Date:   Sun, 4 Mar 2018 19:50:34 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>, virtio-dev@...ts.oasis-open.org,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        Jakub Kicinski <kubakici@...pl>
Subject: Re: [PATCH v4 2/2] virtio_net: Extend virtio to use VF datapath when
 available

Sun, Mar 04, 2018 at 07:24:12PM CET, alexander.duyck@...il.com wrote:
>On Sat, Mar 3, 2018 at 11:13 PM, Jiri Pirko <jiri@...nulli.us> wrote:
>> Sun, Mar 04, 2018 at 01:26:53AM CET, alexander.duyck@...il.com wrote:
>>>On Sat, Mar 3, 2018 at 1:25 PM, Jiri Pirko <jiri@...nulli.us> wrote:
>>>> Sat, Mar 03, 2018 at 07:04:57PM CET, alexander.duyck@...il.com wrote:
>>>>>On Sat, Mar 3, 2018 at 3:31 AM, Jiri Pirko <jiri@...nulli.us> wrote:
>>>>>> Fri, Mar 02, 2018 at 08:42:47PM CET, mst@...hat.com wrote:
>>>>>>>On Fri, Mar 02, 2018 at 05:20:17PM +0100, Jiri Pirko wrote:
>>>>>>>> >Yeah, this code essentially calls out the "shareable" code with a
>>>>>>>> >comment at the start and end of the section what defines the
>>>>>>>> >virtio_bypass functionality. It would just be a matter of mostly
>>>>>>>> >cutting and pasting to put it into a separate driver module.
>>>>>>>>
>>>>>>>> Please put it there and unite the use of it with netvsc.
>>>>>>>
>>>>>>>Surely, adding this to other drivers (e.g. might this be handy for xen
>>>>>>>too?) can be left for a separate patchset. Let's get one device merged
>>>>>>>first.
>>>>>>
>>>>>> Why? Let's do the generic infra alongside with the driver. I see no good
>>>>>> reason to rush into merging driver and only later, if ever, to convert
>>>>>> it to generic solution. On contrary. That would lead into multiple
>>>>>> approaches and different behavious in multiple drivers. That is plain
>>>>>> wrong.
>>>>>
>>>>>If nothing else it doesn't hurt to do this in one driver in a generic
>>>>>way, and once it has been proven to address all the needs of that one
>>>>>driver we can then start moving other drivers to it. The current
>>>>>solution is quite generic, that was my contribution to this patch set
>>>>>as I didn't like how invasive it was being to virtio and thought it
>>>>>would be best to keep this as minimally invasive as possible.
>>>>>
>>>>>My preference would be to give this a release or two in virtio to
>>>>>mature before we start pushing it onto other drivers. It shouldn't
>>>>>take much to cut/paste this into a new driver file once we decide it
>>>>>is time to start extending it out to other drivers.
>>>>
>>>> I'm not talking about cut/paste and in fact that is what I'm worried
>>>> about. I'm talking about common code in net/core/ or somewhere that
>>>> would take care of this in-driver bonding. Each driver, like virtio_net,
>>>> netvsc would just register some ops to it and the core would do all
>>>> logic. I believe it is essential take this approach from the start.
>>>
>>>Sorry, I didn't mean cut/paste into another driver, I meant to make it
>>>a driver of its own. My thought was to eventually create a shared/core
>>>driver module that is then used by the other drivers.
>>>
>>>My concern right now is that Stephen has indicated he doesn't want
>>>this approach taken with netvsc, and most of the community doesn't
>>
>> IIUC, he only does not like the extra netdev. Is there anything else?
>
>Nope that is pretty much it. It doesn't seem like a big deal for
>virtio, but for netvsc it is significant since they don't have any
>"backup" bit feature differentiation, so they would likely be stuck
>with 2 netdevs even in their basic setup.

Okay. If that is a strict "no-go" for netvsc, this should be
just a flag passed down to the in-driver bond code.


>
>>>want the netvsc approach applied to virtio. Until that impasse can be
>>>resolved there isn't much value in trying to split this up so it is
>>>available to other drivers. In addition I would imagine it would make
>>>it a pain for others to back-port into distros since it would break
>>>legacy netvsc driver behavior. Patches are always welcome. Once this
>>>is in you are free to try fighting to get this made into a generic
>>>module and applied to both drivers, but we have already spent close to
>>>3 months on this and it seems like there has been significantly more
>>
>> Alex, time is never a good argument for poor design and shortcuts.
>
>I'm not saying we should go with a poor design due to time. But
>expecting us to implement something where the maintainer of said
>driver has not agreed to is pointless, and I don't see it as a design

He just does not like the third netdev, not the fact that the code
for in-driver bonding would be shared.


>shortcut to implement something in one driver with the expectation
>that we will then make it core later once it has proven itself and has
>use elsewhere. In the meantime I would imagine it also makes it easier
>for things like backports and such for us to do it this way since we
>are only impacting one driver.

When you are working on upstream kernel, you should not care about
backports. That leads to poor design. Not an argument.


>
>You are telling us to do something that not everyone has agreed to.

Who did not?


>Currently we only have agreement from Michael on taking this code, as
>such we are working with virtio only for now. When the time comes that

If you do duplication of netvsc in-driver bonding in virtio_net, it will
stay there forever. So what you say is: "We will do it halfway now
and promise to fix it later". That later will never happen, I'm pretty
sure. That is why I push for in-driver bonding shared code as a part of
this patchset.

+ if you would be pushing first driver to do this, I would understand.
But the first driver is already in. You are pushing second. This is the
time to do the sharing, unification of behaviour. Next time is too late.


>we can get other maintainers, specifically Stephen, to agree to it
>then we can cut/paste this code into a core file or into a module of
>its own. Alternatively I suppose we could take this up to Dave if you
>can't get Stephen to agree. If you can get Dave to say we need to
>change netvsc then we will go ahead with it, but generally I prefer to
>respect when the maintainer of something says they don't want us
>modifying their code in some way.

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