From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp.tuxdriver.com (charlotte.tuxdriver.com [70.61.120.58]) by dpdk.org (Postfix) with ESMTP id 9892CC6DE for ; Tue, 23 Jun 2015 21:34:03 +0200 (CEST) Received: from hmsreliant.think-freely.org ([2001:470:8:a08:7aac:c0ff:fec2:933b] helo=localhost) by smtp.tuxdriver.com with esmtpsa (TLSv1:AES128-SHA:128) (Exim 4.63) (envelope-from ) id 1Z7TxE-0005uX-3L; Tue, 23 Jun 2015 15:34:01 -0400 From: Neil Horman To: dev@dpdk.org Date: Tue, 23 Jun 2015 15:33:34 -0400 Message-Id: <1435088014-18973-2-git-send-email-nhorman@tuxdriver.com> X-Mailer: git-send-email 2.1.0 In-Reply-To: <1435088014-18973-1-git-send-email-nhorman@tuxdriver.com> References: <1435088014-18973-1-git-send-email-nhorman@tuxdriver.com> X-Spam-Score: -2.9 (--) X-Spam-Status: No Subject: [dpdk-dev] [PATCH 2/2] ABI: Add some documentation X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 23 Jun 2015 19:34:04 -0000 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 CC: john.mcnamara@intel.com CC: thomas.monjalon@6wind.com --- 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 b2b0a92..251062e 100644 --- a/doc/guides/guidelines/index.rst +++ b/doc/guides/guidelines/index.rst @@ -6,3 +6,4 @@ Guidelines :numbered: coding_style + versioning diff --git a/doc/guides/guidelines/versioning.rst b/doc/guides/guidelines/versioning.rst new file mode 100644 index 0000000..79af924 --- /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 acheive 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__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 requries 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 alow 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), whcih 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);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 origional rte_acl_create symbol to the origional 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 fuction 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 +reimplementing 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. + +Thats 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 annoucement 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 ponited 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 neeed 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 +`_. + +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 + +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