[Xen-users] Re: kexec into xen

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> >> I have a question: Is it possible to kexec into a xen vmm from a non-xen
> >> kernel? In my vision it would be interesting to boot from a generic
> >> linux installer, then analyze the hardware and boot into a suitable xen
> >> kernel, that loads a dom0 linux kernel.
> >
> > Yes, that should be doable.  Tim Deegan wrote a patch that allowed kexec
> > to boot multiboot images like Xen.  I believe it's been in the upstream
> > kexec tools for quite a while.  You specify Xen as the kernel, followed
> > by XenLinux as a module and (optionally) an initrd (as another module).
>
> That sounds very interesting.

This stuff was in place for Xenoboot.  Given your chosen use case you might 
actually want to take a look at that - it's basically a livecd based 
bootloader using kexec (with multiboot support).  It might give you some 
ideas, and perhaps some of it will fit with the sorts of things you want to 
do...  It's not designed to host VMs itself, though.  VMknoppix may be worth 
checking out in that respect, or the Xen demo CD (I think Tim did this too, 
actually!)

> >> Do you know anybody interested in this?
> >
> > What sort of applications were you thinking of?  Selecting between
> > 32-bit, PAE and 64-bit perhaps?
>
> I am trying to build a cheap xen cluster. And I thought of booting
> various computers using a generic bootable image (.e.g live-cd) and then
> prepare the network environment and boot into a xen vmm form the
> existing system. This allows us to deploy updates on the shared storage
> (e.g. nfs) without adding the need to update the cd-roms.

Ah, cool :-)

So dom0 would be running entirely off the CD-ROM?  Where are the VMs booting 
from?

Bear in mind that NFS root on Linux is reputedly a bit dodgy...  And using 
loopfile-backed VM disks off NFS is a definite no-no...  Although with 
tap:aio devices maybe it would be OK...  In any case: you might want to look 
at network block stores if you want migration to work well.  There's a gnbd 
hotplug script included in the Xen distribution which should automatically 
manage making connections to the server and handle migration.

There were some theoretical problems with shared block stores and migration 
(including Write-After-Write hazards, I think...) but these may have been 
eliminated now.

Good luck!

Cheers,
Mark


-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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