First, Microsoft submitted source code to the Linux kernel. Then, one of its executives claimed “We love open source.” And just last night, Microsoft said it is now adding interoperability support for CentOS Linux so it can be run on Hyper-V, the virtualization platform that comes with Windows Server.

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There’s a healthy does of self-interest here, as Microsoft is trying to improve the viability of Hyper-V in an all-out war to topple VMware from its place at the top of the virtualization market, while also fending off open source competitors Xen and KVM. Plus, Linux is widely used in the Web server market. But you also might call supporting CentOS an indirect compliment paid to rival Red Hat, because CentOS is a version of Linux based on the Red Hat OS.

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Microsoft marketing GM Sandy Gupta explains that Microsoft is targeting CentOS because it “is a popular Linux distribution for hosters,” and therefore an important entry point for Microsoft to get its software into cloud networks.

“This development enables our Hosting partners to consolidate their mixed Windows + Linux infrastructure on Windows Server Hyper-V; reducing cost and complexity, while betting on an enterprise class virtualization platform,” Gupta writes. Microsoft is set to share more details about Hyper-V’s support for CentOS at this week’s Open Source Business Conference.

Microsoft has a long, mostly contentious relationship with Linux and open source software users, often claiming that Linux software infringes on its patents. Linux-based Android is one of Microsoft’s latest targets in enforcing patent claims. But if Linux and open source can be integrated with Microsoft software in a way that helps Microsoft sell more licenses, then Redmond officials become a little more forgiving, even supporting Linux servers (including Red Hat’s) in its System Center management suite.

We’ll keep a close eye on Microsoft’s evolving stance toward open source software.