[dpdk-dev] [PATCH v7 3/4] doc: updates to versioning guide for abi versions

[Ray Kinsella <mdr@ashroe.eu>]

Updates to the ABI versioning guide, to account for the changes to the DPDK
ABI/API policy. Fixes for references to abi versioning and policy guides.

Signed-off-by: Ray Kinsella <mdr@ashroe.eu>
---
 doc/guides/contributing/abi_versioning.rst | 248 +++++++++++++++++++----------
 doc/guides/contributing/patches.rst        |   6 +-
 doc/guides/contributing/stable.rst         |   2 +-
 doc/guides/rel_notes/deprecation.rst       |   5 +-
 4 files changed, 175 insertions(+), 86 deletions(-)

diff --git a/doc/guides/contributing/abi_versioning.rst b/doc/guides/contributing/abi_versioning.rst
index 53e6ac0..a7cf1b9 100644
--- a/doc/guides/contributing/abi_versioning.rst
+++ b/doc/guides/contributing/abi_versioning.rst
@@ -1,44 +1,134 @@
 ..  SPDX-License-Identifier: BSD-3-Clause
     Copyright 2018 The DPDK contributors
 
-.. library_versioning:
+.. _abi_versioning:
 
-Library versioning
+ABI Versioning
+==============
+
+This document details the mechanics of ABI version management in DPDK.
+
+.. _what_is_soname:
+
+What is a library's soname?
+---------------------------
+
+System libraries usually adopt the familiar major and minor version naming
+convention, where major versions (e.g. ``librte_eal 20.x, 21.x``) are presumed
+to be ABI incompatible with each other and minor versions (e.g. ``librte_eal
+20.1, 20.2``) are presumed to be ABI compatible. A library's `soname
+<https://en.wikipedia.org/wiki/Soname>`_. is typically used to provide backward
+compatibility information about a given library, describing the lowest common
+denominator ABI supported by the library. The soname or logical name for the
+library, is typically comprised of the library's name and major version e.g.
+``librte_eal.so.20``.
+
+During an application's build process, a library's soname is noted as a runtime
+dependency of the application. This information is then used by the `dynamic
+linker <https://en.wikipedia.org/wiki/Dynamic_linker>`_ when resolving the
+applications dependencies at runtime, to load a library supporting the correct
+ABI version. The library loaded at runtime therefore, may be a minor revision
+supporting the same major ABI version (e.g. ``librte_eal.20.2``), as the library
+used to link the application (e.g ``librte_eal.20.0``).
+
+.. _major_abi_versions:
+
+Major ABI versions
 ------------------
 
-Downstreams might want to provide different DPDK releases at the same time to
-support multiple consumers of DPDK linked against older and newer sonames.
+An ABI version change to a given library, especially in core libraries such as
+``librte_mbuf``, may cause an implicit ripple effect on the ABI of it's
+consuming libraries, causing ABI breakages. There may however be no explicit
+reason to bump a dependent library's ABI version, as there may have been no
+obvious change to the dependent library's API, even though the library's ABI
+compatibility will have been broken.
+
+This interdependence of DPDK libraries, means that ABI versioning of libraries
+is more manageable at a project level, with all project libraries sharing a
+**single ABI version**. In addition, the need to maintain a stable ABI for some
+number of releases as described in the section :doc:`abi_policy`, means
+that ABI version increments need to carefully planned and managed at a project
+level.
+
+Major ABI versions are therefore declared typically aligned with an LTS release
+and is then supported some number of subsequent releases, shared across all
+libraries. This means that a single project level ABI version, reflected in all
+individual library's soname, library filenames and associated version maps
+persists over multiple releases.
+
+.. code-block:: none
+
+ $ head ./lib/librte_acl/rte_acl_version.map
+ DPDK_20 {
+        global:
+ ...
 
-Also due to the interdependencies that DPDK libraries can have applications
-might end up with an executable space in which multiple versions of a library
-are mapped by ld.so.
+ $ head ./lib/librte_eal/rte_eal_version.map
+ DPDK_20 {
+        global:
+ ...
 
-Think of LibA that got an ABI bump and LibB that did not get an ABI bump but is
-depending on LibA.
+When an ABI change is made between major ABI versions to a given library, a new
+section is added to that library's version map describing the impending new ABI
+version, as described in the section :ref:`example_abi_macro_usage`. The
+library's soname and filename however do not change, e.g. ``libacl.so.20``, as
+ABI compatibility with the last major ABI version continues to be preserved for
+that library.
 
-.. note::
+.. code-block:: none
 
-    Application
-    \-> LibA.old
-    \-> LibB.new -> LibA.new
+ $ head ./lib/librte_acl/rte_acl_version.map
+ DPDK_20 {
+        global:
+ ...
 
-That is a conflict which can be avoided by setting ``CONFIG_RTE_MAJOR_ABI``.
-If set, the value of ``CONFIG_RTE_MAJOR_ABI`` overwrites all - otherwise per
-library - versions defined in the libraries ``LIBABIVER``.
-An example might be ``CONFIG_RTE_MAJOR_ABI=16.11`` which will make all libraries
-``librte<?>.so.16.11`` instead of ``librte<?>.so.<LIBABIVER>``.
+ DPDK_21 {
+        global:
+
+ } DPDK_20;
+ ...
 
+ $ head ./lib/librte_eal/rte_eal_version.map
+ DPDK_20 {
+        global:
+ ...
+
+However when a new ABI version is declared, for example DPDK ``21``, old
+depreciated functions may be safely removed at this point and the entire old
+major ABI version removed, see the section :ref:`deprecating_entire_abi` on
+how this may be done.
+
+.. code-block:: none
+
+ $ head ./lib/librte_acl/rte_acl_version.map
+ DPDK_21 {
+        global:
+ ...
+
+ $ head ./lib/librte_eal/rte_eal_version.map
+ DPDK_21 {
+        global:
+ ...
+
+At the same time, the major ABI version is changed atomically across all
+libraries by incrementing the major version in individual library's soname, e.g.
+``libacl.so.21``. This is done by bumping the LIBABIVER number in the libraries
+Makefile to indicate to dynamic linking applications that this is a later, and
+possibly incompatible library version:
+
+.. code-block:: c
+
+   -LIBABIVER := 20
+   +LIBABIVER := 21
 
-ABI versioning
---------------
 
 Versioning Macros
-~~~~~~~~~~~~~~~~~
+-----------------
 
 When a symbol is exported from a library to provide an API, it also provides a
 calling convention (ABI) that is embodied in its name, return type and
 arguments. Occasionally that function may need to change to accommodate new
-functionality or behavior. When that occurs, it is desirable to allow for
+functionality or behavior. When that occurs, it is may be required to allow for
 backward compatibility for a time with older binaries that are dynamically
 linked to the DPDK.
 
@@ -61,8 +151,10 @@ The macros exported are:
   fully qualified function ``p``, so that if a symbol becomes versioned, it
   can still be mapped back to the public symbol name.
 
+.. _example_abi_macro_usage:
+
 Examples of ABI Macro use
-^^^^^^^^^^^^^^^^^^^^^^^^^
+~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Updating a public API
 _____________________
@@ -106,16 +198,16 @@ maintain previous ABI versions that are accessible only to previously compiled
 binaries
 
 The addition of a parameter to the function is ABI breaking as the function is
-public, and existing application may use it in its current form.  However, the
+public, and existing application may use it in its current form. However, the
 compatibility macros in DPDK allow a developer to use symbol versioning so that
 multiple functions can be mapped to the same public symbol based on when an
-application was linked to it.  To see how this is done, we start with the
-requisite libraries version map file.  Initially the version map file for the
-acl library looks like this
+application was linked to it. To see how this is done, we start with the
+requisite libraries version map file. Initially the version map file for the acl
+library looks like this
 
 .. code-block:: none
 
-   DPDK_2.0 {
+   DPDK_20 {
         global:
 
         rte_acl_add_rules;
@@ -141,7 +233,7 @@ This file needs to be modified as follows
 
 .. code-block:: none
 
-   DPDK_2.0 {
+   DPDK_20 {
         global:
 
         rte_acl_add_rules;
@@ -163,16 +255,16 @@ This file needs to be modified as follows
         local: *;
    };
 
-   DPDK_2.1 {
+   DPDK_21 {
         global:
         rte_acl_create;
 
-   } DPDK_2.0;
+   } DPDK_20;
 
 The addition of the new block tells the linker that a new version node is
-available (DPDK_2.1), which contains the symbol rte_acl_create, and inherits the
-symbols from the DPDK_2.0 node.  This list is directly translated into a list of
-exported symbols when DPDK is compiled as a shared library
+available (DPDK_21), which contains the symbol rte_acl_create, and inherits
+the symbols from the DPDK_20 node. This list is directly translated into a
+list of exported symbols when DPDK is compiled as a shared library
 
 Next, we need to specify in the code which function map to the rte_acl_create
 symbol at which versions.  First, at the site of the initial symbol definition,
@@ -191,22 +283,22 @@ with the public symbol name
 
 Note that the base name of the symbol was kept intact, as this is conducive to
 the macros used for versioning symbols.  That is our next step, mapping this new
-symbol name to the initial symbol name at version node 2.0.  Immediately after
+symbol name to the initial symbol name at version node 20.  Immediately after
 the function, we add this line of code
 
 .. code-block:: c
 
-   VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
+   VERSION_SYMBOL(rte_acl_create, _v20, 20);
 
 Remembering to also add the rte_compat.h header to the requisite c file where
-these changes are being made.  The above macro instructs the linker to create a
-new symbol ``rte_acl_create@DPDK_2.0``, which matches the symbol created in older
-builds, but now points to the above newly named function.  We have now mapped
-the original rte_acl_create symbol to the original function (but with a new
-name)
+these changes are being made. The above macro instructs the linker to create a
+new symbol ``rte_acl_create@DPDK_20``, which matches the symbol created in
+older builds, but now points to the above newly named function. We have now
+mapped the original rte_acl_create symbol to the original function (but with a
+new name)
 
-Next, we need to create the 2.1 version of the symbol.  We create a new function
-name, with a different suffix, and  implement it appropriately
+Next, we need to create the 21 version of the symbol. We create a new function
+name, with a different suffix, and implement it appropriately
 
 .. code-block:: c
 
@@ -220,12 +312,12 @@ name, with a different suffix, and  implement it appropriately
         return ctx;
    }
 
-This code serves as our new API call.  Its the same as our old call, but adds
-the new parameter in place.  Next we need to map this function to the symbol
-``rte_acl_create@DPDK_2.1``.  To do this, we modify the public prototype of the call
-in the header file, adding the macro there to inform all including applications,
-that on re-link, the default rte_acl_create symbol should point to this
-function.  Note that we could do this by simply naming the function above
+This code serves as our new API call. Its the same as our old call, but adds the
+new parameter in place. Next we need to map this function to the symbol
+``rte_acl_create@DPDK_21``. To do this, we modify the public prototype of the
+call in the header file, adding the macro there to inform all including
+applications, that on re-link, the default rte_acl_create symbol should point to
+this function. Note that we could do this by simply naming the function above
 rte_acl_create, and the linker would chose the most recent version tag to apply
 in the version script, but we can also do this in the header file
 
@@ -233,11 +325,11 @@ in the version script, but we can also do this in the header file
 
    struct rte_acl_ctx *
    -rte_acl_create(const struct rte_acl_param *param);
-   +rte_acl_create(const struct rte_acl_param *param, int debug);
-   +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
+   +rte_acl_create_v21(const struct rte_acl_param *param, int debug);
+   +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 21);
 
 The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
-header, to link to the rte_acl_create_v21 function and apply the DPDK_2.1
+header, to link to the rte_acl_create_v21 function and apply the DPDK_21
 version node to it.  This method is more explicit and flexible than just
 re-implementing the exact symbol name, and allows for other features (such as
 linking to the old symbol version by default, when the new ABI is to be opt-in
@@ -257,6 +349,7 @@ assumption is that the most recent version of the symbol is the one you want to
 map.  So, back in the C file where, immediately after ``rte_acl_create_v21`` is
 defined, we add this
 
+
 .. code-block:: c
 
    struct rte_acl_ctx *
@@ -270,21 +363,22 @@ That tells the compiler that, when building a static library, any calls to the
 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
 
 That's it, on the next shared library rebuild, there will be two versions of
-rte_acl_create, an old DPDK_2.0 version, used by previously built applications,
-and a new DPDK_2.1 version, used by future built applications.
+rte_acl_create, an old DPDK_20 version, used by previously built applications,
+and a new DPDK_21 version, used by future built applications.
 
 
 Deprecating part of a public API
 ________________________________
 
-Lets assume that you've done the above update, and after a few releases have
-passed you decide you would like to retire the old version of the function.
-After having gone through the ABI deprecation announcement process, removal is
-easy.  Start by removing the symbol from the requisite version map file:
+Lets assume that you've done the above update, and in preparation for the next
+major ABI version you decide you would like to retire the old version of the
+function. After having gone through the ABI deprecation announcement process,
+removal is easy. Start by removing the symbol from the requisite version map
+file:
 
 .. code-block:: none
 
-   DPDK_2.0 {
+   DPDK_20 {
         global:
 
         rte_acl_add_rules;
@@ -306,48 +400,42 @@ easy.  Start by removing the symbol from the requisite version map file:
         local: *;
    };
 
-   DPDK_2.1 {
+   DPDK_21 {
         global:
         rte_acl_create;
-   } DPDK_2.0;
+   } DPDK_20;
 
 
 Next remove the corresponding versioned export.
 
 .. code-block:: c
 
- -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
+ -VERSION_SYMBOL(rte_acl_create, _v20, 20);
 
 
 Note that the internal function definition could also be removed, but its used
-in our example by the newer version _v21, so we leave it in place.  This is a
-coding style choice.
-
-Lastly, we need to bump the LIBABIVER number for this library in the Makefile to
-indicate to applications doing dynamic linking that this is a later, and
-possibly incompatible library version:
-
-.. code-block:: c
+in our example by the newer version v21, so we leave it in place and declare it
+as static. This is a coding style choice.
 
-   -LIBABIVER := 1
-   +LIBABIVER := 2
+.. _deprecating_entire_abi:
 
 Deprecating an entire ABI version
 _________________________________
 
-While removing a symbol from and ABI may be useful, it is often more practical
-to remove an entire version node at once.  If a version node completely
-specifies an API, then removing part of it, typically makes it incomplete.  In
-those cases it is better to remove the entire node
+While removing a symbol from an ABI may be useful, it is more practical to
+remove an entire version node at once, as is typically done at the declaration
+of a major ABI version. If a version node completely specifies an API, then
+removing part of it, typically makes it incomplete. In those cases it is better
+to remove the entire node.
 
 To do this, start by modifying the version map file, such that all symbols from
-the node to be removed are merged into the next node in the map
+the node to be removed are merged into the next node in the map.
 
 In the case of our map above, it would transform to look as follows
 
 .. code-block:: none
 
-   DPDK_2.1 {
+   DPDK_21 {
         global:
 
         rte_acl_add_rules;
@@ -375,8 +463,8 @@ symbols.
 
 .. code-block:: c
 
- -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
- +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
+ -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 20);
+ +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 21);
 
 Lastly, any VERSION_SYMBOL macros that point to the old version node should be
 removed, taking care to keep, where need old code in place to support newer
diff --git a/doc/guides/contributing/patches.rst b/doc/guides/contributing/patches.rst
index 9e1013b..2140303 100644
--- a/doc/guides/contributing/patches.rst
+++ b/doc/guides/contributing/patches.rst
@@ -156,9 +156,9 @@ Make your planned changes in the cloned ``dpdk`` repo. Here are some guidelines
 
   * For other PMDs and more info, refer to the ``MAINTAINERS`` file.
 
-* New external functions should be added to the local ``version.map`` file.
-  See the :doc:`Guidelines for ABI policy and versioning </contributing/versioning>`.
-  New external functions should also be added in alphabetical order.
+* New external functions should be added to the local ``version.map`` file. See
+  the :doc:`ABI policy <abi_policy>` and :ref:`ABI versioning <abi_versioning>`
+  guides. New external functions should also be added in alphabetical order.
 
 * Important changes will require an addition to the release notes in ``doc/guides/rel_notes/``.
   See the :ref:`Release Notes section of the Documentation Guidelines <doc_guidelines>` for details.
diff --git a/doc/guides/contributing/stable.rst b/doc/guides/contributing/stable.rst
index 2b563d4..4d38bb8 100644
--- a/doc/guides/contributing/stable.rst
+++ b/doc/guides/contributing/stable.rst
@@ -54,7 +54,7 @@ After the X.11 release, an LTS branch will be created for it at
 http://git.dpdk.org/dpdk-stable where bugfixes will be backported to.
 
 A LTS release may align with the declaration of a new major ABI version,
-please read the :ref:`abi_policy` for more information.
+please read the :doc:`abi_policy` for more information.
 
 It is anticipated that there will be at least 4 releases per year of the LTS
 or approximately 1 every 3 months. However, the cadence can be shorter or
diff --git a/doc/guides/rel_notes/deprecation.rst b/doc/guides/rel_notes/deprecation.rst
index 237813b..25f71ad 100644
--- a/doc/guides/rel_notes/deprecation.rst
+++ b/doc/guides/rel_notes/deprecation.rst
@@ -4,8 +4,9 @@
 ABI and API Deprecation
 =======================
 
-See the :doc:`guidelines document for details of the ABI policy </contributing/versioning>`.
-API and ABI deprecation notices are to be posted here.
+See the guidelines document for details of the :doc:`ABI policy
+<../contributing/abi_policy>`. API and ABI deprecation notices are to be posted
+here.
 
 
 Deprecation Notices
-- 
2.7.4