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

Thomas Monjalon thomas at monjalon.net
Mon May 25 23:24:15 CEST 2020


Since dynamic fields and flags were added in 19.11,
the idea was to use them for new features, not only PMD-specific.

The rule 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

Signed-off-by: Thomas Monjalon <thomas at monjalon.net>
---
 doc/guides/contributing/design.rst | 13 +++++++++++++
 doc/guides/prog_guide/mbuf_lib.rst | 23 +++++++++++++++++++++++
 lib/librte_mbuf/rte_mbuf_core.h    |  2 ++
 3 files changed, 38 insertions(+)

diff --git a/doc/guides/contributing/design.rst b/doc/guides/contributing/design.rst
index d3dd694b65..508115d5bd 100644
--- a/doc/guides/contributing/design.rst
+++ b/doc/guides/contributing/design.rst
@@ -57,6 +57,19 @@ 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".
+
+
 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



More information about the dev mailing list