The Windows Observer--'Viridian' Hypercall APIs to be Open Source, Microsoft Says

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Volume 4, Number 40 -- October 31, 2007

'Viridian' Hypercall APIs to be Open Source, Microsoft Says

Published: October 31, 2007

by Alex Woodie

Microsoft announced last week that it would deliver key APIs for the Windows Server Virtualization hypervisor it's developing (codenamed "Viridian"), under an open source license. By making the critical "hypercall" APIs available through its Open Specification Promise license, it will ensure a higher level of interoperability with third-party applications and operating systems, including Linux.

The growth in hypervisor usage is changing how software developers work. Instead of writing system calls that act against the kernel of operating systems, developers must also master the task of writing hypercalls to hypervisors to gain privileged access to hardware and low-level resources that were previously managed by operating systems, such as updating page tables.

Microsoft's WSV hypervisor, which is still in development, features a documented hypercall API, just as the XenSource and VMWare hypervisors feature their own hypercall APIs. Microsoft first documented this API over a year ago, when it unveiled Viridian at the WinHEC 2006 conference in Seattle.

By putting the hypercall API for Windows Server Virtualization in the public realm, Microsoft will be fostering a greater level of collaboration among the software development community. In particular, distributing the APIs for free via OSP will boost interoperability with Linux when both Windows and Linux are running on the same server, an increasingly common scenario.

Microsoft lined up some praise from its buddies over at SuSE Linux developer Novell for its hypercall announcement. "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 virtualization solutions that deliver true interoperability between Windows and Linux," said Roger Levy, Novell's senior vice president and general manager of open platform solutions in a press release.

For example, Levy noted, as a result of the Viridian hypercall APIs being available free of charge to any developer, Novell will be able to give its customers everything they need to be able to run a "paravirtualized" version of Windows Server 2008 (codename "Longhorn," which will ship next year) as a guest on the Xen hypervisor (which is available now). Presumably, SuSE Linux Enterprise Server will be running next to Longhorn on that virtualized box, and Xen will keep things running smoothly, like one big happy family.

Praise was also heaped on Microsoft's open source decision by XenSource, the developers of the open source Xen hypervisor, which last year entered into a partnership with Microsoft to help it develop Windows Server Virtualization.

Simon Crosby, CTO of the virtualization and management division at Citrix Systems, which bought XenSource earlier this year, said the availability of Microsoft hypervisor APIs under open source license 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's plans call for WSV to be released 180 days after the release to manufacturing (RTM) of Windows Server 2008. Both products are currently in the test stage--Windows Server 2008 is currently at the Release Candidate 0 (RCO) stage, while WSV is undergoing its first community technology preview (CTP).

The hypercall APIs will not be the first bit of virtualization technology that Microsoft has licensed under the OSP, which institutes a legally binding agreement that prohibits Microsoft from suing people or organizations adopting a technology it put under the licensing scheme. A year ago Microsoft's Virtual Hard Disk (VHD) format became the first technology it chose to license under its then-new OSP. VHD allows customs to easily migrate their data from one virtualization technology to another, and is thus a critical element for avoiding vendor lock-in.