netdev - Re: [PATCH net-next v9 3/4] virtio_net: Extend virtio to use VF datapath when available

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Date:   Mon, 30 Apr 2018 09:12:08 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     "Samudrala, Sridhar" <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, kubakici@...pl, jasowang@...hat.com,
        loseweigh@...il.com, aaron.f.brown@...el.com
Subject: Re: [PATCH net-next v9 3/4] virtio_net: Extend virtio to use VF
 datapath when available

Mon, Apr 30, 2018 at 05:00:33AM CEST, sridhar.samudrala@...el.com wrote:
>
>On 4/28/2018 1:24 AM, Jiri Pirko wrote:
>> Fri, Apr 27, 2018 at 07:06:59PM CEST, sridhar.samudrala@...el.com wrote:
>> > This patch enables virtio_net to switch over to a VF datapath when a VF
>> > netdev is present with the same MAC address. It allows live migration
>> > of a VM with a direct attached VF without the need to setup a bond/team
>> > between a VF and virtio net device in the guest.
>> > 
>> > The hypervisor needs to enable only one datapath at any time so that
>> > packets don't get looped back to the VM over the other datapath. When a VF
>> Why? Both datapaths could be enabled at a time. Why the loop on
>> hypervisor side would be a problem. This in not an issue for
>> bonding/team as well.
>
>Somehow the hypervisor needs to make sure that the broadcasts/multicasts from the VM
>sent over the VF datapath don't get looped back to the VM via the virtio-net datapth.

Why? Please see below.


>This can happen if both datapaths are enabled at the same time.
>
>I would think this is an issue even with bonding/team as well when virtio-net and
>VF are backed by the same PF.
>
>

I believe that the scenario is the same as on an ordinary nic/swich
network:

...................

  host
 
       bond0
      /     \
    eth0   eth1
     |       |
...................
     |       |
     p1      p2

  switch

...................

It is perfectly valid to p1 and p2 be up and "bridged" together. Bond
has to cope with loop-backed frames. "Failover driver" should too,
it's the same scenario.


>> 
>> 
>> > is plugged, the virtio datapath link state can be marked as down. The
>> > hypervisor needs to unplug the VF device from the guest on the source host
>> > and reset the MAC filter of the VF to initiate failover of datapath to
>> "reset the MAC filter of the VF" - you mean "set the VF mac"?
>
>Yes.  the PF should take away the MAC address assigned to the VF so that the PF
>starts receiving those packets.

Okay, got it. Please put this in the description.


>
>> 
>> 
>> > virtio before starting the migration. After the migration is completed,
>> > the destination hypervisor sets the MAC filter on the VF and plugs it back
>> > to the guest to switch over to VF datapath.
>> > 
>> > It uses the generic failover framework that provides 2 functions to create
>> > and destroy a master failover netdev. When STANDBY feature is enabled, an
>> > additional netdev(failover netdev) is created that acts as a master device
>> > and tracks the state of the 2 lower netdevs. The original virtio_net netdev
>> > is marked as 'standby' netdev and a passthru device with the same MAC is
>> > registered as 'primary' netdev.
>> > 
>> > This patch is based on the discussion initiated by Jesse on this thread.
>> > https://marc.info/?l=linux-virtualization&m=151189725224231&w=2
>> [...]
>> 
>