kernel-xen is dead. Long live kernel + paravirt_ops

Posted: July 25th, 2008 | Filed under: Fedora | 4 Comments »

In Fedora 9 we discontinued our long standing forward-port of Xen’s 2.6.18 kernel tree, and switch to a generic LKML tree (which already had i386 Xen DomU pv_ops), and added a set of patches to support x86_64 Xen DomU pv_ops. While it lacks functionality compared to the previous Xen kernels, and was certainly less stable for a while, overall this was a great success in terms of maintainability. It was still a separate kernel RPM though…

Jeremy Fitzhardinge has meanwhile continued to improve the Xen i386 pv_ops tree stability & functionality in upstream LKML, and has also taken the hacky Fedora x86_64 pv_ops patches, majorly cleaned them up & worked them into a form that was acceptable for upstream. A couple of days ago Ingo sent Jeremy’s work onto Linus who promptly merged it for 2.6.27

Fedora 10 Rawhide of course is tracking 2.6.27 so yesterday Mark McLoughlin turned on Xen pv_ops in the main kernel RPM, and killed off ‘kernel-xen’.

So for Fedora 10 we’ll have one kernel RPM to rule them all. By the magic of pv_ops, auto-detecting whether its running bare metal, Xen, VMWare (VMI) or KVM at boot and optimizing itself for each platform!

There’s only one small wrinkle, that isn’t really Xen’s fault. Anaconda install images on 32-bit Fedora are a i586 non-PAE kernel. Xen 32-bit is i686, PAE only, so we still need to have a separate initrd and vmlinux for installation – but at least its derived from the general purpose ‘kernel-PAE’ binary, instead of ‘kernel-xen’. Of course 64-bit doesn’t have this complication. Someone just needs to fix 32-bit Linux so it can auto-switch between non-PAE and PAE at runtime. It was always said to be impossible to unify UP & SMP kernels…until someone actually did it. Now just need someone to do the impossible for PAE and all will be right with the world :-)

It’s taken a long time & alot of work by many many people to get Xen’s DomU kernel bits merged upstream, so congratulations to all involved on getting another architecture merged, enabling us to finally take full advantage of paravirt_ops in Fedora’s Xen kernels.