Linux gets broader virtualisation support

Xen and lguest virtualisation technologies have been folded into the Linux kernel, giving future Linux distributions a more direct link to the virtualisation techniques.

KVM (Kernel-based Virtual Machine), a relatively new technique relying on hardware-based virtualisation support built into more recent Intel and AMD chips, was introduced into the Linux kernel in December 2006.

Both Xen and lguest were merged into version 2.6.23 of the Linux kernel last week, according to mailing lists. Virtualisation allows multiple operating system instances to run on a single hardware platform, but there are many different approaches to the idea.

Xen is by far the better known of the two. It was initially created as a research project at Cambridge University, and is now promoted in commercial products by XenSource, which also sponsors development of the open-source version of the software.

Xen is based around a hypervisor, which oversees the guest operating systems, and a host operating system, which is typically Linux. Guests must be modified to support Xen, but the software can also make use of hardware-based virtualisation support to run unmodified guests. That comes in handy in the case of operating systems that can't be modified, such as Windows.

The introduction of Xen support into the kernel means that Linux distributors will no longer need to maintain Xen guest support themselves.

Lguest is far simpler than either Xen or KVM, with fewer high-end features — for instance, it doesn't yet feature 64-bit support. The technology is designed specifically to appeal to programmers, according to Rusty Russell, the high-profile developer behind the project. "Most of all, lguest is awesome fun! Too much of the kernel is a big ball of hair. Lguest is simple enough to dive into and hack," he wrote in notes accompanying the lguest patch.

He acknowledged, however, that features such as performance and high-end features need plenty of work.

Lguest doesn't require hardware virtualisation support, as KVM does, Russell said. It will only run a guest kernel with lguest support, but the process of launching a guest OS is simpler than under Xen, since no hypervisor is involved, he said.