Microsoft has announced that a key application programming interface of its hypervisor technology will be open sourced.
It has added to its Open Specification Promise the hypercall API that is central to Windows Server virtualisation (code named Viridian) which will be available when Windows Server virtualisation is released to manufacturing.
In the interim, Microsoft says it has posted an updated draft of the hypercall API so that developers can gain early access to the interface. Microsoft first distributed hypercall API draft documentation to attendees of Windows Hardware Engineering Conference (WinHEC) 2006.
The hypercall API enables guest OSes to call for resources from the hypervisor. According to Microsoft, it enables development of software that builds on Viridian and is "available for use by any organisation seeking to integrate or extend their software with Windows Server 2008 and Windows Server virtualisation".
Microsoft continues to maintain that Viridian will release to manufacturing within 180 days of the release of Windows Server 2008, currently scheduled for the first quarter of 2008.
Two of Microsoft's close partners have endorsed the OSP move.
"The majority of our customers have mixed-source environments, and they want their platform vendors to make things work together," says Roger Levy, Novell's open source general manager. "That's why we entered into a technical collaboration agreement with Microsoft. As a result, Novell is the first vendor to develop and ship technology that will allow a para-virtualised Windows Server 2008 to be hosted as a guest on the Xen hypervisor.
"Microsoft's decision to put the hypercall API under their Open Specifications Promise will make it even easier for Novell, our customers and partners, and the entire open source community to develop high-quality virtualisation solutions that deliver true interoperability between Windows and Linux."
Citrix CTO Simon Crosby says his company "is committed to the delivery of value-added virtualisation solutions for the Windows platform, so interoperability with Microsoft's virtualisation solutions is key to our success. This is made possible by Microsoft's open and progressive approach to licensing key technologies such as its VHD image format and the Windows Server Virtualisation hypercall API.
"This will allow us to ensure that virtual machines created on XenServer will be compatible with Microsoft WSV when it is delivered as a component of Windows Server 2008."
Microsoft applied the OSP to its virtual hard disk image format in October 2006, mirroring a similar initiative by market-leader VMware, which has also opened its virtual machine format, some of its management interfaces, and a number of its hypervisor APIs. VMware also pointed out that: "Microsoft's licensing policy has some restrictions on transferring licences from one server to another. Under Microsoft licensing, Windows Server licences are tied to physical hosts and do not move with virtual machine environments."