netdev - Re: Let's do P4

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Date:   Wed, 2 Nov 2016 16:27:11 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Thomas Graf <tgraf@...g.ch>, Jakub Kicinski <kubakici@...pl>,
        netdev@...r.kernel.org, davem@...emloft.net, jhs@...atatu.com,
        roopa@...ulusnetworks.com, simon.horman@...ronome.com,
        ast@...nel.org, prem@...efootnetworks.com,
        hannes@...essinduktion.org, jbenc@...hat.com, tom@...bertland.com,
        mattyk@...lanox.com, idosch@...lanox.com, eladr@...lanox.com,
        yotamg@...lanox.com, nogahf@...lanox.com, ogerlitz@...lanox.com,
        linville@...driver.com, andy@...yhouse.net, f.fainelli@...il.com,
        dsa@...ulusnetworks.com, vivien.didelot@...oirfairelinux.com,
        andrew@...n.ch, ivecera@...hat.com,
        Maciej Żenczykowski <zenczykowski@...il.com>
Subject: Re: Let's do P4

Wed, Nov 02, 2016 at 04:22:50PM CET, john.fastabend@...il.com wrote:

[...]

>>>
>>> What is your compilerA? Is that part of tc in user space? Maybe linked
>> 
>> It is something that transforms original p4 source to some intermediate
>> form, easy to be processed by in-kernel compilers.
>> 
>> 
>>> against LLVM lib, for example? If you really want some sw path, can't tc
>>> do this transparently from user space instead when it gets a netlink error
>>> that it cannot get offloaded (and thus switch internally to f_bpf's loader)?
>> 
>> In real life, user will most probably use p4 for hw programming, but the
>> sw fallback will be done in bpf directly. In that case, he would use
>> cls_bfp SKIP_HW
>> cls_p4 SKIP_SW
>> 
>> But in order to allow cls_p4 offloading to hw, we need in-kernel
>> interpreter. That is purpose of compilerB to take agvantage of bpf, but
>> the in-kernel interpreter could be implemented differently.
>> 
>
>But this is the issue. We openly acknowledge it wont actually be used.
>We have multiple user space compilers that generate at least half way
>reasonable ebpf code that is being used in real deployments and
>works great for testing. This looks like pure overhead to satisfy this
>hw/sw parity checkbox and I can't see why anyone would use it or even
>maintain it. Looks like a checkbox and I like to avoid useless work that
>is likely to bit rot.

That's how it works I'm afraid, unless something changed from the last
time we discussed this. Without in-kernel implementation, it's a bypass.

Dave?