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



On Thu, Feb 02, 2017 at 07:09:24PM +0200, Michael S. Tsirkin wrote:
> On Thu, Feb 02, 2017 at 11:47:57AM -0500, Laine Stump wrote:
> > On 02/02/2017 10:06 AM, 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.
> > 
> > (aside note: the code that exec's ovs-vsctl was written back when there was
> > no standardized API for performing such operations. libvirt would prefer to
> > not exec external programs though, and I've heard that OVS may now have an
> > official API of some sort for doing things like this (maybe via netlink or
> > dbus or something?) If that's the case, can someone point me in the right
> > direction?)
> > 
> > > > >   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.
> > 
> > This brings up another tangentially related question I came up against last
> > night - qemu now has an option to report the host's MTU to the guest for
> > virtio and vhost-user interfaces, and Michal Privoznik recently pushed
> > patches to set the MTU sent to the guest via an explicit <mtu size='n'/> in
> > libvirt's interface config:
> > 
> >    https://bugzilla.redhat.com/1408701
> > 
> > But it would be much nicer if libvirt could learn the MTU of [that stuff at
> > the other end of the unix socket] without requiring intervention in
> > libvirt's config. For example, I'm just now testing patches for tap-based
> > interfaces (connecting to Linux host bridges or OVS switches) that query the
> > current MTU of the bridge and report that to qemu; this eliminates the
> > burden of configuring each interface of each guest individually (and
> > changing that config in all those places if someone ever wants to change the
> > MTU of the bridge).
> > 
> > As Dan says, though, libvirt's only interaction in the case of vhost-user is
> > with the unix socket. Is there any way to learn what is the appropriate MTU
> > from OVS in these cases? Or must Nova (or ovirt or some poor user) set that
> > up in the libvirt config for every single interface?
> 
> We could add commands for all kind of queries to the vhost-user
> protocol.  libvirt would have to learn the vhost-user protocol though.
> Interested?

No, I really don't think libvirt should implement the vhost-user protocol

Regards,
Daniel
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