netdev - Re: Focusing the XDP project

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Date:   Wed, 22 Feb 2017 00:40:46 +0200
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Tom Herbert <tom@...bertland.com>
Cc:     David Miller <davem@...emloft.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        john fastabend <john.fastabend@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Brenden Blanco <bblanco@...il.com>
Subject: Re: Focusing the XDP project

On Tue, Feb 21, 2017 at 7:40 PM, Tom Herbert <tom@...bertland.com> wrote:
> On Tue, Feb 21, 2017 at 8:46 AM, David Miller <davem@...emloft.net> wrote:
>> From: Tom Herbert <tom@...bertland.com>
>> Date: Tue, 21 Feb 2017 08:35:41 -0800
>>
>>> We already have good APIs for similar datapath functionality (GRO,
>>> GSO, xmit_more, etc.), and I don't see why XDP is so special that we
>>> can't come up with a reasonable API for batching and implement it.
>>
>> What you are missing is that it wasn't always this way.
>>
>> The initial TSO support was a hodge-podge of weird driver APIs and
>> simple heuristics thrown all over the place.  It was ugly but worked
>> and allowed us to experiment.  We had to adjust driver internals
>> a lot until on the way towards getting things how they are today.
>>
>> This is the natural course of things, so please don't suggest that XDP
>> shouldn't evolve in the same way.
>>
>> I think we really need to be fast and loose right now and only try to
>> constrict and perfect the API after some of this initial activity has
>> died down.
>>
>> Yes it's more work for the brave drivers that add XDP support, but
>> unfortunately that's how we figure out what's really needed and works
>> in the long term.
>
> I'd be more supportive of this line of thinking if we (e.g. FB) didn't
> have to spend the majority time over the past few months trying to
> deal with all the complexity being thrown into drivers for all these
> new features such as XDP. Case in point, Mellanox drivers are
> completely non-modular and have a horrible directory structure. They
> tried to fix, this but the patch set was rejected because it would
> break people trying to do backports. That's a fair argument, but the
> lesson I gather from that is that we should put more time in up front
> thinking about how to structure code the right way instead of just
> throwing it in and trying to deal with the consequences later.
>

I also gathered the same lesson :) I will do my best to separate
between regular RX path and XDP-enabled RX path as much as possible in
my next RX staging/bulking patches as the current mlx5 design allows
me to do so with a little code duplication.

> Tom