July 18th was an important day in the history of the Xen project accord to XenSource CTO, Simon Crosby. Why you ask?  It seems Linus Torvalds merged the XenSource patches into upstream for release as part of the 2.6.23 kernel.   

Linus merged XenSource's Jeremy Fitzhardinge's latest xen-upstream branch into the Linux kernel which included adding a virtual network device driver, a virtual block device driver, the Xenbus sysfs and virtual device hot-plug driver, support for preemption, and SMP guest support.  There were 44 commits overall. 

Simon Crosby writes, "About a year ago, XenSource, the Linux kernel community and VMware set out to develop a common interface into the Linux kernel that would allow for optimal execution on a hypervisor, taking advantage of paravirtualization - the key innovation of the Xen project that will also be adopted into Solaris 10, and that can be expected in the forthcoming Windows Server, Longhorn. The patches in Linux allow a kernel to directly plug into the hypercall interface of the hypervisor, bypassing the need for the tedious code-rewriting technology used historically in all emulation based hypervisors, and even today in VMware’s product line. Instead of having to trap, and emulate virtualization-unsafe instructions in guests, and thereafter patch the guests by re-writing the instruction sequences, the guest uses a virtualization-aware API to directly call into the hypervisor for certain non-virtualization safe operations. This gives a tremendous performance boost - of the order of 25% over the trap, emulate and patch technology.

...

This work will allow future Linux distro kernels to simply base of kernel.org, and automatically inherit Xen support, without needing to pull the Xen paravirtualization patches into their kernel as a separate effort."

Next up is 64 bit support, according to Jeremy, who is also working on Dom0 support.