[dpdk-dev] [PATCH v2] mbuf: document guideline for new fields and flags

[Jerin Jacob]

On Thu, Jun 11, 2020 at 12:02 PM Thomas Monjalon <thomas at monjalon.net> wrote:
>
> Since dynamic fields and flags were added in 19.11,
> the idea was to use them for new features, not only PMD-specific.
>
> The guideline is made more explicit in doxygen, in the mbuf guide,
> and in the contribution design guidelines.
>
> For more information about the original design, see the presentation
> https://www.dpdk.org/wp-content/uploads/sites/35/2019/10/DynamicMbuf.pdf
>
> This decision was discussed in the Technical Board:
> http://mails.dpdk.org/archives/dev/2020-June/169667.html
>
> Signed-off-by: Thomas Monjalon <thomas at monjalon.net>
> Acked-by: Olivier Matz <olivier.matz at 6wind.com>

Acked-by: Jerin Jacob <jerinj at marvell.com>



> ---
>
> v2:
>         - replace "rule" with "guideline"
>         - add few exception criterias
>         - add URL of the techboard minutes
>
> ---
>  doc/guides/contributing/design.rst | 16 ++++++++++++++++
>  doc/guides/prog_guide/mbuf_lib.rst | 23 +++++++++++++++++++++++
>  lib/librte_mbuf/rte_mbuf_core.h    |  2 ++
>  3 files changed, 41 insertions(+)
>
> diff --git a/doc/guides/contributing/design.rst b/doc/guides/contributing/design.rst
> index d3dd694b65..4b9fb8a84d 100644
> --- a/doc/guides/contributing/design.rst
> +++ b/doc/guides/contributing/design.rst
> @@ -57,6 +57,22 @@ The following config options can be used:
>  * ``CONFIG_RTE_EXEC_ENV`` is a string that contains the name of the executive environment.
>  * ``CONFIG_RTE_EXEC_ENV_FREEBSD`` or ``CONFIG_RTE_EXEC_ENV_LINUX`` are defined only if we are building for this execution environment.
>
> +Mbuf features
> +-------------
> +
> +The ``rte_mbuf`` structure must be kept small (128 bytes).
> +
> +In order to add new features without wasting buffer space for unused features,
> +some fields and flags can be registered dynamically in a shared area.
> +The "dynamic" mbuf area is the default choice for the new features.
> +
> +The "dynamic" area is eating the remaining space in mbuf,
> +and some existing "static" fields may need to become "dynamic".
> +
> +Adding a new static field or flag must be an exception matching many criterias
> +like (non exhaustive): wide usage, performance, size.
> +
> +
>  Library Statistics
>  ------------------
>
> diff --git a/doc/guides/prog_guide/mbuf_lib.rst b/doc/guides/prog_guide/mbuf_lib.rst
> index 0d3223b081..c3dbfb9221 100644
> --- a/doc/guides/prog_guide/mbuf_lib.rst
> +++ b/doc/guides/prog_guide/mbuf_lib.rst
> @@ -207,6 +207,29 @@ The list of flags and their precise meaning is described in the mbuf API
>  documentation (rte_mbuf.h). Also refer to the testpmd source code
>  (specifically the csumonly.c file) for details.
>
> +Dynamic fields and flags
> +~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +The size of the mbuf is constrained and limited;
> +while the amount of metadata to save for each packet is quite unlimited.
> +The most basic networking information already find their place
> +in the existing mbuf fields and flags.
> +
> +If new features need to be added, the new fields and flags should fit
> +in the "dynamic space", by registering some room in the mbuf structure:
> +
> +dynamic field
> +   named area in the mbuf structure,
> +   with a given size (at least 1 byte) and alignment constraint.
> +
> +dynamic flag
> +   named bit in the mbuf structure,
> +   stored in the field ``ol_flags``.
> +
> +The dynamic fields and flags are managed with the functions ``rte_mbuf_dyn*``.
> +
> +It is not possible to unregister fields or flags.
> +
>  .. _direct_indirect_buffer:
>
>  Direct and Indirect Buffers
> diff --git a/lib/librte_mbuf/rte_mbuf_core.h b/lib/librte_mbuf/rte_mbuf_core.h
> index b9a59c879c..22be41e520 100644
> --- a/lib/librte_mbuf/rte_mbuf_core.h
> +++ b/lib/librte_mbuf/rte_mbuf_core.h
> @@ -12,6 +12,8 @@
>   * packet offload flags and some related macros.
>   * For majority of DPDK entities, it is not recommended to include
>   * this file directly, use include <rte_mbuf.h> instead.
> + *
> + * New fields and flags should fit in the "dynamic space".
>   */
>
>  #include <stdint.h>
> --
> 2.26.2
>