RE: [PATCH v2] mempool cache: add zero-copy get and put functions

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> Zero-copy access to mempool caches is beneficial for PMD performance, and
> must be provided by the mempool library to fix [Bug 1052] without a
> performance regression.

LGTM in general, thank you for working on it.
Few comments below.
 
> 
> [Bug 1052]: https://bugs.dpdk.org/show_bug.cgi?id=1052
> 
> v2:
> * Fix checkpatch warnings.
> * Fix missing registration of trace points.
> * The functions are inline, so they don't go into the map file.
> v1 changes from the RFC:
> * Removed run-time parameter checks. (Honnappa)
>   This is a hot fast path function; requiring correct application
>   behaviour, i.e. function parameters must be valid.
> * Added RTE_ASSERT for parameters instead.

RTE_ASSERT(n <= RTE_MEMPOOL_CACHE_MAX_SIZE);
I think it is too excessive.
Just:
if (n <= RTE_MEMPOOL_CACHE_MAX_SIZE) return NULL;
seems much more convenient for the users here and
more close to other mempool/ring API behavior.
In terms of performance - I don’t think one extra comparison here
would really count.

I also think would be really good to add:
add zc_(get|put)_bulk_start(),  zc_(get|put)_bulk_finish().
Where _start would check/fill the cache and return the pointer,
while _finsih would updathe cache->len.
Similar to what we have for rte_ring _peek_ API.
That would allow to extend this API usage - let say inside PMDs
it could be used not only for MBUF_FAST_FREE case,  but for generic
TX code path (one that have to call rte_mbuf_prefree()) also.  

>   Code for this is only generated if built with RTE_ENABLE_ASSERT.
> * Removed fallback when 'cache' parameter is not set. (Honnappa)
> * Chose the simple get function; i.e. do not move the existing objects in
>   the cache to the top of the new stack, just leave them at the bottom.
> * Renamed the functions. Other suggestions are welcome, of course. ;-)
> * Updated the function descriptions.
> * Added the functions to trace_fp and version.map.

Would be great to add some test-cases in app/test to cover this new API.