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Subject: Re: Support pure Xen as a hypervisor follow-up
From: sebgoa <runseb@gmail.com>
In-Reply-To: <EAF31A00-5145-486F-9580-022A6964EB36@citrix.com>
Date: Wed, 14 May 2014 10:53:34 +0200
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On Apr 9, 2014, at 2:37 PM, Dave Scott <Dave.Scott@citrix.com> wrote:

> Hi,
>=20
> 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 Denver.
>=20
> 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.

Dave, just to let you know that Tim has done some important =
"refactoring" to split up XenServer hypervisor in CS between Xen and =
XenServer. That way we could keep using xapi for XS but start moving to =
libvirt for Xen.

Tim worked in the xen2server branch (don't ask about the name, I messed =
it up=85:) ).

=
https://git-wip-us.apache.org/repos/asf?p=3Dcloudstack.git;a=3Dshortlog;h=3D=
refs/heads/xen2server

Would be nice to see some of the libvirt stuff in that branch to handle =
a new driver for Xen.

Since the two hypervisors will be split up, we could still drop in some =
early libvirt patches to handle Xen and put this in 4.5 as a wip.

-Sebastien

>=20
> Some of the areas are:
>=20
> 1. hypervisor detection
>=20
> 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 =93kvm=94 or =93xen=94. Or we could query libvirt directly =
(perhaps via 'virsh capabilities'?)
>=20
> 2. fiddling with the domain.xml
>=20
> When starting a domain via libvirt the XML configuration has =
hypervisor-specific stuff in it. Some of this is easy to change like:
>=20
>  <domain type=3D'kvm'>
>=20
> obviously becomes
>=20
>  <domain type=3D'xen'>
>=20
> and
>=20
>  <emulator>/usr/libexec/qemu-kvm</emulator>
>=20
> should probably be
>=20
>  <emulator>/some/other/path/qemu-dm</emulator>
>=20
> 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
>=20
>  <channel type=3D'unix'> ...
>=20
> -- I presume this is a control channel into the system VM. Perhaps we =
could implement this in libvirt/libxl using vchan?
>=20
> 3. system VMs?
>=20
> 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?
>=20
> =85 and I=92m sure there=92s more.
>=20
> Anyway, feedback would be welcome. If anyone else in Denver wants to =
chat, then come grab me later!
>=20
> Cheers,
> Dave Scott
>=20
> [1] =
http://mail-archives.apache.org/mod_mbox/cloudstack-users/201403.mbox/%3cC=
AJGXtBNbmQTQ81rALgH2kMA7V5WJYZKr3xnyasMKC_br+UKzOw@mail.gmail.com%3e
>=20
> [2] =
https://github.com/jamesbulpin/cloudstack/commits/jamesb_xen_exploratory