netdev - Re: [RFC PATCH v3 0/3] Enable virtio_net to act as a backup for a passthru device

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Date:   Fri, 16 Feb 2018 18:38:17 -0800
From:   Jakub Kicinski <kubakici@...pl>
To:     Sridhar Samudrala <sridhar.samudrala@...el.com>
Cc:     mst@...hat.com, stephen@...workplumber.org, davem@...emloft.net,
        netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org, jesse.brandeburg@...el.com,
        alexander.h.duyck@...el.com, jasowang@...hat.com,
        loseweigh@...il.com
Subject: Re: [RFC PATCH v3 0/3] Enable virtio_net to act as a backup for a
 passthru device

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.