netdev - Re: [virtio-dev] [RFC PATCH net-next v2 1/2] virtio_net: Introduce VIRTIO_NET_F

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Date:   Mon, 22 Jan 2018 17:13:01 -0800
From:   Jakub Kicinski <kubakici@...pl>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Alexander Duyck <alexander.duyck@...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>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        achiad shochat <achiad.mellanox@...il.com>,
        Achiad Shochat <achiad@...lanox.com>
Subject: Re: [virtio-dev] [RFC PATCH net-next v2 1/2] virtio_net: Introduce
 VIRTIO_NET_F_BACKUP feature bit

On Tue, 23 Jan 2018 02:47:57 +0200, Michael S. Tsirkin wrote:
> On Mon, Jan 22, 2018 at 04:16:23PM -0800, Jakub Kicinski wrote:
> > On Tue, 23 Jan 2018 02:05:48 +0200, Michael S. Tsirkin wrote:  
> > > > As we are using virtio_net to control and manage the VF data path, it is not
> > > > clear to me
> > > > what is the advantage of creating a new device rather than extending
> > > > virtio_net to manage
> > > > the VF datapath via transparent bond mechanism.  
> > > 
> > > So that XDP redirect actions can differentiate between virtio, PT
> > > device and the bond. Without it there's no way to redirect
> > > to virtio specifically.  
> > 
> > Let's make a list :P
> > 
> > separate netdev:
> > 1. virtio device being a bond master is confusing/unexpected.
> > 2. virtio device being both a master and a slave is confusing.  
> 
> vlans are like this too, aren't they?

Perhaps a bad wording.  Both master and member would be better.

> > 3. configuration of a master may have different semantics than
> >    configuration of a slave.
> > 4. two para-virt devices may create a loop (or rather will be bound 
> >    to each other indeterministically, depending on which spawns first).  
> 
> For 2 virtio devices, we can disable the bond to make it deterministic.

Do you mean the hypervisor can or is there a knob in virtio_net to mask
off features?  Would that require re-probe of the virtio device?

> > 5. there is no user configuration AFAIR in existing patch, VM admin
> >    won't be able to prevent the bond.  Separate netdev we can make 
> >    removable even if it's spawned automatically.  
> 
> That's more or less a feature. If you want configurability, just use
> any of the existing generic solutions (team,bond,bridge,...).

The use case in mind is that VM admin wants to troubleshoot a problem
and temporarily disable the auto-bond without touching the hypervisor 
(and either member preferably).

> > 6. XDP redirect use-case (or any explicit use of the virtio slave)
> >    (from MST)
> > 
> > independent driver:
> > 7. code reuse.  
> 
> With netvsc? That precludes a separate device though because of
> compatibility.

Hopefully with one of the established bonding drivers (fingers
crossed).  We may see proliferation of special bonds (see Achiad's
presentation from last netdev about NIC-NUMA-node-bonds).

> > separate device:  
> 
> I'm not sure I understand how "separate device" is different from
> "separate netdev". Do you advocate for a special device who's job is
> just to tell the guest "bind these two devices together"?
> 
> Yea, sure, that works. However for sure it's more work to
> implement and manage at all levels. Further
> 
> - it doesn't answer the question
> - a feature bit in a virtio device is cheap enough that
>   I wouldn't worry too much about this feature
>   going unused later.

Right, I think we are referring to different things as device.  I mean
a bus device/struct device, but no strong preference on that one.  I'll
be happy as long as there is a separate netdev, really :)

> > 8. bond any netdev with any netdev.
> > 9. reuse well-known device driver model.
> > a. natural anchor for hypervisor configuration (switchdev etc.)  
> 
> saparate netdev has the same property
>
> > b. next-gen silicon may be able to disguise as virtio device, and the
> >    loop check in virtio driver will prevent the legitimate bond it such
> >    case.  AFAIU that's one of the goals of next-gen virtio spec as well.  
> 
> In fact we have a virtio feature bit for the fallback.
> So this part does not depend on how software in guest works
> and does not need software solutions.

You mean in the new spec?  Nice.  Still I think people will try to
implement the old one too given sufficiently capable HW.