Re: [libvirt] [RFC] Vhost-user backends cross-version migration support

On Thu, Feb 02, 2017 at 05:29:08PM +0000, Daniel P. Berrange wrote:

On Thu, Feb 02, 2017 at 07:20:35PM +0200, Michael S. Tsirkin wrote:

quoted

On Thu, Feb 02, 2017 at 05:10:28PM +0000, Daniel P. Berrange wrote:

quoted

On Thu, Feb 02, 2017 at 06:21:55PM +0200, Michael S. Tsirkin wrote:

quoted

On Thu, Feb 02, 2017 at 03:06:21PM +0000, Daniel P. Berrange wrote:

quoted

On Thu, Feb 02, 2017 at 03:14:01PM +0100, Maxime Coquelin wrote:

quoted

On 02/01/2017 12:41 PM, Daniel P. Berrange wrote:

quoted

It depends where / how in OVS it needs to be set. The only stuff libvirt
does with OVS is to run 'add-port' and 'del-port' commands via the ovs
cli tool. We pass through arguments from the port profile stored in the
XML config.

  <interface type='bridge'>
    <source bridge='ovsbr'/>
    <virtualport type='openvswitch'>
      <parameters profileid='menial' interfaceid='09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f'/>
    </virtualport>
  </interface>

eg those things in <parameters/> get passed as cli args to the 'add-port'
command. Soo if add-port needs this new version string, then we'd need
to add the version to the openvswitch virtualport XML.

If the version is provided to OVS in a different command, then it would
probably be outside scope of libvirt.

I think it would make sense to be a parameter of the add-port command.
But it would be for vhost-user related add-port command, I didn't find
where/if this is managed in libvirt XML.

For vhost-user, libvirt does not have any interaction with OVS at
all. If the thing that's using the vhost-user UNIX socket, in turn
connects to OVS, that's outside scope of libvirt. IOW, for vhost-user
OVS it seems like that job is for Nova / os-vif to solve.

I don't think they currently understand the issues involved in
cross-version migration though.  This is a complex subject and easy to
get wrong.  It would be significantly better to figure it out at libvirt
level since it already deals with cross-version migration issues.

Libvirt considers vhost-user to be a blackbox though - it just exposes
a UNIX socket, and whatever is on the other end is completely opaque.
The fact that the other end might plumb the data stream into openvswitch
is not something libvirt should know, as we don't want to end up having
to add custom code to libvirt for every different vhost-user server
impl.

IOW, if the version str can be passed to QEMU, and then onto vhost-user
backend in QEMU, then libvirt can be involved. If the version str has
to be given to openvswitch that's not for libvirt to deall with.

Regards,
Daniel

It's not just about passing it to QEMU. The following is needed:
- need to query version when creating VM/device
- need to query supported versions when migrating VM

Those are both things that nova can do, since it knows what the vhost-user
device in question is connected to, and so can query the versions, and check
versions before triggering migration with libvirt

Regards,
Daniel

It can, but then it will need to query libvirt or source for the version
string since it's in the XML.

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