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 <tho...@monjalon.net> Acked-by: Olivier Matz <olivier.m...@6wind.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
