1 .. SPDX-License-Identifier: BSD-3-Clause
2 Copyright 2018 The DPDK contributors
10 This document details some methods for handling ABI management in the DPDK.
11 Note this document is not exhaustive, in that C library versioning is flexible
12 allowing multiple methods to achieve various goals, but it will provide the user
13 with some introductory methods
18 #. Whenever possible, ABI should be preserved
19 #. Libraries or APIs marked in ``experimental`` state may change without constraint.
20 #. New APIs will be marked as ``experimental`` for at least one release to allow
21 any issues found by users of the new API to be fixed quickly
22 #. The addition of symbols is generally not problematic
23 #. The modification of symbols can generally be managed with versioning
24 #. The removal of symbols generally is an ABI break and requires bumping of the
26 #. Updates to the minimum hardware requirements, which drop support for hardware which
27 was previously supported, should be treated as an ABI change.
32 An ABI (Application Binary Interface) is the set of runtime interfaces exposed
33 by a library. It is similar to an API (Application Programming Interface) but
34 is the result of compilation. It is also effectively cloned when applications
35 link to dynamic libraries. That is to say when an application is compiled to
36 link against dynamic libraries, it is assumed that the ABI remains constant
37 between the time the application is compiled/linked, and the time that it runs.
38 Therefore, in the case of dynamic linking, it is critical that an ABI is
39 preserved, or (when modified), done in such a way that the application is unable
40 to behave improperly or in an unexpected fashion.
45 ABI versions are set at the time of major release labeling, and the ABI may
46 change multiple times, without warning, between the last release label and the
47 HEAD label of the git tree.
49 ABI versions, once released, are available until such time as their
50 deprecation has been noted in the Release Notes for at least one major release
51 cycle. For example consider the case where the ABI for DPDK 2.0 has been
52 shipped and then a decision is made to modify it during the development of
53 DPDK 2.1. The decision will be recorded in the Release Notes for the DPDK 2.1
54 release and the modification will be made available in the DPDK 2.2 release.
56 ABI versions may be deprecated in whole or in part as needed by a given
59 Some ABI changes may be too significant to reasonably maintain multiple
60 versions. In those cases ABI's may be updated without backward compatibility
61 being provided. The requirements for doing so are:
63 #. At least 3 acknowledgments of the need to do so must be made on the
64 dpdk.org mailing list.
66 - The acknowledgment of the maintainer of the component is mandatory, or if
67 no maintainer is available for the component, the tree/sub-tree maintainer
68 for that component must acknowledge the ABI change instead.
70 - It is also recommended that acknowledgments from different "areas of
71 interest" be sought for each deprecation, for example: from NIC vendors,
72 CPU vendors, end-users, etc.
74 #. The changes (including an alternative map file) must be gated with
75 the ``RTE_NEXT_ABI`` option, and provided with a deprecation notice at the
77 It will become the default ABI in the next release.
79 #. A full deprecation cycle, as explained above, must be made to offer
80 downstream consumers sufficient warning of the change.
82 #. At the beginning of the next release cycle, every ``RTE_NEXT_ABI``
83 conditions will be removed, the ``LIBABIVER`` variable in the makefile(s)
84 where the ABI is changed will be incremented, and the map files will
87 Note that the above process for ABI deprecation should not be undertaken
88 lightly. ABI stability is extremely important for downstream consumers of the
89 DPDK, especially when distributed in shared object form. Every effort should
90 be made to preserve the ABI whenever possible. The ABI should only be changed
91 for significant reasons, such as performance enhancements. ABI breakage due to
92 changes such as reorganizing public structure fields for aesthetic or
93 readability purposes should be avoided.
97 Updates to the minimum hardware requirements, which drop support for hardware
98 which was previously supported, should be treated as an ABI change, and
99 follow the relevant deprecation policy procedures as above: 3 acks and
100 announcement at least one release in advance.
105 APIs marked as ``experimental`` are not considered part of the ABI and may
106 change without warning at any time. Since changes to APIs are most likely
107 immediately after their introduction, as users begin to take advantage of
108 those new APIs and start finding issues with them, new DPDK APIs will be
109 automatically marked as ``experimental`` to allow for a period of stabilization
110 before they become part of a tracked ABI.
112 Note that marking an API as experimental is a multi step process.
113 To mark an API as experimental, the symbols which are desired to be exported
114 must be placed in an EXPERIMENTAL version block in the corresponding libraries'
116 Secondly, the corresponding definitions of those exported functions, and
117 their forward declarations (in the development header files), must be marked
118 with the ``__rte_experimental`` tag (see ``rte_compat.h``).
119 The DPDK build makefiles perform a check to ensure that the map file and the
120 C code reflect the same list of symbols.
121 This check can be circumvented by defining ``ALLOW_EXPERIMENTAL_API``
122 during compilation in the corresponding library Makefile.
124 In addition to tagging the code with ``__rte_experimental``,
125 the doxygen markup must also contain the EXPERIMENTAL string,
126 and the MAINTAINERS file should note the EXPERIMENTAL libraries.
128 For removing the experimental tag associated with an API, deprecation notice
129 is not required. Though, an API should remain in experimental state for at least
130 one release. Thereafter, normal process of posting patch for review to mailing
131 list can be followed.
133 Examples of Deprecation Notices
134 -------------------------------
136 The following are some examples of ABI deprecation notices which would be
137 added to the Release Notes:
139 * The Macro ``#RTE_FOO`` is deprecated and will be removed with version 2.0,
140 to be replaced with the inline function ``rte_foo()``.
142 * The function ``rte_mbuf_grok()`` has been updated to include a new parameter
143 in version 2.0. Backwards compatibility will be maintained for this function
144 until the release of version 2.1
146 * The members of ``struct rte_foo`` have been reorganized in release 2.0 for
147 performance reasons. Existing binary applications will have backwards
148 compatibility in release 2.0, while newly built binaries will need to
149 reference the new structure variant ``struct rte_foo2``. Compatibility will
150 be removed in release 2.2, and all applications will require updating and
151 rebuilding to the new structure at that time, which will be renamed to the
152 original ``struct rte_foo``.
154 * Significant ABI changes are planned for the ``librte_dostuff`` library. The
155 upcoming release 2.0 will not contain these changes, but release 2.1 will,
156 and no backwards compatibility is planned due to the extensive nature of
157 these changes. Binaries using this library built prior to version 2.1 will
158 require updating and recompilation.
163 When a symbol is exported from a library to provide an API, it also provides a
164 calling convention (ABI) that is embodied in its name, return type and
165 arguments. Occasionally that function may need to change to accommodate new
166 functionality or behavior. When that occurs, it is desirable to allow for
167 backward compatibility for a time with older binaries that are dynamically
170 To support backward compatibility the ``lib/librte_compat/rte_compat.h``
171 header file provides macros to use when updating exported functions. These
172 macros are used in conjunction with the ``rte_<library>_version.map`` file for
173 a given library to allow multiple versions of a symbol to exist in a shared
174 library so that older binaries need not be immediately recompiled.
176 The macros exported are:
178 * ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
179 versioned symbol ``b@DPDK_n`` to the internal function ``b_e``.
181 * ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
182 the linker to bind references to symbol ``b`` to the internal symbol
185 * ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the
186 fully qualified function ``p``, so that if a symbol becomes versioned, it
187 can still be mapped back to the public symbol name.
189 Setting a Major ABI version
190 ---------------------------
192 Downstreams might want to provide different DPDK releases at the same time to
193 support multiple consumers of DPDK linked against older and newer sonames.
195 Also due to the interdependencies that DPDK libraries can have applications
196 might end up with an executable space in which multiple versions of a library
199 Think of LibA that got an ABI bump and LibB that did not get an ABI bump but is
206 \-> LibB.new -> LibA.new
208 That is a conflict which can be avoided by setting ``CONFIG_RTE_MAJOR_ABI``.
209 If set, the value of ``CONFIG_RTE_MAJOR_ABI`` overwrites all - otherwise per
210 library - versions defined in the libraries ``LIBABIVER``.
211 An example might be ``CONFIG_RTE_MAJOR_ABI=16.11`` which will make all libraries
212 ``librte<?>.so.16.11`` instead of ``librte<?>.so.<LIBABIVER>``.
214 Examples of ABI Macro use
215 -------------------------
217 Updating a public API
218 ~~~~~~~~~~~~~~~~~~~~~
220 Assume we have a function as follows
225 * Create an acl context object for apps to
229 rte_acl_create(const struct rte_acl_param *param)
235 Assume that struct rte_acl_ctx is a private structure, and that a developer
236 wishes to enhance the acl api so that a debugging flag can be enabled on a
237 per-context basis. This requires an addition to the structure (which, being
238 private, is safe), but it also requires modifying the code as follows
243 * Create an acl context object for apps to
247 rte_acl_create(const struct rte_acl_param *param, int debug)
253 Note also that, being a public function, the header file prototype must also be
254 changed, as must all the call sites, to reflect the new ABI footprint. We will
255 maintain previous ABI versions that are accessible only to previously compiled
258 The addition of a parameter to the function is ABI breaking as the function is
259 public, and existing application may use it in its current form. However, the
260 compatibility macros in DPDK allow a developer to use symbol versioning so that
261 multiple functions can be mapped to the same public symbol based on when an
262 application was linked to it. To see how this is done, we start with the
263 requisite libraries version map file. Initially the version map file for the
264 acl library looks like this
274 rte_acl_classify_alg;
275 rte_acl_classify_scalar;
278 rte_acl_find_existing;
280 rte_acl_ipv4vlan_add_rules;
281 rte_acl_ipv4vlan_build;
285 rte_acl_set_ctx_classify;
290 This file needs to be modified as follows
300 rte_acl_classify_alg;
301 rte_acl_classify_scalar;
304 rte_acl_find_existing;
306 rte_acl_ipv4vlan_add_rules;
307 rte_acl_ipv4vlan_build;
311 rte_acl_set_ctx_classify;
322 The addition of the new block tells the linker that a new version node is
323 available (DPDK_2.1), which contains the symbol rte_acl_create, and inherits the
324 symbols from the DPDK_2.0 node. This list is directly translated into a list of
325 exported symbols when DPDK is compiled as a shared library
327 Next, we need to specify in the code which function map to the rte_acl_create
328 symbol at which versions. First, at the site of the initial symbol definition,
329 we need to update the function so that it is uniquely named, and not in conflict
330 with the public symbol name
335 -rte_acl_create(const struct rte_acl_param *param)
336 +rte_acl_create_v20(const struct rte_acl_param *param)
339 struct rte_acl_ctx *ctx;
342 Note that the base name of the symbol was kept intact, as this is conducive to
343 the macros used for versioning symbols. That is our next step, mapping this new
344 symbol name to the initial symbol name at version node 2.0. Immediately after
345 the function, we add this line of code
349 VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
351 Remembering to also add the rte_compat.h header to the requisite c file where
352 these changes are being made. The above macro instructs the linker to create a
353 new symbol ``rte_acl_create@DPDK_2.0``, which matches the symbol created in older
354 builds, but now points to the above newly named function. We have now mapped
355 the original rte_acl_create symbol to the original function (but with a new
358 Next, we need to create the 2.1 version of the symbol. We create a new function
359 name, with a different suffix, and implement it appropriately
364 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
366 struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
373 This code serves as our new API call. Its the same as our old call, but adds
374 the new parameter in place. Next we need to map this function to the symbol
375 ``rte_acl_create@DPDK_2.1``. To do this, we modify the public prototype of the call
376 in the header file, adding the macro there to inform all including applications,
377 that on re-link, the default rte_acl_create symbol should point to this
378 function. Note that we could do this by simply naming the function above
379 rte_acl_create, and the linker would chose the most recent version tag to apply
380 in the version script, but we can also do this in the header file
385 -rte_acl_create(const struct rte_acl_param *param);
386 +rte_acl_create(const struct rte_acl_param *param, int debug);
387 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
389 The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
390 header, to link to the rte_acl_create_v21 function and apply the DPDK_2.1
391 version node to it. This method is more explicit and flexible than just
392 re-implementing the exact symbol name, and allows for other features (such as
393 linking to the old symbol version by default, when the new ABI is to be opt-in
396 One last thing we need to do. Note that we've taken what was a public symbol,
397 and duplicated it into two uniquely and differently named symbols. We've then
398 mapped each of those back to the public symbol ``rte_acl_create`` with different
399 version tags. This only applies to dynamic linking, as static linking has no
400 notion of versioning. That leaves this code in a position of no longer having a
401 symbol simply named ``rte_acl_create`` and a static build will fail on that
404 To correct this, we can simply map a function of our choosing back to the public
405 symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro. Generally the
406 assumption is that the most recent version of the symbol is the one you want to
407 map. So, back in the C file where, immediately after ``rte_acl_create_v21`` is
413 rte_acl_create_v21(const struct rte_acl_param *param, int debug)
417 MAP_STATIC_SYMBOL(struct rte_acl_ctx *rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
419 That tells the compiler that, when building a static library, any calls to the
420 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
422 That's it, on the next shared library rebuild, there will be two versions of
423 rte_acl_create, an old DPDK_2.0 version, used by previously built applications,
424 and a new DPDK_2.1 version, used by future built applications.
427 Deprecating part of a public API
428 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
430 Lets assume that you've done the above update, and after a few releases have
431 passed you decide you would like to retire the old version of the function.
432 After having gone through the ABI deprecation announcement process, removal is
433 easy. Start by removing the symbol from the requisite version map file:
443 rte_acl_classify_alg;
444 rte_acl_classify_scalar;
447 rte_acl_find_existing;
449 rte_acl_ipv4vlan_add_rules;
450 rte_acl_ipv4vlan_build;
454 rte_acl_set_ctx_classify;
465 Next remove the corresponding versioned export.
469 -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
472 Note that the internal function definition could also be removed, but its used
473 in our example by the newer version _v21, so we leave it in place. This is a
476 Lastly, we need to bump the LIBABIVER number for this library in the Makefile to
477 indicate to applications doing dynamic linking that this is a later, and
478 possibly incompatible library version:
485 Deprecating an entire ABI version
486 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
488 While removing a symbol from and ABI may be useful, it is often more practical
489 to remove an entire version node at once. If a version node completely
490 specifies an API, then removing part of it, typically makes it incomplete. In
491 those cases it is better to remove the entire node
493 To do this, start by modifying the version map file, such that all symbols from
494 the node to be removed are merged into the next node in the map
496 In the case of our map above, it would transform to look as follows
506 rte_acl_classify_alg;
507 rte_acl_classify_scalar;
510 rte_acl_find_existing;
512 rte_acl_ipv4vlan_add_rules;
513 rte_acl_ipv4vlan_build;
517 rte_acl_set_ctx_classify;
522 Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
523 updated to point to the new version node in any header files for all affected
528 -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
529 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
531 Lastly, any VERSION_SYMBOL macros that point to the old version node should be
532 removed, taking care to keep, where need old code in place to support newer
533 versions of the symbol.
535 Running the ABI Validator
536 -------------------------
538 The ``devtools`` directory in the DPDK source tree contains a utility program,
539 ``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
541 <http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
543 This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
544 utilities which can be installed via a package manager. For example::
546 sudo yum install abi-compliance-checker
547 sudo yum install abi-dumper
549 The syntax of the ``validate-abi.sh`` utility is::
551 ./devtools/validate-abi.sh <REV1> <REV2> <TARGET>
553 Where ``REV1`` and ``REV2`` are valid gitrevisions(7)
554 https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
555 on the local repo and target is the usual DPDK compilation target.
559 # Check between the previous and latest commit:
560 ./devtools/validate-abi.sh HEAD~1 HEAD x86_64-native-linuxapp-gcc
562 # Check between two tags:
563 ./devtools/validate-abi.sh v2.0.0 v2.1.0 x86_64-native-linuxapp-gcc
565 # Check between git master and local topic-branch "vhost-hacking":
566 ./devtools/validate-abi.sh master vhost-hacking x86_64-native-linuxapp-gcc
568 After the validation script completes (it can take a while since it need to
569 compile both tags) it will create compatibility reports in the
570 ``./compat_report`` directory. Listed incompatibilities can be found as
573 grep -lr Incompatible compat_reports/