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

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?