Microsoft was the fifth-largest corporate contributor to the 3.0 Linux kernel as measured by the number of changes to its previous release. Only Red Hat, Intel, Novell, and IBM had more contributions. Microsoft was fifteenth as measured by number of lines changed.
The work done by Microsoft was to enable the Microsoft Hyper-V driver to be included in the mainline Linux kernel. Microsoft originally submitted the necessary code changes back in July 2009, but there were a lot of problems with the code, and the Linux kernel developers insisted that the code be fixed before being accepted into the mainline kernel.