[dpdk-dev] RFC: hiding struct rte_eth_dev

Ray Kinsella mdr at ashroe.eu
Mon Sep 23 18:19:27 CEST 2019


Hi folks,

The ABI Stability proposals should be pretty well known at this point.
The latest rev is here ...

http://inbox.dpdk.org/dev/1565864619-17206-1-git-send-email-mdr@ashroe.eu/

As has been discussed public data structure's are risky for ABI
stability, as any changes to a data structure can change the ABI. As a
general rule you want to expose as few as possible (ideally none), and
keep them as small as possible.

One of the key data structures in DPDK is `struct rte_eth_dev`. In this
case, rte_eth_dev is exposed public-ally, as a side-effect of the
inlining of the [rx,tx]_burst functions.

Marcin Zapolski has been looking at what to do about it, with no current
consensus on a path forward. The options on our table is:-

1. Do nothing, live with the risk to DPDK v20 ABI stability.

2. Pad rte_eth_dev, add some extra bytes to the structure "in case" we
need to add a field during the v20 ABI (through to 20.11).

3. Break rte_eth_dev into public and private structs.
  - See
http://inbox.dpdk.org/dev/20190906131813.1343-1-marcinx.a.zapolski@intel.com/
  - This ends up quiet an invasive patch, late in the cycle, however it
does have no performance penalty.

4. Uninline [rx,tx]_burst functions
 -  See
http://inbox.dpdk.org/dev/20190730124950.1293-1-marcinx.a.zapolski@intel.com/
 - This has a performance penalty of ~2% with testpmd, impact on a "real
workload" is likely to be in the noise.

We need to agree an approach for v19.11, and that may be we agree to do
nothing. My personal vote is 4. as the simplest with minimal impact.

Thanks,

Ray K


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