netdev - Re: [PATCH v4 net-next 0/8] flow_dissector: Protocol specific flow dissector offload

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Message-ID: <20171003074632.GD1916@nanopsycho>
Date:   Tue, 3 Oct 2017 09:46:32 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Tom Herbert <tom@...bertland.com>
Cc:     David Miller <davem@...emloft.net>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Tom Herbert <tom@...ntonium.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Rohit Seth <rohit@...ntonium.net>
Subject: Re: [PATCH v4 net-next 0/8] flow_dissector: Protocol specific flow
 dissector offload

Fri, Sep 29, 2017 at 07:59:35PM CEST, tom@...bertland.com wrote:
>On Fri, Sep 29, 2017 at 10:42 AM, David Miller <davem@...emloft.net> wrote:
>> From: Tom Herbert <tom@...bertland.com>
>> Date: Fri, 29 Sep 2017 08:48:55 -0700
>>
>>> The flow_dissector interface is not a uAPI.
>>
>> That's not true, insofar as cls_flower.c uses the flow_dissector
>> therefore if you change the flow_dissector in certain ways then
>> cls_flower.c might have it's behavior changed and that is in fact UAPI
>> facing.
>
>Then I would suggest adding another flag like FLOW_DISSECTOR_F_FLOWER
>and when anyone puts new code into flow_dissector they can wrap it
>with "if !(flags & FLOW_DISSECTOR_F_FLOWER)". If the flower uAPI is
>subsequently update then the conditional can be removed. This way
>flower can support maintain its APIs, but we can still still extend
>and improve flow_dissector for othersuse cases.

This is not flower-specific problem. Flow_dissector is a servant of many.
As such, it is instructed what should it do. If you want to
change the way inner headers are parsed, you should either:
1) change the callers so they are behaving the same as before
2) make the flow_dissection change optional so the caller can say if he
   wants original or new behaviour.