mbuf: document guideline for new fields and flags

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@monjalon.net>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>

diff --git a/doc/guides/contributing/design.rst b/doc/guides/contributing/design.rst
index d3dd694..5fe7f63 100644 (file)
--- 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 criteria
+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 0d3223b..c3dbfb9 100644 (file)
--- 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 b9a59c8..22be41e 100644 (file)
--- 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>