[libvirt] RFC: a <uuid> for every <interface> in each domain

[Laine Stump]

On 08/15/2012 01:07 PM, Daniel P. Berrange wrote:
> On Wed, Aug 15, 2012 at 12:27:58PM -0400, Laine Stump wrote:
>> A couple of situations have come up recently that could be solved by
>> every interface in every domain always having a unique identifier
>> associated with it:
>>
>> 1) In order to properly track information as a guest is disconnected and
>> reconnected to an Open vSwitch bridge, the initial Open vSwitch support
>> added an "interfaceid" parameter to virtualport. If the interfaceid
>> isn't specified by the user when the interface is defined, one is
>> automatically generated and placed into the persistent definition.
>>
>> But if the guest interface is just <interface type='network'>, it isn't
>> known at the time of definition whether this interface will be using an
>> Open vSwitch connection, or something else. Then, by the time we get
>> into the network driver and decide that we're going to use Open vSwitch,
>> it's too late to conveniently/cleanly generate an interfaceid and plug
>> it back into the <virtualport> of the interface's persistent config. The
>> result is that each time the guest is restarted, Open vSwitch gets a new
>> interfaceid for it, and can't properly track things.
>>
>> If every interface had a uuid, the code that connects to an Open vSwitch
>> network could just use the interface's uuid if interfaceid wasn't specified.
> Can't we just query the network at time of domain definitiion to
> determine what type it is, and then just fill in a interfaceid ?

That doesn't work because the network on the host where the domain is
defined isn't necessarily the network on the host where it will be
running at some random time in the future. Even the same host might
change its network setup. A large part of the motivation for creating
the new network types was to eliminate this hard connection between the
domain's definition and the hardware where it currently resides.

> In any case if someon really cares about stable interfaceids they
> ought to just be providing that info themselves in the domain XML,
> because nothing can make this work with transient VMs.


You are correct about transient domains.

The code that I just checked in solves the problem (for persistent
domains of course) by allowing a <virtualport/> element to be added to
the interface with no type and no parameters. If this is done, both the
interfaceid and the instanceid are filled in with autogenerated uuids in
anticipation that some time in the future there may be a need to connect
to an Open vSwitch network or 802.1Qbg network. (or of course, those
could be filled in beforehand with known data by whatever
application/person is creating the XML)

It just seems more elegant to have a single identifier that's guaranteed
to be available, and can be used for multiple tasks, rather than tacking
on a new one each time we come up with a new use case.

>
>
>> 2) I'm about to write some patches that will allow enacting config
>> changes to a network without needing to destroy/re-start the network.
>> One thing that came up while I was mulling over what could and couldn't
>> be modified without a restart is that it will be possible to allow
>> removing physical interfaces from a network's interface pool (used for
>> macvtap and hostdev modes), but only if that physical interface isn't
>> currently in use. There is a use counter on each interface so we can
>> easily tell when that situation occurs, but once we know of the failure,
>> we have no way of pointing to which guest is causing the problem. If
>> each interface had a uuid, we could save a list of the uuid's of all
>> interfaces currently connected to a particular physical interface, and
>> report that in the failure message. It would then be a fairly mechanical
>> task to find that uuid in the guests' config.
> I would have thought this could be solved just by having a reference
> back to the VM that is using the device, rather than the <interface>.

Yeah, I could add that to the (currently internal) API instead.

>> Does this sound like a reasonable idea? Any reasons *not* to do it?
>> Problems we'll need to take care of if we add it (for example, existing
>> guest interfaces will all need to get a uuid during the upgrade process,
>> similar to the way we add a mac address to all existing networks that
>> don't have one). Any other things you can think of doing with uuid if we
>> add one?
> I'm not too sure really, though my gut feeling is to not have a UUID
> against the interface, and I don't think the two scenarios above really
> require it

Okay. Since it's not 100% and there's not 100% consensus that it's
worthwhile, I'll drop it unless/until another situation comes up. When
in doubt, inaction is usually a good policy :-)