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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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?

Regards,
Maxime