Linux 2.6.37: first upstream Linux kernel to work as Dom0

by Stefano Stabellini
Linux 2.6.37, released just few days ago, is the first upstream Linux kernel that can boot on Xen as Dom0: Linus pulled my “xen initial domain” patch series on the 28th of October and on the 5th of January the first Linux kernel was released with early Dom0 support!
Dom0 is the first domain started by the Xen hypervisor on boot and until now adding domain 0 support to the Linux kernel has required out of tree patches (note that NetBSD and Solaris have had Dom0 support for a very long time). This means that every Linux distro supporting Xen as virtualization platform has to maintain an additional kernel patch series.
Distro maintainers, worry no more: Dom0 support is upstream! It is now very easy to enable and support Xen in the standard kernel distro images and I hope this will lead to an upsurge in distribution support for Xen. Just enabling CONFIG_XEN in the kernel config of a 2.6.37 Linux kernel allows the very same Linux kernel image to boot on native, on Xen as Dom0, on Xen as normal PV guest and on Xen as PV on HVM guest!
That said, the kernel backends, in particular netback and blkback, are not yet available in the upstream kernel. Therefore a 2.6.37 vanilla kernel can only be used to start VMs on the very latest xen-unstable. In fact xen-unstable contains additional functionalities that allow qemu-xen to offer a userspace fallback for the missing backends. This support will become part of the Xen 4.1 release which is due in the next couple of months.
In the short term the out of tree patch set has been massively reduced. It is expected that the xen.git kernel tree will soon contain the proposed upstreamable versions of the backend drivers. I strongly encourage everyone to pull these and start testing upstream dom0 support!
I want to thank Jeremy Fitzharding, Konrad Rzeszutek Wilk, Ian Campbell and everyone else who was involved for the major contributions and general help that made this possible.