Re: [ovs-dev] [PATCH RFC] dpif-netdev: Add support Intel DPDK based ports.

[Vincent JARDIN <vincent.jardin-pdR9zngts4EAvxtiuMwx3w@public.gmane.org>]

Thomas,

First and easy answer: it is open source, so anyone can recompile. So, 
what's the issue?

> Without a concept of stable interfaces, it will be difficult to
> package and distribute RTE libraries, PMD, and DPDK applications. Right
> now, the obvious path would include packaging the PMD bits together
> with each DPDK application depending on the version of DPDK the binary
> was compiled against. This is clearly not ideal.

>
>> I agree that some areas could be improved since they are not into the
>> critical datapath of packets, but still other areas remain very CPU
>> constraints. For instance:
>> http://dpdk.org/browse/dpdk/commit/lib/librte_ether/rte_ethdev.h?id=c3d0564cf0f00c3c9a61cf72bd4bd1c441740637
>>
>> is bad:
>>     struct eth_dev_ops
>> is churned, no comment, and a #ifdef that changes the structure
>> according to compilation!
>
> This is a very good example as it outlines the difference between
> control structures and the fast path. We have this same exact trade off
> in the kernel a lot where we have highly optimized internal APIs
> towards modules and drivers but want to provide binary compatibility to
> a certain extend.

As long as we agree on this limited scope, we'll think about it and 
provide a proposal on dev-VfR2kkLFssw@public.gmane.org mailing list.

> As for the specific example you mention, it is relatively trivial to
> make eth_dev_ops backwards compatible by appending appropriate padding
> to the struct before a new major release and ensure that new members
> are added by replacing the padding accordingly. Obviously no ifdefs
> would be allowed anymore.

Of course, it is basic C!

>> Should an application use the librte libraries of the DPDK:
>>    - you can use RTE_VERSION and RTE_VERSION_NUM :
>> http://dpdk.org/doc/api/rte__version_8h.html#a8775053b0f721b9fa0457494cfbb7ed9
>
> Right. This would be more or less identical to requiring a specific
> DPDK version in OVS_CHEC_DPDK. It's not ideal to require application to
> clutter their code with #ifdefs all over for every new minor release
> though.
>
>>    - you can write your own wrapper (with CPU overhead) in order to have
>> a stable ABI, that wrapper should be tight to the versions of the librte
>> => the overhead is part of your application instead of the DPDK,
>>    - *otherwise recompile your software, it is opensource, what's the
>> issue?*
>>
>> We are opened to any suggestion to have stable ABI, but it should never
>> remove the options to have fast/efficient/compilation/CPU execution
>> processing.
>
> Absolutely agreed. We also don't want to add tons of abstraction and
> overcomplicate everything. Still, I strongly believe that the definition
> of stable interfaces towards applications and especially PMD is
> essential.
>
> I'm not proposing to standardize all the APIs towards applications on
> the level of POSIX. DPDK is in early stages and disruptive changes will
> come along. What I would propose on an abstract level is:
>
> 1. Extend but not break API between minor releases. Postpone API
>     breakages to the next major release. High cadence of major
>     releases initially, lower cadence as DPDK matures.
>
> 2. Define ABI stability towards PMD for minor releases to allow
>     isolated packaging of PMD by padding control structures and keeping
>     functions ABI stable.

I get lost: do you mean ABI + API toward the PMDs or towards the 
applications using the librte ?

Best regards,
   Vincent