Re: [Xen-users] vanilla kernel and xen (in general), vanilla 2.6.24 and xen

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> >> There shall be nothing special. Take a config for 2.6.18 kernel and just
> >> change the kernel and initrd lines. as long as the build is matching the
> >> Xen capability, PAE nonPAE etc.
> >
> > The Xen code that's in 2.6.24 from kernel.org is rather different to that
> > in 2.6.18 XenLinux from XenSource.  So just reusing the same config file
> > will probably not work on its own, sorry :-(
>
> Can you comment on that a bit more? Are there now like 2 different
> branches of kernel Xen for a domain user?

Essentially, yes.

The XenLinux kernel which XenSource hosts and develops and that is the basis 
of the existing distro kernels is based on a source tree whose history dates 
back to the really early days of Xen.  This has full support for all the cool 
bits of functionality Xen supports and can run as dom0 or domU; however it 
can't be booted as a "native" kernel on bare-metal and it's based on the 
2.6.18 kernel, which is getting old.

kernel.org Linux has support for running as a Xen domU since Linux 2.6.23 was 
released.  The Xen-aware code in that is derived from that in the XenSource 
XenLinux kernel - in many places it's a port of the same code, not a 
different implementation.  However, quite a lot of changes were required to 
make the addition of Xen support acceptable to the mainline kernel developers 
(using paravirt-ops) to the XenLinux 2.6.18 (which uses a separate Xen 
subarchitecture).  Jeremy Fitzhardinge at XenSource took on the difficult 
task of getting Xen support in mainline Linux and managed to get a basic set 
of functionality merged into 2.6.23.

The current situation is still not ideal, since the XenLinux kernel is still 
based on 2.6.18 and it's hard work for distros to forward-port Xen support to 
whatever kernel they're running.  Meanwhile the kernel.org port doesn't 
support all the juicy features that XenLinux 2.6.18 supports.

A number of people - Red Hat are helping spearhead this - are working on 
turning all the juicy features of XenLinux 2.6.18 into patches on top of 
mainline Linux's existing Xen support.  This includes support for running as 
dom0.

The end goal, I believe, is to get as much Xen support upstream as possible 
(e.g. paravirt framebuffer, live migration support, maybe dom0 support).  
Anything that's left can be maintained by the Xen developers as a patch on 
top of mainline linux, rather than maintaining their own separate kernel.

This involves some short term pain but the end result should be that Xen 
support is available for newer kernels quicker, kernel.org Linux will come 
with more Xen functionality by default, and it may even be possible for 
distros to ship the same kernel for use as the native kernel, as dom0 and as 
domU (and for lguest, and for VMI).  Which would be awesome :-)

Does that clear things up a bit?

Cheers,
Mark

-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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