Linux drivers for the community (GPLv2)
Today, in a break from the ordinary, Microsoft released 20,000 lines of device driver code to the Linux community. The code, which includes three Linux device drivers, has been submitted to the Linux kernel community for inclusion in the Linux tree. The drivers will be available to the Linux community and customers alike, and will enhance the performance of the Linux operating system when virtualized on Windows Server 2008 Hyper-V or Windows Server 2008 R2 Hyper-V.
https://www.microsoft.com/presspass/features/2009/Jul09/07-20LinuxQA.mspx
Matt and the Virtualization Team blog have extra details.