(pruned to reduce thread)

On Wed, 21 Feb 2018 16:17:19 -0800
Alexander Duyck [off-list ref] wrote:

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FWIW two solutions that immediately come to mind is to export "backup"
as phys_port_name of the backup virtio link and/or assign a name to the
master like you are doing already.  I think team uses team%d and bond
uses bond%d, soft naming of master devices seems quite natural in this
case.  

I figured I had overlooked something like that.. Thanks for pointing
this out. Okay so I think the phys_port_name approach might resolve
the original issue. If I am reading things correctly what we end up
with is the master showing up as "ens1" for example and the backup
showing up as "ens1nbackup". Am I understanding that right?

The problem with the team/bond%d approach is that it creates a new
netdevice and so it would require guest configuration changes.
 

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IMHO phys_port_name == "backup" if BACKUP bit is set on slave virtio
link is quite neat.  

I agree. For non-"backup" virio_net devices would it be okay for us to
just return -EOPNOTSUPP? I assume it would be and that way the legacy
behavior could be maintained although the function still exists.
 

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- When the 'active' netdev is unplugged OR not present on a destination
  system after live migration, the user will see 2 virtio_net netdevs.  

That's necessary and expected, all configuration applies to the master
so master must exist.  

With the naming issue resolved this is the only item left outstanding.
This becomes a matter of form vs function.

The main complaint about the "3 netdev" solution is a bit confusing to
have the 2 netdevs present if the VF isn't there. The idea is that
having the extra "master" netdev there if there isn't really a bond is
a bit ugly.  

Is it this uglier in terms of user experience rather than
functionality? I don't want it dynamically changed between 2-netdev
and 3-netdev depending on the presence of VF. That gets back to my
original question and suggestion earlier: why not just hide the lower
netdevs from udev renaming and such? Which important observability
benefits users may get if exposing the lower netdevs?

Thanks,
-Siwei  

The only real advantage to a 2 netdev solution is that it looks like
the netvsc solution, however it doesn't behave like it since there are
some features like XDP that may not function correctly if they are
left enabled in the virtio_net interface.

As far as functionality the advantage of not hiding the lower devices
is that they are free to be managed. The problem with pushing all of
the configuration into the upper device is that you are limited to the
intersection of the features of the lower devices. This can be
limiting for some setups as some VFs support things like more queues,
or better interrupt moderation options than others so trying to make
everything work with one config would be ugly.

Let's not make XDP the blocker for doing the best solution
from the end user point of view. XDP is just yet another offload
thing which needs to be handled.  The current backup device solution
used in netvsc doesn't handle the full range of offload options
(things like flow direction, DCB, etc); no one but the HW vendors
seems to care.