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

[Laine Stump <laine@laine.org>]

On 02/03/2017 04:12 AM, Michal Privoznik wrote:
> 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.

Except that (as I just noted in the BZ) libvirt doesn't know the name of 
the bridge in the case of vhost-user. It only knows the name of the 
socket. So maybe in this case the only solution is for Nova (which 
*does* know about the bridge) needs to query the MTU and populate 
libvirt's XML appropriately.