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

[Michal Privoznik]

On 02/02/2017 06:16 PM, Maxime Coquelin wrote:
> 
> 
> On 02/02/2017 06:09 PM, 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?
>>
> 
> I think it could be possible to query the MTU value from the OVS DB
> using its JSON RPC-like API, but this is something I haven't tried.
> I guess it would need to resolve the ovs interface from the vhost-user
> socket path.
> 
> Can people familiar with OVS confirm this is something possible?

Libvirt does not use OVS' JSON RPC yet (it'll be long way to go anyway).
Therefore, for any OVS interaction it uses ovs-vsctl directly. Therefore:

ovs-vsctl get Interface ovsbr0 mtu

is what Laine is looking for. However, something fishy is happening here:

lisa ~ # ovs-vsctl set Interface ovsbr0 mtu=9000
lisa ~ # ovs-vsctl get Interface ovsbr0 mtu
1500
lisa ~ # ifconfig ovsbr0 mtu 9000
lisa ~ # ovs-vsctl get Interface ovsbr0 mtu
9000

(yes, Lisa Simpson)

But since we just want to query the bridge's MTU and not set it, we
should be safe.

Michal