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



On Thu, Feb 02, 2017 at 06:21:55PM +0000, Daniel P. Berrange wrote:
> On Thu, Feb 02, 2017 at 07:31:49PM +0200, Michael S. Tsirkin wrote:
> > 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:
> > > > On Thu, Feb 02, 2017 at 05:10:28PM +0000, Daniel P. Berrange wrote:
> > > > > On Thu, Feb 02, 2017 at 06:21:55PM +0200, Michael S. Tsirkin wrote:
> > > > > > On Thu, Feb 02, 2017 at 03:06:21PM +0000, Daniel P. Berrange wrote:
> > > > > > > On Thu, Feb 02, 2017 at 03:14:01PM +0100, Maxime Coquelin wrote:
> > > > > > > > 
> > > > > > > > 
> > > > > > > > On 02/01/2017 12:41 PM, Daniel P. Berrange wrote:
> > > > > > > > > 
> > > > > > > > > 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
> > 
> > It can, but then it will need to query libvirt or source for the version
> > string since it's in the XML.
> 
> No, it wouldn't be in the XML at all. Nova on the source queries what
> vhostuser version it has and what the target host has, and can prevent
> the migration if they're incompatible.

This is not sufficient. Exactly the same as qemu machine type,
this must be preserved from time of install and moved
wherever VM goes.


> I dont think libvirt has to be
> involved at all for this, as all the info can be obtained by nova/os-vif
> from the vhostuser impl it has configured.
> 
> Regards,
> Daniel

Given we are already confused at libvirt level, I find it
highly unlikely upper levels will do the right thing.

Generally I simply don't understand why would we
expose to higher levels something which is fundamentally
not a policy decision.


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