netdev - Re: Let's do P4

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Date:   Sun, 30 Oct 2016 15:39:05 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Thomas Graf <tgraf@...g.ch>,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kubakici@...pl>, netdev@...r.kernel.org,
        davem@...emloft.net, jhs@...atatu.com, roopa@...ulusnetworks.com,
        simon.horman@...ronome.com, ast@...nel.org, daniel@...earbox.net,
        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

On Sun, Oct 30, 2016 at 05:38:36PM +0100, Jiri Pirko wrote:
> Sun, Oct 30, 2016 at 11:26:49AM CET, tgraf@...g.ch wrote:
> >On 10/30/16 at 08:44am, Jiri Pirko wrote:
> >> Sat, Oct 29, 2016 at 06:46:21PM CEST, john.fastabend@...il.com wrote:
> >> >On 16-10-29 07:49 AM, Jakub Kicinski wrote:
> >> >> On Sat, 29 Oct 2016 09:53:28 +0200, Jiri Pirko wrote:
> >> >>> Hi all.
> >> >>>

sorry for delay. travelling to KS, so probably missed something in
this thread and comments can be totally off...

the subject "let's do P4" is imo misleading, since it reads like
we don't do P4 at the moment, whereas the opposite is true.
Several p4->bpf compilers is a proof.

> The network world is divided into 2 general types of hw:
> 1) network ASICs - network specific silicon, containing things like TCAM
>    These ASICs are suitable to be programmed by P4.

i think the opposite is the case in case of P4.
when hw asic has tcam it's still far far away from being usable with P4
which requires fully programmable protocol parser, arbitrary tables and so on.
P4 doesn't even define TCAM as a table type. The p4 program can declare
a desired algorithm of search in the table and compiler has to figure out
what HW resources to use to satisfy such p4 program.

> 2) network processors - basically a general purpose CPUs
>    These processors are suitable to be programmed by eBPF.

I think this statement is also misleading, since it positions
p4 and bpf as competitors whereas that's not the case.
p4 is the language. bpf is an instruction set.

> Exactly. Following drawing shows p4 pipeline setup for SW and Hw:
> 
>                                  |
>                                  |               +--> ebpf engine
>                                  |               |
>                                  |               |
>                                  |           compilerB
>                                  |               ^
>                                  |               |
> p4src --> compilerA --> p4ast --TCNL--> cls_p4 --+-> driver -> compilerC -> HW
>                                  |
>                        userspace | kernel
>                                  |

frankly this diagram smells very much like kernel bypass to me,
since I cannot see how one can put the whole p4 language compiler
into the driver, so this last step of p4ast->hw, I presume, will be
done by firmware, which will be running full compiler in an embedded cpu
on the switch. To me that's precisely the kernel bypass, since we won't
have a clue what HW capabilities actually are and won't be able to fine
grain control them.
Please correct me if I'm wrong.

> Plus the thing I cannot imagine in the model you propose is table fillup.
> For ebpf, you use maps. For p4 you would have to have a separate HW-only
> API. This is very similar to the original John's Flow-API. And therefore
> a kernel bypass.

I think John's flow api is a better way to expose mellanox switch capabilities.
I also think it's not fair to call it 'bypass'. I see nothing in it
that justify such 'swear word' ;)
The goal of flow api was to expose HW features to user space, so that
user space can program it. For something simple as mellanox switch
asic it fits perfectly well.
Unless I misunderstand the bigger goal of this discussion and it's
about programming ezchip devices.

If the goal is to model hw tcam in the linux kernel then just introduce
tcam bpf map type. It will be dog slow in user space, but it will
match exactly what is happnening in the HW and user space can make
sensible trade-offs.