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

["Zhang, Helin" <helin.zhang@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Neil Horman
> Sent: Thursday, June 25, 2015 2:35 AM
> To: dev@dpdk.org
> Subject: [dpdk-dev] [PATCHv2 2/2] ABI: Add some documentation
> 
> 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;
One question, does it need a line of "local: *;", like it did in
librte_ether/rte_ether_version.map?

> +
> +   } 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