Highlighting the Linux 2.6.21 update is the inclusion of the paravirt-ops paravirtualization interface, which enables multiple hypervisors to hook directly into Linux.
VMware, XenSource, IBM and Red Hat first launched the effort to provide expansive virtualization in paravirt-ops in August to make it easier for all virtualization vendors to work in Linux. VMware's VMI (virtual machine interface) is also included in 2.6.21 taking advantage of paravirt-ops.
"It is absolutely a milestone for us," Jack Lo, senior director of R&D at VMware, told internetnews.com. "We definitely wanted to be able to work with the Linux community; from the beginning we thought it was good for Linux and we're quite excited to see it in the Linux kernel."
Paravirt ops improves Linux virtualization capabilities by allowing the kernel to directly communicate with the hypervisor. By being part of the mainline kernel.org Linux kernel, it is expected to help improve the adoption and use of hypervisors.
"It will dramatically ease the maintenance of the kernel hypervisor interface, which is something that has consumed excessively and also put additional burden on the distros that package Xen," XenSource CTO Simon Crosby told internetnews.com.
Though VMware's VMI is in 2.6.21, Xen's hooks into paravirt-ops are not in the 2.6.21 kernel. Crosby said he expected the actual Xen hooks to be in the 2.6.22 kernel.
Crosby explained that XenSource had to work with Novell SLES 10 and Red Hat RHEL 5 to build a kernel that is Xen ready from a stock kernel.org kernel. "It's always been our desire for distros to simply take a kernel from kernel.org and know that it will run Xen," Crosby said.
According to Crosby, it is "tremendously important" that Xen, like VMI, end up in the kernel. The fact that VMI is first with paravirt-ops hooks doesn't really matter either, according to Crosby.
"It's not a big deal at all that VMware is first; it's really just a sequencing issue," Crosby said. "Xen is a more expansive interface; it touches more of the kernel."
Overall for both VMware and Xen the effort to get paravirt-ops into the kernel was a learning process.
"Certainly at the beginning we were Linux kernel newbies," VMware's Lo said. "During the process we've had a lot of discussions on the mailing list and we learned a lot."
Fundamentally, though, the effort was a collaborative one in which multiple vendors were able to come together for the common good.
"When we first started going about this, I have to be frank, we were a little naïve," Crosby said. "Once we sat down with VMware what I saw was tremendous goodwill on all sides."
Though paravirt-ops is just now landing in the mainline 2.6.21 kernel.org Linux kernel, it was actually backported by Ubuntu in the recent Feisty Fawn release. Other distros, including Red Hat's Fedora, are expected to rebase to the 2.6.21 kernel in the coming weeks.