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From: Thomas Monjalon <thomas@monjalon.net>
To: dev@dpdk.org
Cc: techboard@dpdk.org, Olivier Matz <olivier.matz@6wind.com>,
	John McNamara <john.mcnamara@intel.com>,
	Marko Kovacevic <marko.kovacevic@intel.com>
Subject: [dpdk-dev] [PATCH v2] mbuf: document guideline for new fields and flags
Date: Thu, 11 Jun 2020 08:32:29 +0200
Message-ID: <20200611063229.3591726-1-thomas@monjalon.net> (raw)
In-Reply-To: <20200525212415.3173817-1-thomas@monjalon.net>

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>
---

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


  parent reply	other threads:[~2020-06-11  6:32 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-25 21:24 [dpdk-dev] [PATCH] mbuf: document rule " Thomas Monjalon
2020-05-26  7:39 ` Jerin Jacob
2020-05-26 16:06   ` Olivier Matz
2020-05-26 16:29     ` Jerin Jacob
2020-05-27  7:09       ` Olivier Matz
2020-05-27  7:31         ` Jerin Jacob
2020-05-27  9:51           ` Thomas Monjalon
2020-05-27 11:43             ` Jerin Jacob
2020-05-27 11:56               ` Thomas Monjalon
2020-05-27 12:07                 ` Jerin Jacob
2020-05-27 12:23                   ` Olivier Matz
2020-05-27 12:34                     ` Jerin Jacob
2020-06-11  6:32 ` Thomas Monjalon [this message]
2020-06-11  6:37   ` [dpdk-dev] [PATCH v2] mbuf: document guideline " Jerin Jacob
2020-06-11  7:30     ` Thomas Monjalon

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