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:

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On Tue, 23 Jan 2018 02:05:48 +0200, Michael S. Tsirkin wrote:  

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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.

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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?

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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).

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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).

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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 :)

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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

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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.