[libvirt] RFC: disconnecting guest/domain interface config from host config (aka migration with macvtap)

[Laine Stump]

On 04/27/2011 09:58 AM, Oved Ourfalli wrote:
> Laine, hello.
>
> We read your proposal for abstraction of guest<-->  host network 
> connection in libvirt.
>
> You has an open issue there regarding the vepa/vnlink attributes:
> "3) What about the parameters in the<virtualport>  element that are 
> currently used by vepa/vnlink. Do those belong with the host, or with 
> the guest?"
>
> The parameters for the virtualport element should be on the guest, and 
> not the host, because a specific interface can run multiple profiles,

Are you talking about host interface or guest interface? If you mean 
that multiple different profiles can be used when connecting to a 
particular switch - as long as there are only a few different profiles, 
rather than each guest having its own unique profile, then it still 
seems better to have the port profile live with the network definition 
(and just define multiple networks, one for each port profile).


>   so it will be a mistake to define a profile to be interface specific 
> on the host. Moreover, putting it in the guest level will enable us in 
> the future (if supported by libvirt/qemu) to migrate a vm from a host 
> with vepa/vnlink interfaces, to another host with a bridge, for example.

It seems to me like doing exactly the opposite would make it easier to 
migrate to a host that used a different kind of switching (from vepa to 
vnlink, or from a bridged interface to vepa, etc), since the port 
profile required for a particular host's network would be at the host 
waiting to be used.

> So, in the networks at the host level you will have:
> <network type='direct'>
> <name>red-network</name>
> <source mode='vepa'>
> <pool>
> <interface>
> <name>eth0</name>
>        .....
> </interface>
> <interface>
> <name>eth4</name>
>        .....
> </interface>
> <interface>
> <name>eth18</name>
>        .....
> </interface>
> </pool>
> </source>
> </network>
>
> And in the guest you will have (for vepa):
> <interface type='network'>
> <source network='red-network'/>
> <virtualport type="802.1Qbg">
> <parameters managerid="11" typeid="1193047" typeidversion="2" 
> instanceid="09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f"/>
> </virtualport>
> </interface>
>
> Or (for vnlink):
> <interface type='network'>
> <source network='red-network'/>
> <virtualport type="802.1Qbh">
> <parameters profile_name="profile1"/>
> </virtualport>
> </interface>

This illustrates the problem I was wondering about - in your example it 
would not be possible for the guest to migrate from the host using a 
vepa switch to the host using a vnlink switch (and it would be possible 
to migrate to a host using a standard bridge only if the virtualport 
element was ignored). If the virtualport element lived with the network 
definition of red-network on each host, it could be migrated without 
problem.

The only problematic thing would be if any of the attributes within 
<parameters> was unique for each guest (I don't know anything about the 
individual attributes, but "instanceid" sounds like it might be 
different for each guest).

> Then, when migrating from a vepa/vnlink host to another vepa/vnlink 
> host containing red-network, the profile attributes will be available 
> at the guest domain xml.
> In case the target host has a red-network, which isn't vepa/vnlink, we 
> want to be able to choose whether to make the use of the profile 
> attributes optional (i.e., libvirt won't fail in case of migrating to 
> a network of another type), or mandatory (i.e., libvirt will fail in 
> case of migration to a non-vepa/vnlink network).
>
> We have something similar in CPU flags:
> <cpu match="exact">
> <model>qemu64</model>
> <topology sockets="S" cores="C" threads="T"/>
> <feature policy="require/optional/disable......" name="sse2"/>
> </cpu>

In this analogy, does "CPU flags" == "mode (vepa/vnlink/bridge)" or does 
"CPU flags" == "virtualport parameters" ? It seems like what you're 
wanting can be satisfied by simply not defining "red-network" on the 
hosts that don't have the proper networking setup available (maybe what 
you *really* want to call it is "red-vnlink-network").