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From Dave Scott <>
Subject Support pure Xen as a hypervisor follow-up
Date Wed, 09 Apr 2014 12:37:34 GMT

Following up from Tim's "Support pure Xen as a hypervisor" proposal last month[1] I'd like
to start working on this and maybe even make a little bit of progress while I'm at CCC in

Helpfully James Bulpin managed to get CS + libvirt + xen to start an instance in a simple
configuration. Although the patches[2] are not intended to be production-ready :) they help
highlight some of the areas we need to change.

Some of the areas are:

1. hypervisor detection

Where we currently look for KVM specifically ("lsmod | grep kvm") we could switch to either
detecting any Linux hypervisor (by reading /sys/hypervisor/type) and assuming if a hypervisor
is present then we can use libvirt on it (is this a fair assumption?) Or we could white-list
“kvm” or “xen”. Or we could query libvirt directly (perhaps via 'virsh capabilities'?)

2. fiddling with the domain.xml

When starting a domain via libvirt the XML configuration has hypervisor-specific stuff in
it. Some of this is easy to change like:

  <domain type='kvm'>

obviously becomes

  <domain type='xen'>



should probably be


Some is a bit more invasive (to the VM) such as the virtual hardware type should be switched
from "virtio" to "xen" (and the block device in Linux will change from /dev/vd* to /dev/xvd*)
and we'll have to either implement or work around the lack of

  <channel type='unix'> ...

-- I presume this is a control channel into the system VM. Perhaps we could implement this
in libvirt/libxl using vchan?

3. system VMs?

It would be very convenient if the system VM images could work on both xen and KVM. This is
probably doable as long as we don't bake in virtual hardware specific information (such as
/dev/vda) in the image. We could use the qcow2 format in both cases. What do you think?

… and I’m sure there’s more.

Anyway, feedback would be welcome. If anyone else in Denver wants to chat, then come grab
me later!

Dave Scott



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