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diff --git a/doc/guides/contributing/versioning.rst b/doc/guides/contributing/versioning.rst new file mode 100644 index 00000000..ae10a984 --- /dev/null +++ b/doc/guides/contributing/versioning.rst @@ -0,0 +1,494 @@ +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 libraries marked in experimental state may change without constraint. +#. 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. + +#. The changes (including an alternative map file) must be gated with + the ``RTE_NEXT_ABI`` option, and provided with a deprecation notice at the + same time. + It will become the default ABI in the next release. + +#. A full deprecation cycle, as explained above, must be made to offer + downstream consumers sufficient warning of the change. + +#. At the beginning of the next release cycle, every ``RTE_NEXT_ABI`` + conditions will be removed, the ``LIBABIVER`` variable in the makefile(s) + where the ABI is changed will be incremented, and the map files will + be updated. + +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 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 +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 <REV1> <REV2> <TARGET> + +Where ``REV1`` and ``REV2`` are valid gitrevisions(7) +https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html +on the local repo and target is the usual DPDK compilation target. + +For example: + + # Check between the previous and latest commit: + ./scripts/validate-abi.sh HEAD~1 HEAD x86_64-native-linuxapp-gcc + + # Check between two tags: + ./scripts/validate-abi.sh v2.0.0 v2.1.0 x86_64-native-linuxapp-gcc + + # Check between git master and local topic-branch "vhost-hacking": + ./scripts/validate-abi.sh master vhost-hacking 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/ |