The Xen team has been working hard for years to get everything together that they needed to run the Linux kernel as Dom0. Some of these components were added last year to handle memory management, grant table stuff, network pv driver and block pv driver back-end code, and now the final elements have been added to handle everything else.
Another addition that will make the mainline Linux kernel more Xen friendly is something called pvops. According to Coekaerts, pvops is a mode that will enable the kernel to switch between paravirtualization (pv), hardware virtualization (hvm), or paravirtual-hardware virtualization (pv-hvm) at boot time. Instead of having multiple kernel binaries, he explains, "There is just one and it will lay out its operations at boot time when it detects on what platform it runs."
Simon Crosby, CTO for the data center and cloud division at Citrix Systems, added another perspective to this announcement:
In practical terms, this means that every Linux kernel from 2.6.39 onwards, will contain every piece of code to seamlessly take the role of guest, host (in the case of KVM) or Domain 0 (in the case of Xen). For the key consumers of Xen, including open source powered clouds such as AWS, this achievement permits easy adoption of the latest and greatest Linux kernels, their drivers and other key architectural innovations, both to power the virtual infrastructure and as guest VMs. Whereas historically this required real engineering and code, this is no longer the case - it just runs.
So now you can use KVM as a component of your Linux distro, or you can use your Linux distro to run Dom0 (platform services) for Xen. This decoupling is unique to Linux, and to my mind gives Linux and the open source community the ability to support a far richer set of virtualized infrastructure abstractions.
One of the questions worth asking is if this Linux mainline win after eight years is too little, too late? Or is it just what the doctor ordered to allow the Xen community to further respond to the challenges that have been put forth by its open source virtualization rival Kernel-based Virtual Machine (KVM)?
The latest move does in theory put Xen back on a level playing field with the KVM hypervisor on at least one point. KVM has been enjoying the fact that it has been included in the Linux kernel going all the way back to 2007 with version 2.6.20; as you might expect, it has leveraged that fact as a key differentiator throughout its marketing material and competitive analysis against Xen and other hypervisor platforms.