Subject: Re: Let's do P4

From: John Fastabend <[email-masked]>
Date: Mon, 31 Oct 2016 12:35:00 -0700

[...]

>>>
>>
>> I think the issue with offloading a P4-AST will be how much work goes
>> into mapping this onto any particular hardware instance. And how much
>> of the P4 language feature set is exposed.
>>
>> For example I suspect MLX switch has a different pipeline than MLX NIC
>> and even different variations of the product lines. The same goes for
>> Intel pipeline in NIC and switch and different products in same line.
>>
>> If P4-ast describes the exact instance of the hardware its an easy task
>> the map is 1:1 but isn't exactly portable. Taking an N table onto a M
>> table pipeline on the other hand is a bit more work and requires various
>> transformations to occur in the runtime API. I'm guessing the class of
>> devices we are talking about here can not reconfigure themselves to
>> match the P4-ast.
> 
> I believe we can assume that. the p4ast has to be generic as the
> original p4source is. It would be a terrible mistake to couple it with
> some specific hardware. I only want to use p4ast because it would be easy
> parse in kernel, unlike p4source.

Sure but in the fixed ASIC cases the universe of P4 programs is much
larger than the handful of ones that can be 'accepted' by the device. So
you really need to have some knowledge of the hardware. However if you
believe (guessing from last bullet) that devices will be configurable
in the future then its more likely that the hardware can 'accept' the
program.

> 
> 
>>
>> In the naive implementation only pipelines that map 1:1 will work. Maybe
>> this is what Alexei is noticing?
> 
> P4 is ment to program programable hw, not fixed pipeline.
> 

I'm guessing there are no upstream drivers at the moment that support
this though right? The rocker universe bits though could leverage this.

> 
>>
>>>
>>>> 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
>>>
>>> In case of mlxsw, that compiler would be in driver.
>>>
>>>
>>>> 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.
>>>
>>> You are wrong. By your definition, everything has to be figured out in
>>> driver and FW does nothing. Otherwise it could do "something else" and
>>> that would be a bypass? Does not make any sense to me whatsoever.
>>>
>>>
>>>>
>>>>> 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.
>>>
>>> We are under impression that p4 suits us nicely. But it is not about
>>> us, it is about finding the common way to do this.
>>>
>>
>> I'll just poke at my FlowAPI question again. For fixed ASICS what is
>> the Flow-API missing. We have a few proof points that show it is both
>> sufficient and usable for the handful of use cases we care about.
> 
> Yeah, it is most probably fine. Even for flex ASICs to some point. The
> question is how it stands comparing to other alternatives, like p4
> 

Just to be clear the Flow-API _was_ generated from the initial P4 spec.
The header files and tools used with it were autogenerated ("compiled"
in a loose sense) from the P4 program. The piece I never exposed
was the set_* operations to reconfigure running systems. I'm not sure
how valuable this is in practice though.

Also there is a P4-16 spec that will be released shortly that is more
flexible and also more complex.

> 
>>
>>>
>>>> I also think it's not fair to call it 'bypass'. I see nothing in it
>>>> that justify such 'swear word' ;)
>>>
>>> John's Flow-API was a kernel bypass. Why? It was a API specifically
>>> designed to directly work with HW tables, without kernel being involved.
>>
>> I don't think that is a fair definition of HW bypass. The SKIP_SW flag
>> does exactly that for 'tc' based offloads and it was not rejected.
> 
> No, no, no. You still have possibility to do the same thing in kernel,
> same functionality, with the same API. That is a big difference.
> 
> 
>>
>> The _real_ reason that seems to have fallen out of this and other
>> discussion is the Flow-API didn't provide an in-kernel translation into
>> an emulated patch. Note we always had a usermode translation to eBPF.
>> A secondary reason appears to be overhead of adding yet another netlink
>> family.
> 
> Yeah. Maybe you remember, back then when Flow-API was being discussed,
> I suggested to wrap it under TC as cls_xflows and cls_xflowsaction of
> some sort and do in-kernel datapath implementation. I believe that after
> that, it would be acceptable.
> 

As I understand the thread here that is exactly the proposal here right?
With a discussion around if the structures/etc are sufficient or any
alternative representations exist.

> 
>>
>>>
>>>
>>>> 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.
>>>
>>> Again, this is not mlx-asic-specific. And again, that is a kernel bypass.
>>>
>>>
>>>> Unless I misunderstand the bigger goal of this discussion and it's
>>>> about programming ezchip devices.
>>>
>>> No. For network processors, I believe that BPF is nicely offloadable, no
>>> need to do the excercise for that.
>>>
>>>
>>>>
>>>> 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.
>>>
>>> No, you got me completely wrong. This is not about the TCAM. This is
>>> about differences in the 2 words (p4/bpf).
>>> Again, for "p4-ish" devices, you have to translate BPF. And as you
>>> noted, it's an instruction set. Very hard if not impossible to parse in
>>> order to get back the original semantics.
>>>
>>
>> I think in this discussion "p4-ish" devices means devices with multiple
>> tables in a pipeline? Not devices that have programmable/configurable
>> pipelines right? And if we get to talking about reconfigurable devices
>> I believe this should be done out of band as it typically means
>> reloading some ucode, etc.
> 
> I'm talking about both. But I think we should focus on reconfigurable
> ones, as we probably won't see that much fixed ones in the future.
> 

hmm maybe but the 10/40/100Gbps devices are going to be around for some
time. So we need to ensure these work well.

.John