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

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

Hi Thomas,

On 29/01/2014 09:15, Thomas Graf wrote:

 > The obvious and usual best practise would be for DPDK to guarantee
 > ABI stability between minor releases.
 >
 > Since dpdk-dev is copied as well, any comments?

DPDK's ABIs are not Kernel's ABIs, they are not POSIX, there is no 
standard. Currently, there is no such plan to have a stable ABI since we 
need to keep freedom to chase CPU cycles over having a stable ABI. For 
instance, some applications on top of the DPDK process the packets in 
less than 150 CPU cycles (have a look at testpmd:
   http://dpdk.org/browse/dpdk/tree/app/test-pmd )

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!

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
   - 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.

Best regards,
   Vincent