Re: [PATCH v4 2/2] virtio_net: Extend virtio to use VF datapath when available

[Alexander Duyck <alexander.duyck@gmail.com>]

On Tue, Mar 6, 2018 at 6:38 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Tue, Mar 06, 2018 at 03:27:46PM -0800, Alexander Duyck wrote:
>> > I definitelly vote for a separate common shared code for both netvsc and
>> > virtio_net - even if you use 2 and 3 netdev model, you could share the
>> > common code. Strict checks and limitation should be in place.
>>
>> Noted. But as I also mentioned there isn't that much "common" code
>> between the two models. I think if anything we could probably look at
>> peeling out a few bits such as "get_<iface>_bymac" which really would
>> become dev_get_by_mac_and_ops in order to find the device for the
>> notifiers. I probably wouldn't even put that in our driver and would
>> instead put it in the core code since it almost makes more sense
>> there. Beyond that sharing becomes much more challenging due to the
>> differences in the Rx and Tx paths that build out of the difference
>> between the 2 driver and 3 driver models.
>
> At this point it might be worth it to articulate the advantages
> of the 3 netdev model.

The main advantages are performance on the lower devices. Specifically
we can run LLTX and IFF_NOQUEUE on the upper device, meaning the VF
doesn't take a performance hit when we start transmitting through it.
In addition the paravirtual device is able to fully expose it's
features, so for example virtio_net can maintain in-driver XDP, and we
can provide generic XDP on the upper device. We can also have an
asymmetric configuration where the number of queues or feature sets
can be different between the ports.

Basically what it comes down to is in the 3 netdev model we never have
to deal with the issues of going from being a standard netdev to being
a master netdev. The role of each netdev is clearly defined.

As far as doing a generic solution it is the preferred way to go as
well since we don't have to rewrite functionality of the lower device.
Currently the only changes really needed are to add a phys_port_name
operation so that you can distinguish between the bypass interface and
the original paravirtual one. As such you have no confusion about what
you are running.

> If they are compelling, why wouldn't netvsc users want them?

I believe part of the logic for Stephen's choice is that netvsc
doesn't have a "BACKUP" bit like what we have virtio_net. As a result
they have no way of knowing if a VF will ever show up or not. That
makes the 3 netdev solution less appealing as they always end up in
the bonding mode. In addition they have intentionally limited
themselves to avoid features such as XDP that would cause issues with
the 2 netdev model.

> Alex, I think you were one of the strongest proponents of this model,
> you should be well placed to provide a summary.

Hopefully the information provided helps. In my mind the issue is that
netvsc was not designed correctly, and as such it is using the 2
netdev model as a bit of a crutch to handle the case where they wanted
to add a VF bypass. At this point it has become a part of their
architecture so it would be confusing for their user base to deal with
having 2 netdevs spawn every time their driver sees a new device. In
the case of virtio we only spawn 2 netdevs if the backup bit is set
which implies a specific use case. When the bit isn't set we are only
spawning one device, and as a result we can get much better
performance out of the resultant combination of devices, and can
expose functionality such as in-driver XDP in virtio.