[dpdk-dev,PATCHv4,4/4] ABI: Add some documentation
diff mbox

Message ID 1435586380-1673-4-git-send-email-nhorman@tuxdriver.com
State Accepted, archived
Headers show

Commit Message

Neil Horman June 29, 2015, 1:59 p.m. UTC
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

v3)
     * Fixed in tact -> intact
     * Added docs to address static linking
     * Removed duplicate documentation from release notes

v4)
     * Fixed more grammatical errors / etc
     * removed BASE_SYMBOL from documentation
---
 doc/guides/guidelines/index.rst      |   1 +
 doc/guides/guidelines/versioning.rst | 480 +++++++++++++++++++++++++++++++++++
 doc/guides/rel_notes/abi.rst         |  30 +--
 3 files changed, 483 insertions(+), 28 deletions(-)
 create mode 100644 doc/guides/guidelines/versioning.rst

Comments

Thomas Monjalon June 29, 2015, 3:07 p.m. UTC | #1
2015-06-29 09:59, Neil Horman:
> 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
> 
> v3)
>      * Fixed in tact -> intact
>      * Added docs to address static linking
>      * Removed duplicate documentation from release notes
> 
> v4)
>      * Fixed more grammatical errors / etc
>      * removed BASE_SYMBOL from documentation

Series applied with minor fixes, thanks

This doc should help to produce patches with new ABI without breaking
the old one.
For cases where the backward compatibility is impossible to achieve,
a patch amending this doc to suggest a NEXT_ABI option can be discussed.

Patch
diff mbox

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..63526db
--- /dev/null
+++ b/doc/guides/guidelines/versioning.rst
@@ -0,0 +1,480 @@ 
+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
+  versioned symbol ``b@DPDK_n`` 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``.
+
+* ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the fully
+qualified function ``p``, so that if a symbol becomes versioned, it can still be
+mapped back to the public symbol name.
+
+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 intact, 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.
+
+One last thing we need to do.  Note that we've taken what was a public symbol,
+and duplicated it into two uniquely and differently named symbols.  We've then
+mapped each of those back to the public symbol ``rte_acl_create`` with different
+version tags.  This only applies to dynamic linking, as static linking has no
+notion of versioning.  That leaves this code in a position of no longer having a
+symbol simply named ``rte_acl_create`` and a static build will fail on that
+missing symbol.
+
+To correct this, we can simply map a function of our choosing back to the public
+symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro.  Generally the
+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_create_v21(const struct rte_acl_param *param, int debug)
+   {
+        ...
+   }
+   MAP_STATIC_SYMBOL(struct rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
+
+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.
+
+
+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/
diff --git a/doc/guides/rel_notes/abi.rst b/doc/guides/rel_notes/abi.rst
index f00a6ee..4086198 100644
--- a/doc/guides/rel_notes/abi.rst
+++ b/doc/guides/rel_notes/abi.rst
@@ -1,33 +1,7 @@ 
 ABI policy
 ==========
-ABI versions are set at the time of major release labeling, and ABI may change
-multiple times between the last labeling and the HEAD label of the git tree
-without warning.
-
-ABI versions, once released are available until such time as their
-deprecation has been noted here for at least one major release cycle, after it
-has been tagged.  E.g. the ABI for DPDK 2.0 is shipped, and then the decision to
-remove it is made during the development of DPDK 2.1.  The decision will be
-recorded here, shipped with the DPDK 2.1 release, and actually removed when DPDK
-2.2 ships.
-
-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 of.  In those events ABI's may be updated without backward
-compatibility provided.  The requirements for doing so are:
-
-#. At least 3 acknowledgments of the need on the dpdk.org
-#. A full deprecation cycle must be made to offer downstream consumers sufficient warning of the change.  E.g. if dpdk 2.0 is under development when the change is proposed, a deprecation notice must be added to this file, and released with dpdk 2.0.  Then the change may be incorporated for dpdk 2.1
-#. 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 ABI whenever possible.  For instance, reorganizing public
-structure field for aesthetic or readability purposes should be avoided as it will
-cause ABI breakage.  Only significant (e.g. performance) reasons should be seen
-as cause to alter ABI.
+See the guidelines document for details of the ABI policy.  ABI deprecation
+notices are to be posted here
 
 Examples of Deprecation Notices
 -------------------------------