VMware has slammed Microsoft's tie-up with open source virtualisation developer XenSource, announced earlier this week, calling it a "one-way street," and accusing XenSource of betraying its open source roots.

The agreement between the two companies will allow a para-virtualised Windows to inter-operate with XenSource's Xen, and significantly add to Xen's appeal by making Windows Server virtualisation able to run open-source Xen-based guest operating systems.

VMware has criticised the deal however saying that while it will optimise Microsoft code to run on the Xen hypervisor it does not allow Microsoft-XenSource developed code to be used by the open source community.

Brian Byun, the company's VP of products and alliances, pointed out in his blog that the deal is "a one-way street that favours Microsoft and Windows running Linux.

The arrangement will allow Linux to run on future Microsoft hypervisors through translated calls to the hypervisor when Windows is controlling the hardware, but not the other way around.

That means there is no mention of Longhorn optimisations or 'enlightenments' being ported to Xen or licensed to XenSource to enable a Xen hypervisor to run full optimisations with Longhorn OS."

Byun accused XenSource of abandoning its open source roots, with the result that its code would become proprietary.

He said that XenSource was enabling Microsoft to build proprietary layers and APIs, adding: "It stands to reason that in order to protect Windows from GPL contamination, XenSource will need to undertake a lot of non-GPL development to translate and buffer the Linux kernel from Windows hypervisor interfaces.

"And nothing that Microsoft licenses to, or develops with, XenSource is GPL and can be used directly by the Xen or Linux communities and commercial distributions."

Byun commented too on the timing, saying that: "It’s odd to trumpet future inter-operability for the Windows hypervisor whose first release is roughly two years away or more, while the Linux hypervisor interfaces are still being actively discussed in the open source community."

He mentioned too that there was no sign in the XenSource-Microsoft announcement of "support for this scheme from major Linux commercial distributions, OEMs and customers."

Contrasting VMware's approach, Byun said it hoped "there will soon be a standard Linux interface for para-virtualisation, which will simplify and standardise how Linux is supported on various hypervisors, including VMware and Xen."

The company has been working publicly with Linux kernel developers on an open interface, aiming to allow the kernel to run on a choice of hypervisors, the interface which would be available to any OS.

VMware has already proposed how this could work.