netdev - Re: [PATCH net] failover: eliminate callback hell

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Date:   Thu, 7 Jun 2018 00:47:52 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Jiri Pirko <jiri@...nulli.us>, kys@...rosoft.com,
        haiyangz@...rosoft.com, davem@...emloft.net,
        sridhar.samudrala@...el.com, netdev@...r.kernel.org,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH net] failover: eliminate callback hell

On Wed, Jun 06, 2018 at 02:24:47PM -0700, Stephen Hemminger wrote:
> On Wed, 6 Jun 2018 15:30:27 +0300
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > On Wed, Jun 06, 2018 at 09:25:12AM +0200, Jiri Pirko wrote:
> > > Tue, Jun 05, 2018 at 05:42:31AM CEST, stephen@...workplumber.org wrote:  
> > > >The net failover should be a simple library, not a virtual
> > > >object with function callbacks (see callback hell).  
> > > 
> > > Why just a library? It should do a common things. I think it should be a
> > > virtual object. Looks like your patch again splits the common
> > > functionality into multiple drivers. That is kind of backwards attitude.
> > > I don't get it. We should rather focus on fixing the mess the
> > > introduction of netvsc-bonding caused and switch netvsc to 3-netdev
> > > model.  
> > 
> > So it seems that at least one benefit for netvsc would be better
> > handling of renames.
> > 
> > Question is how can this change to 3-netdev happen?  Stephen is
> > concerned about risk of breaking some userspace.
> > 
> > Stephen, this seems to be the usecase that IFF_HIDDEN was trying to
> > address, and you said then "why not use existing network namespaces
> > rather than inventing a new abstraction". So how about it then? Do you
> > want to find a way to use namespaces to hide the PV device for netvsc
> > compatibility?
> > 
> 
> Netvsc can't work with 3 dev model. MS has worked with enough distro's and
> startups that all demand eth0 always be present. And VF may come and go.

Well failover seems to maintain this invariant with the 3 dev model.

> After this history, there is a strong motivation not to change how kernel
> behaves. Switching to 3 device model would be perceived as breaking
> existing userspace.

I feel I'm misunderstood. I was asking whether a 3-rd device can be
hidden so that userspace does not know that you switched to a 3 device
model. It will think there are 2 devices and will keep working.

If you do that, then there won't be anything that
would be perceived as breaking existing userspace, will there?


> With virtio you can  work it out with the distro's yourself.
> There is no pre-existing semantics to deal with.
> 
> For the virtio, I don't see the need for IFF_HIDDEN.
> With 3-dev model as long as you mark the PV and VF devices
> as slaves, then userspace knows to leave them alone. Assuming userspace
> is already able to deal with team and bond devices.

That's clear enough.

> Any time you introduce new UAPI behavior something breaks.

Not if we do it right.

> On the rename front, I really don't care if VF can be renamed.

OK that's nice.

> And for
> netvsc want to allow the PV device to be renamed.

That's because of the 2 device model, right?  So that explains why even
if the delayed hack is good for the goose it might not be good for the
gander :)

> Udev developers want that
> but have not found a stable/persistent value to expose to userspace
> to allow it.