Red Hat and Microsoft might seem unlikely bedfellows, but they have something in common. Both believe virtualization is a feature of the operating system -- Microsoft through the addition of Hyper-V to Windows Server 2008, and Red Hat through the addition of the KVM hypervisor to the Linux kernel.

And they both want to compete with market leader VMware, which views virtualization as a layer of software that's separate from the operating system. When VMware is done building out its vCloud product set, it may describe Windows and Linux as "features" of its "data center operating system."

To make headway against VMware, Red Hat and Microsoft paired up earlier this week to remove a barrier to adoption of their respective forms of virtualization. As things stood before Feb. 17, neither one of them would provide technical support for the other's operating system as a guest in a virtual machine under their own virtualized host.

Now Microsoft has agreed to support running Red Hat Enterprise Linux 5.2 or 5.3 as a virtual machine guest under Windows Server 2008.

Red Hat has agreed to support running Windows Server 2000 Service Pack 4, Server 2003 Service Pack 2, and Server 2008 as a virtualized guest under Red Hat Enterprise Linux, once Red Hat includes the KVM hypervisor in its enterprise operating system. It hasn't done so yet, but KVM's addition is considered imminent.

What's prompted this mutual nonaggression pact? Red Hat resisted a patent agreement in 2006 and 2007, when it came under pressure from Microsoft to sign one. At one point, CEO Steve Ballmer declared that Linux users with no Microsoft assurance of IP safety were carrying "an undisclosed balance sheet liability." Novell, distributor of SUSE Linux, signed a patent pact with Microsoft, along with several smaller Linux vendors; Red Hat refused.

"We didn't believe licensing of IP should be made a condition of interoperability," said Brian Stevens, CTO of Red Hat, in an interview during a visit to San Francisco this week. In the Microsoft/Novell pact, Novell paid Microsoft for intellectual property and Microsoft bought Linux support coupons from Novell. There's been no exchange of intellectual property or payments in the virtualization agreement, said Stevens. "This virtualization agreement wouldn't have happened if, two years ago, we had signed a patent agreement," he added.

Cooperating with Microsoft on virtualization, on the other hand, cuts close to the heart of operating system internals. In effect, the two companies have agreed to exchange their batteries of tests that confirm their operating systems will run on a new chip architecture. The same tests can be used to prove an operating system will perform predictably in a virtualization environment.