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

[Laine Stump <laine@redhat.com>]

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?