virtualization generally refers to software and hardware that let a single computer run multiple simultaneous operating system instances. It is useful for making servers more efficient and isolating desktop applications into non-interfering partitions. Xen, Microsoft and VMware all are working on software called a hypervisor that governs how those virtual machines get access to the hardware resources.

Morton said he prefers a neutral interface that works with any hypervisor, rather than the Xen-specific patch to Linux that had been envisioned.

"For a long time, it was thought that we'd just merge the Xen patches as-is and be happy. But then, Linux would only run on Xen," Morton said. Instead, VMware programmers suggested a documented, stable interface between the kernel and the hypervisor — and they're preparing one, he said.

"From a high-level design perspective, I think that VMware's point is a good one, and that a general kernel-to-virtual machine interface is a better thing than a Xen-only one," Morton said.

XenSource and VMware both are fine with the change, but VMware gets a place at the table it lacked before.

"Anything that levels the playing field for different people — that's going to be good for everyone, but certainly good for us as well," said Dan Chu, the senior director of developer products at EMC subsidiary VMware.

The issue comes up with a new generation of "paravirtualisation" technology that offers better performance than VMware's current approach, but that requires changes to the operating system. If software companies adopt the kernel interface, that means that they and their customers won't have to worry about different versions of software for real or virtual machines, or different hypervisors, said Jack Lo, VMware's senior director of research and development.

Morton said the Xen programmers haven't been active in the interface work. "This has been floating around for a year, and I've heard precious little from the Xen team on the topic," Morton said.

But Xen has a similar approach to the VMware interface, called VMI, and the two are converging, said Xen founder Ian Pratt. "About 75 or 80 percent of the code structures are identical. It's that common ground that hopefully should make its way into Linus' and Andrew's kernels shortly," he said. "The discussion on all of those has, I think, yet to begin."

Part of the problem has stemmed from branding issues, Pratt said. "They object to putting patches into Linux having the 'Xen' prefix on all the function names. We're miffed about it being called VMI, since we did the hard work of interfaces and defining the patches," he said.

VMware didn't mean for VMI to stand for VMware — it's an abbreviation for the generic Virtual Machine Interface — but the company is happy to adjust names as necessary, Lo said.

The Open Source Development Labs has taken an active role in trying to clean up the situation, Pratt added. "OSDL has volunteered to set up meetings to get this stuff discussed," he said, noting they would possibly be set up through a virtualisation task force.

How long until Xen merges?
Morton said the Xen programmers have work to do before they can expect their patches to be incorporated into Linux. That inclusion into the mainline kernel makes programming and certification tasks much easier for Linux sellers such as Red Hat and Novell's SuSE.

"I've seen little from the Xen team, period. I see that Xen has been snuck into Red Hat and possibly SuSE kernels, but right now I'd say it's a long way from making it into mainline," he said. "We really haven't even started looking at the code and discussing it."

The merge work is being handled by programmers at Red Hat and Novell's SuSE who have closer ties with the main kernel programmers, Pratt said, adding that he's optimistic it will be uncontroversial and take place soon.

"There's a big block of code everyone can agree on. I think that will go in very soon," Pratt said. "By September, I would hope that everything is in there."