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The next revision of the Linux kernel is to include a virtualisation feature developed by VMware, called VMI.

The stable update, version 2.6.21, will then allow para-virtualised operating systems -- Linux itself -- to run inside virtual machines on top of, and to some extent sharing, the Linux kernel. Para-virtualisation is a technique that offers virtual machine performance in exchange for OS flexibility; the guest OS has to be modified to understand that it's running inside a virtual machine, and must be the same as the host.

VMware introduced VMI (Virtual Machine Interface) in July 2005, intended as an interface open for any virtualisation vendor to hook into in order to communicate with the Linux kernel. The move is sure to spark controversy, since virtualisation, as an increasingly ubiquitous and mission-critical technology, is a very sensitive issue.

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The idea behind VMI was that it could be a single standard, rather than there being different interfaces, depending on which virtualisation hypervisor you chose to deploy.

On its introduction, VMare took care about the claims it made for its code. The company's Zachary Amsden said: "This is not an attempt to force a proprietary interface into the Linux kernel. This is an attempt to find a common interface that can be used by many hypervisors by isolating hypervisor specific idioms into a neutral layer. This new layer is just what is claims to be - a virtual machine interface, which allows hypervisor dependent code to be abstracted in a way that benefits both Linux and hypervisor development."

However, some in the open source kernel development community reckoned that VMI hampered performance, and raised doubts over VMware’s motives. They argued that a closed source, commercial vendor such as VMware -- no matter how open many of its interfaces are -- could end up in control of a crucial kernel component at some future point.

One observer, Pierre LeFranc, reckoned that the reactions were because: "XenSource didn't like the VMware patches to Linux, but mostly they disliked the fact that VMware had patches ready to be integrated in Linux before XenSource did."

Come what may, as a result of this feedback VMware modified VMI to support multiple hypervisors and hook into a para-virtualisation interface in the kernel known as paravirt-ops.

The company said that the new VMI "incorporates many of the concepts of VMI including the support of transparent para-virtualisation. Using this interface, a para-virtualised Linux operating system will be able to run on any hypervisor that supports it." According to VMware, VMI is being adapted by developers from IBM, RedHat, and XenSource.

Expect to see VMware's forthcoming Workstation 6.0 product making use of VMI-enabled Linux kernels -- coming a distro near you, soon.

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