Re: [PATCH net] failover: eliminate callback hell

From: "Samudrala, Sridhar" <sridhar.samudrala@intel.com>
To: Stephen Hemminger <stephen@networkplumber.org>,
	"Michael S. Tsirkin" <mst@redhat.com>
Cc: Jiri Pirko <jiri@resnulli.us>,
	kys@microsoft.com, haiyangz@microsoft.com, davem@davemloft.net,
	netdev@vger.kernel.org,
	Stephen Hemminger <sthemmin@microsoft.com>
Subject: Re: [PATCH net] failover: eliminate callback hell
Date: Wed, 6 Jun 2018 14:54:04 -0700	[thread overview]
Message-ID: <e20a6cdf-34b4-cbc9-1dc9-75c436d6c2fe@intel.com> (raw)
In-Reply-To: <20180606142447.3c5072d8@xeon-e3>

On 6/6/2018 2:24 PM, Stephen Hemminger wrote:
> On Wed, 6 Jun 2018 15:30:27 +0300
> "Michael S. Tsirkin" <mst@redhat.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@networkplumber.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.
> 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 think it should be possible for netvsc to work with 3 dev model if the only
requirement is that eth0 will always be present. With net_failover, you will
see eth0 and eth0nsby OR with older distros eth0 and eth1.  It may be an issue
if somehow there is userspace requirement that there can be only 2 netdevs, not 3
when VF is plugged.

eth0 will be the net_failover device and eth0nsby/eth1 will be the netvsc device
and the IP address gets configured on eth0. Will this be an issue?

>
> 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.
> Any time you introduce new UAPI behavior something breaks.
>
> On the rename front, I really don't care if VF can be renamed. And for
> netvsc want to allow the PV device to be renamed. Udev developers want that
> but have not found a stable/persistent value to expose to userspace
> to allow it.