[dpdk-dev] [PATCHv2 2/2] ABI: Add some documentation

[Neil Horman <nhorman@tuxdriver.com>]

People have been asking for ways to use the ABI macros, heres some docs to
clarify their use.  Included is:

* An overview of what ABI is
* Details of the ABI deprecation process
* Details of the versioning macros
* Examples of their use
* Details of how to use the ABI validator

Thanks to John Mcnamara, who duplicated much of this effort at Intel while I was
working on it.  Much of the introductory material was gathered and cleaned up by
him

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: john.mcnamara@intel.com
CC: thomas.monjalon@6wind.com

Change notes:

v2)
     * Fixed RST indentations and spelling errors
     * Rebased to upstream to fix index.rst conflict
---
 doc/guides/guidelines/index.rst      |   1 +
 doc/guides/guidelines/versioning.rst | 456 +++++++++++++++++++++++++++++++++++
 2 files changed, 457 insertions(+)
 create mode 100644 doc/guides/guidelines/versioning.rst

diff --git a/doc/guides/guidelines/index.rst b/doc/guides/guidelines/index.rst
index 0ee9ab3..bfb9fa3 100644
--- a/doc/guides/guidelines/index.rst
+++ b/doc/guides/guidelines/index.rst
@@ -7,3 +7,4 @@ Guidelines
 
     coding_style
     design
+    versioning
diff --git a/doc/guides/guidelines/versioning.rst b/doc/guides/guidelines/versioning.rst
new file mode 100644
index 0000000..2aef526
--- /dev/null
+++ b/doc/guides/guidelines/versioning.rst
@@ -0,0 +1,456 @@
+Managing ABI updates
+====================
+
+Description
+-----------
+
+This document details some methods for handling ABI management in the DPDK.
+Note this document is not exhaustive, in that C library versioning is flexible
+allowing multiple methods to achieve various goals, but it will provide the user
+with some introductory methods
+
+General Guidelines
+------------------
+
+#. Whenever possible, ABI should be preserved
+#. The addition of symbols is generally not problematic
+#. The modification of symbols can generally be managed with versioning
+#. The removal of symbols generally is an ABI break and requires bumping of the
+   LIBABIVER macro
+
+What is an ABI
+--------------
+
+An ABI (Application Binary Interface) is the set of runtime interfaces exposed
+by a library. It is similar to an API (Application Programming Interface) but
+is the result of compilation.  It is also effectively cloned when applications
+link to dynamic libraries.  That is to say when an application is compiled to
+link against dynamic libraries, it is assumed that the ABI remains constant
+between the time the application is compiled/linked, and the time that it runs.
+Therefore, in the case of dynamic linking, it is critical that an ABI is
+preserved, or (when modified), done in such a way that the application is unable
+to behave improperly or in an unexpected fashion.
+
+The DPDK ABI policy
+-------------------
+
+ABI versions are set at the time of major release labeling, and the ABI may
+change multiple times, without warning, between the last release label and the
+HEAD label of the git tree.
+
+ABI versions, once released, are available until such time as their
+deprecation has been noted in the Release Notes for at least one major release
+cycle. For example consider the case where the ABI for DPDK 2.0 has been
+shipped and then a decision is made to modify it during the development of
+DPDK 2.1. The decision will be recorded in the Release Notes for the DPDK 2.1
+release and the modification will be made available in the DPDK 2.2 release.
+
+ABI versions may be deprecated in whole or in part as needed by a given
+update.
+
+Some ABI changes may be too significant to reasonably maintain multiple
+versions. In those cases ABI's may be updated without backward compatibility
+being provided. The requirements for doing so are:
+
+#. At least 3 acknowledgments of the need to do so must be made on the
+   dpdk.org mailing list.
+
+#. A full deprecation cycle, as explained above, must be made to offer
+   downstream consumers sufficient warning of the change.
+
+#. The ``LIBABIVER`` variable in the makefile(s) where the ABI changes are
+   incorporated must be incremented in parallel with the ABI changes
+   themselves.
+
+Note that the above process for ABI deprecation should not be undertaken
+lightly. ABI stability is extremely important for downstream consumers of the
+DPDK, especially when distributed in shared object form. Every effort should
+be made to preserve the ABI whenever possible. The ABI should only be changed
+for significant reasons, such as performance enhancements. ABI breakage due to
+changes such as reorganizing public structure fields for aesthetic or
+readability purposes should be avoided.
+
+Examples of Deprecation Notices
+-------------------------------
+
+The following are some examples of ABI deprecation notices which would be
+added to the Release Notes:
+
+* The Macro ``#RTE_FOO`` is deprecated and will be removed with version 2.0,
+  to be replaced with the inline function ``rte_foo()``.
+
+* The function ``rte_mbuf_grok()`` has been updated to include a new parameter
+  in version 2.0. Backwards compatibility will be maintained for this function
+  until the release of version 2.1
+
+* The members of ``struct rte_foo`` have been reorganized in release 2.0 for
+  performance reasons. Existing binary applications will have backwards
+  compatibility in release 2.0, while newly built binaries will need to
+  reference the new structure variant ``struct rte_foo2``. Compatibility will
+  be removed in release 2.2, and all applications will require updating and
+  rebuilding to the new structure at that time, which will be renamed to the
+  original ``struct rte_foo``.
+
+* Significant ABI changes are planned for the ``librte_dostuff`` library. The
+  upcoming release 2.0 will not contain these changes, but release 2.1 will,
+  and no backwards compatibility is planned due to the extensive nature of
+  these changes. Binaries using this library built prior to version 2.1 will
+  require updating and recompilation.
+
+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
+backward compatibility for a time with older binaries that are dynamically
+linked to the DPDK.
+
+To support backward compatibility the ``lib/librte_compat/rte_compat.h``
+header file provides macros to use when updating exported functions. These
+macros are used in conjunction with the ``rte_<library>_version.map`` file for
+a given library to allow multiple versions of a symbol to exist in a shared
+library so that older binaries need not be immediately recompiled.
+
+The macros exported are:
+
+* ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
+  unversioned symbol ``b`` to the internal function ``b_e``.
+
+
+* ``BASE_SYMBOL(b, e)``: Creates a symbol version table entry binding
+  unversioned symbol ``b`` to the internal function ``b_e``.
+
+* ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
+  the linker to bind references to symbol ``b`` to the internal symbol
+  ``b_e``.
+
+
+Examples of ABI Macro use
+-------------------------
+
+Updating a public API
+~~~~~~~~~~~~~~~~~~~~~
+
+Assume we have a function as follows
+
+.. code-block:: c
+
+ /*
+  * Create an acl context object for apps to 
+  * manipulate
+  */
+ struct rte_acl_ctx *
+ rte_acl_create(const struct rte_acl_param *param)
+ {
+        ...
+ }
+
+
+Assume that struct rte_acl_ctx is a private structure, and that a developer
+wishes to enhance the acl api so that a debugging flag can be enabled on a
+per-context basis.  This requires an addition to the structure (which, being
+private, is safe), but it also requires modifying the code as follows
+
+.. code-block:: c
+
+ /*
+  * Create an acl context object for apps to 
+  * manipulate
+  */
+ struct rte_acl_ctx *
+ rte_acl_create(const struct rte_acl_param *param, int debug)
+ {
+        ...
+ }
+
+
+Note also that, being a public function, the header file prototype must also be
+changed, as must all the call sites, to reflect the new ABI footprint.  We will
+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
+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
+
+.. code-block:: none 
+
+   DPDK_2.0 {
+        global:
+
+        rte_acl_add_rules;
+        rte_acl_build;
+        rte_acl_classify;
+        rte_acl_classify_alg;
+        rte_acl_classify_scalar;
+        rte_acl_create;
+        rte_acl_dump;
+        rte_acl_find_existing;
+        rte_acl_free;
+        rte_acl_ipv4vlan_add_rules;
+        rte_acl_ipv4vlan_build;
+        rte_acl_list_dump;
+        rte_acl_reset;
+        rte_acl_reset_rules;
+        rte_acl_set_ctx_classify;
+
+        local: *;
+   };
+
+This file needs to be modified as follows
+
+.. code-block:: none
+
+   DPDK_2.0 {
+        global:
+
+        rte_acl_add_rules;
+        rte_acl_build;
+        rte_acl_classify;
+        rte_acl_classify_alg;
+        rte_acl_classify_scalar;
+        rte_acl_create;
+        rte_acl_dump;
+        rte_acl_find_existing;
+        rte_acl_free;
+        rte_acl_ipv4vlan_add_rules;
+        rte_acl_ipv4vlan_build;
+        rte_acl_list_dump;
+        rte_acl_reset;
+        rte_acl_reset_rules;
+        rte_acl_set_ctx_classify;
+
+        local: *;
+   };
+
+   DPDK_2.1 {
+        global:
+        rte_acl_create;
+
+   } DPDK_2.0;
+
+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
+
+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,
+we need to update the function so that it is uniquely named, and not in conflict
+with the public symbol name
+
+.. code-block:: c
+
+  struct rte_acl_ctx *
+ -rte_acl_create(const struct rte_acl_param *param)
+ +rte_acl_create_v20(const struct rte_acl_param *param)
+ {
+        size_t sz;
+        struct rte_acl_ctx *ctx;
+        ...
+
+Note that the base name of the symbol was kept in tact, as this is condusive 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
+the function, we add this line of code
+
+.. code-block:: c
+
+   VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
+
+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)
+
+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
+
+.. code-block:: c
+
+   struct rte_acl_ctx *
+   rte_acl_create_v21(const struct rte_acl_param *param, int debug);
+   {
+        struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
+
+        ctx->debug = debug;
+
+        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
+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
+
+.. code-block:: c
+
+   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);
+
+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
+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
+for a period.
+
+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.
+
+
+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:
+
+.. code-block:: none
+
+   DPDK_2.0 {
+        global:
+
+        rte_acl_add_rules;
+        rte_acl_build;
+        rte_acl_classify;
+        rte_acl_classify_alg;
+        rte_acl_classify_scalar;
+        rte_acl_dump;
+ -      rte_acl_create
+        rte_acl_find_existing;
+        rte_acl_free;
+        rte_acl_ipv4vlan_add_rules;
+        rte_acl_ipv4vlan_build;
+        rte_acl_list_dump;
+        rte_acl_reset;
+        rte_acl_reset_rules;
+        rte_acl_set_ctx_classify;
+
+        local: *;
+   };
+
+   DPDK_2.1 {
+        global:
+        rte_acl_create;
+   } DPDK_2.0;
+
+
+Next remove the corresponding versioned export
+.. code-block:: c
+
+ -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
+
+
+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
+
+   -LIBABIVER := 1
+   +LIBABIVER := 2
+
+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
+ 
+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
+
+In the case of our map above, it would transform to look as follows
+
+.. code-block:: none
+
+   DPDK_2.1 {              
+        global:
+              
+        rte_acl_add_rules;
+        rte_acl_build;
+        rte_acl_classify;
+        rte_acl_classify_alg;
+        rte_acl_classify_scalar;
+        rte_acl_dump;
+        rte_acl_create
+        rte_acl_find_existing;
+        rte_acl_free;
+        rte_acl_ipv4vlan_add_rules;
+        rte_acl_ipv4vlan_build;
+        rte_acl_list_dump;
+        rte_acl_reset;
+        rte_acl_reset_rules;
+        rte_acl_set_ctx_classify;
+              
+        local: *;
+ };           
+
+Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
+updated to point to the new version node in any header files for all affected
+symbols.
+
+.. code-block:: c
+
+ -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
+ +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
+
+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
+versions of the symbol.
+
+Running the ABI Validator
+-------------------------
+
+The ``scripts`` directory in the DPDK source tree contains a utility program,
+``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
+Compliance Checker
+<http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
+
+This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
+utilities which can be installed via a package manager. For example::
+
+   sudo yum install abi-compliance-checker
+   sudo yum install abi-dumper
+
+The syntax of the ``validate-abi.sh`` utility is::
+
+   ./scripts/validate-abi.sh <TAG1> <TAG2> <TARGET>
+
+Where ``TAG1`` and ``TAG2`` are valid git tags on the local repo and target is
+the usual DPDK compilation target.
+
+For example to test the current committed HEAD against a previous release tag
+we could add a temporary tag and run the utility as follows::
+
+   git tag MY_TEMP_TAG
+   ./scripts/validate-abi.sh v2.0.0 MY_TEMP_TAG x86_64-native-linuxapp-gcc
+
+After the validation script completes (it can take a while since it need to
+compile both tags) it will create compatibility reports in the
+``./compat_report`` directory. Listed incompatibilities can be found as
+follows::
+
+  grep -lr Incompatible compat_reports/
-- 
2.1.0