Date:   Sat, 29 Oct 2016 13:28:34 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Thomas Graf <tgraf@...g.ch>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jhs@...atatu.com,
        roopa@...ulusnetworks.com, john.fastabend@...il.com,
        jakub.kicinski@...ronome.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
Subject: Re: Let's do P4

Sat, Oct 29, 2016 at 01:15:48PM CEST, tgraf@...g.ch wrote:
>On 10/29/16 at 12:10pm, Jiri Pirko wrote:
>> Sat, Oct 29, 2016 at 11:39:05AM CEST, tgraf@...g.ch wrote:
>> >On 10/29/16 at 09:53am, Jiri Pirko wrote:
>> >> 3) Expose the p4ast in-kernel interpreter to userspace
>> >>    As the easiest way I see in to introduce a new TC classifier cls_p4.
>> >> 
>> >>    This can work in a very similar way cls_bpf is:
>> >>    $ tc filter add dev eth0 ingress p4 da ast example.ast
>> >> 
>> >>    The TC cls_p4 will be also used for runtime table manipulation.
>> >
>> >I think this is a great model for the case where HW can provide all
>> >of the required capabilities. Thinking about the case where HW
>> >provides a subset and SW provides an extended version, i.e. the
>> >reality we live in for hosts with ASIC NICs ;-) The hand off point
>> >requires some understanding between p4ast and eBPF.
>> 
>> It can be the other way around. The p4>ebpf compiler won't be complete
>> at the beginning so it is possible that HW could provide more features.
>> I don't think it is a problem. With SKIP_SW and SKIP_HW flags in TC,
>> the user can set different program to each. I think in real life, that
>> would be the most common case anyway.
>
>So given the SKIP_SW flag, the in-kernel compiler is optional anyway.
>Why even risk including a possibly incomplete compiler? Older kernels
>must be capable of running along newer hardware as long as eBPF can
>represent the software path. Having to upgrade to latest and greatest
>kernels is not an option for most people so they would simply have to
>fall back to SKIP_SW and do it in user space anyway.

The thing is, if we needo to offload something, it needs to be
implemented in kernel first. Also, I believe that it is good to have
in-kernel p4 engine for testing and development purposes.


>
>> >Therefore another idea would be to use cls_bpf directly for this. The
>> >p4ast IR could be stored in a separate ELF section in the same object
>> >file with an existing eBPF program. The p4ast IR will match the
>> 
>> I don't like this idea. The kernel API should be clean and simple.
>> Bundling p4ast with bpf.o code, so the bpf.o is for kernel and p4ast is
>> for driver does not look clean at all. The bundle does not make really
>> sense as the programs may do different things for BPF and p4.
>
>I don't care strongly for the bundle. Let's forget about it for now.
>
>> Plus, it's up to user to set this up like he wants. If he wants SW
>> processing by BPF and at the same time HW processing by P4, he will use:
>> cls_bpf instance with SKIP_HW
>> cls_p4 instance with SKIP_SW.
>> 
>> This is much more variable, clean and non-confusing approach, I believe.
>
>Non ASIC hardware will want to do offload based on BPF though so your
>model would require the user to be aware of what is the preferred
>model for his hardware and then either load a cls_bpf only to work
>with a Netronome NIC or a cls_p4 + cls_bpf to work with an ASIC NIC,
>correct?

Correct


>
>I'm not seeing how either of them is more or less variable. The main
>difference is whether to require configuring a single cls with both
>p4ast + bpf or two separate cls, one for each. I'd prefer the single
>cls approach simply because it is cleaner wither regard to offload
>directly off bpf vs off p4ast.

That's the bundle that you asked me to forget earlier in this email? :)

>
>My main point is to not include a IR to eBPF compiler in the kernel
>and let user space handle this instead.

It we do it as you describe, we would be using 2 different APIs for
offloaded and non-offloaded path. I don't believe it is acceptable as
the offloaded features has to have kernel implementation. Therefore, I
believe that p4ast as a kernel API is the only possible option.

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