netdev - Re: [virtio-dev] Re: [RFC PATCH net-next v2 2/2] virtio_net: Extend virtio to use VF datapath when available

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Message-ID: <20180126215829.7b3c6bac@cakuba.netronome.com>
Date:   Fri, 26 Jan 2018 21:58:29 -0800
From:   Jakub Kicinski <kubakici@...pl>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Siwei Liu <loseweigh@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>
Subject: Re: [virtio-dev] Re: [RFC PATCH net-next v2 2/2] virtio_net: Extend
 virtio to use VF datapath when available

On Fri, 26 Jan 2018 21:33:01 -0800, Samudrala, Sridhar wrote:
> >> 3 netdev model breaks this configuration starting with the creation
> >> and naming of the 2 devices to udev needing to be aware of master and
> >> slave virtio-net devices.  
> > I don't understand this comment.  There is one virtio-net device and
> > one "virtio-bond" netdev.  And user space has to be aware of the special
> > automatic arrangement anyway, because it can't touch the VF.  It
> > doesn't make any difference whether it ignores the VF or PV and VF.
> > It simply can't touch the slaves, no matter how many there are.  
> 
> If the userspace is not expected to touch the slaves, then why do we need to
> take extra effort to expose a netdev that is just not really useful.

You said:
"[user space] needs to be aware of master and slave virtio-net devices."

I'm saying:
It has to be aware of the special arrangement whether there is an
explicit bond netdev or not.

> >> Also, from a user experience point of view, loading a virtio-net with
> >> BACKUP feature enabled will now show 2 virtio-net netdevs.  
> > One virtio-net and one virtio-bond, which represents what's happening.  
> This again assumes that we want to represent a bond setup. Can't we 
> treat this
> as virtio-net providing an alternate low-latency datapath by taking over 
> VF datapath?

Bond is just a familiar name, we can call it something else if you
prefer.  The point is there are two data paths which can have
independent low-level settings and a higher level entity with
global settings which represents any path to the outside world.

Hiding low-level netdevs from a lay user requires a generic solution.