On Fri, 16 Feb 2018 10:11:19 -0800, Sridhar Samudrala wrote:

Ppatch 2 is in response to the community request for a 3 netdev
solution.  However, it creates some issues we'll get into in a moment.
It extends virtio_net to use alternate datapath when available and
registered. When BACKUP feature is enabled, virtio_net driver creates
an additional 'bypass' netdev that acts as a master device and controls
2 slave devices.  The original virtio_net netdev is registered as
'backup' netdev and a passthru/vf device with the same MAC gets
registered as 'active' netdev. Both 'bypass' and 'backup' netdevs are
associated with the same 'pci' device.  The user accesses the network
interface via 'bypass' netdev. The 'bypass' netdev chooses 'active' netdev
as default for transmits when it is available with link up and running.

Thank you do doing this.

We noticed a couple of issues with this approach during testing.
- As both 'bypass' and 'backup' netdevs are associated with the same
  virtio pci device, udev tries to rename both of them with the same name
  and the 2nd rename will fail. This would be OK as long as the first netdev
  to be renamed is the 'bypass' netdev, but the order in which udev gets
  to rename the 2 netdevs is not reliable. 

Out of curiosity - why do you link the master netdev to the virtio
struct device?

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.

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

- 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.