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.
15 #. Whenever possible, ABI should be preserved
16 #. ABI/API may be changed with a deprecation process
17 #. The modification of symbols can generally be managed with versioning
18 #. Libraries or APIs marked in ``experimental`` state may change without constraint
19 #. New APIs will be marked as ``experimental`` for at least one release to allow
20 any issues found by users of the new API to be fixed quickly
21 #. The addition of symbols is generally not problematic
22 #. The removal of symbols generally is an ABI break and requires bumping of the
24 #. Updates to the minimum hardware requirements, which drop support for hardware which
25 was previously supported, should be treated as an ABI change.
30 An ABI (Application Binary Interface) is the set of runtime interfaces exposed
31 by a library. It is similar to an API (Application Programming Interface) but
32 is the result of compilation. It is also effectively cloned when applications
33 link to dynamic libraries. That is to say when an application is compiled to
34 link against dynamic libraries, it is assumed that the ABI remains constant
35 between the time the application is compiled/linked, and the time that it runs.
36 Therefore, in the case of dynamic linking, it is critical that an ABI is
37 preserved, or (when modified), done in such a way that the application is unable
38 to behave improperly or in an unexpected fashion.
47 ABI versions are set at the time of major release labeling, and the ABI may
48 change multiple times, without warning, between the last release label and the
49 HEAD label of the git tree.
51 ABI versions, once released, are available until such time as their
52 deprecation has been noted in the Release Notes for at least one major release
53 cycle. For example consider the case where the ABI for DPDK 2.0 has been
54 shipped and then a decision is made to modify it during the development of
55 DPDK 2.1. The decision will be recorded in the Release Notes for the DPDK 2.1
56 release and the modification will be made available in the DPDK 2.2 release.
58 ABI versions may be deprecated in whole or in part as needed by a given
61 Some ABI changes may be too significant to reasonably maintain multiple
62 versions. In those cases ABI's may be updated without backward compatibility
63 being provided. The requirements for doing so are:
65 #. At least 3 acknowledgments of the need to do so must be made on the
66 dpdk.org mailing list.
68 - The acknowledgment of the maintainer of the component is mandatory, or if
69 no maintainer is available for the component, the tree/sub-tree maintainer
70 for that component must acknowledge the ABI change instead.
72 - It is also recommended that acknowledgments from different "areas of
73 interest" be sought for each deprecation, for example: from NIC vendors,
74 CPU vendors, end-users, etc.
76 #. The changes (including an alternative map file) can be included with
77 deprecation notice, in wrapped way by the ``RTE_NEXT_ABI`` option,
78 to provide more details about oncoming changes.
79 ``RTE_NEXT_ABI`` wrapper will be removed when it become the default ABI.
80 More preferred way to provide this information is sending the feature
81 as a separate patch and reference it in deprecation notice.
83 #. A full deprecation cycle, as explained above, must be made to offer
84 downstream consumers sufficient warning of the change.
86 Note that the above process for ABI deprecation should not be undertaken
87 lightly. ABI stability is extremely important for downstream consumers of the
88 DPDK, especially when distributed in shared object form. Every effort should
89 be made to preserve the ABI whenever possible. The ABI should only be changed
90 for significant reasons, such as performance enhancements. ABI breakage due to
91 changes such as reorganizing public structure fields for aesthetic or
92 readability purposes should be avoided.
96 Updates to the minimum hardware requirements, which drop support for hardware
97 which was previously supported, should be treated as an ABI change, and
98 follow the relevant deprecation policy procedures as above: 3 acks and
99 announcement at least one release in advance.
101 Examples of Deprecation Notices
102 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
104 The following are some examples of ABI deprecation notices which would be
105 added to the Release Notes:
107 * The Macro ``#RTE_FOO`` is deprecated and will be removed with version 2.0,
108 to be replaced with the inline function ``rte_foo()``.
110 * The function ``rte_mbuf_grok()`` has been updated to include a new parameter
111 in version 2.0. Backwards compatibility will be maintained for this function
112 until the release of version 2.1
114 * The members of ``struct rte_foo`` have been reorganized in release 2.0 for
115 performance reasons. Existing binary applications will have backwards
116 compatibility in release 2.0, while newly built binaries will need to
117 reference the new structure variant ``struct rte_foo2``. Compatibility will
118 be removed in release 2.2, and all applications will require updating and
119 rebuilding to the new structure at that time, which will be renamed to the
120 original ``struct rte_foo``.
122 * Significant ABI changes are planned for the ``librte_dostuff`` library. The
123 upcoming release 2.0 will not contain these changes, but release 2.1 will,
124 and no backwards compatibility is planned due to the extensive nature of
125 these changes. Binaries using this library built prior to version 2.1 will
126 require updating and recompilation.
128 New API replacing previous one
129 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
131 If a new API proposed functionally replaces an existing one, when the new API
132 becomes non-experimental then the old one is marked with ``__rte_deprecated``.
133 Deprecated APIs are removed completely just after the next LTS.
135 Reminder that old API should follow deprecation process to be removed.
141 APIs marked as ``experimental`` are not considered part of the ABI and may
142 change without warning at any time. Since changes to APIs are most likely
143 immediately after their introduction, as users begin to take advantage of
144 those new APIs and start finding issues with them, new DPDK APIs will be
145 automatically marked as ``experimental`` to allow for a period of stabilization
146 before they become part of a tracked ABI.
148 Note that marking an API as experimental is a multi step process.
149 To mark an API as experimental, the symbols which are desired to be exported
150 must be placed in an EXPERIMENTAL version block in the corresponding libraries'
152 Secondly, the corresponding prototypes of those exported functions (in the
153 development header files), must be marked with the ``__rte_experimental`` tag
154 (see ``rte_compat.h``).
155 The DPDK build makefiles perform a check to ensure that the map file and the
156 C code reflect the same list of symbols.
157 This check can be circumvented by defining ``ALLOW_EXPERIMENTAL_API``
158 during compilation in the corresponding library Makefile.
160 In addition to tagging the code with ``__rte_experimental``,
161 the doxygen markup must also contain the EXPERIMENTAL string,
162 and the MAINTAINERS file should note the EXPERIMENTAL libraries.
164 For removing the experimental tag associated with an API, deprecation notice
165 is not required. Though, an API should remain in experimental state for at least
166 one release. Thereafter, normal process of posting patch for review to mailing
167 list can be followed.
173 Downstreams might want to provide different DPDK releases at the same time to
174 support multiple consumers of DPDK linked against older and newer sonames.
176 Also due to the interdependencies that DPDK libraries can have applications
177 might end up with an executable space in which multiple versions of a library
180 Think of LibA that got an ABI bump and LibB that did not get an ABI bump but is
187 \-> LibB.new -> LibA.new
189 That is a conflict which can be avoided by setting ``CONFIG_RTE_MAJOR_ABI``.
190 If set, the value of ``CONFIG_RTE_MAJOR_ABI`` overwrites all - otherwise per
191 library - versions defined in the libraries ``LIBABIVER``.
192 An example might be ``CONFIG_RTE_MAJOR_ABI=16.11`` which will make all libraries
193 ``librte<?>.so.16.11`` instead of ``librte<?>.so.<LIBABIVER>``.
202 When a symbol is exported from a library to provide an API, it also provides a
203 calling convention (ABI) that is embodied in its name, return type and
204 arguments. Occasionally that function may need to change to accommodate new
205 functionality or behavior. When that occurs, it is desirable to allow for
206 backward compatibility for a time with older binaries that are dynamically
209 To support backward compatibility the ``rte_compat.h``
210 header file provides macros to use when updating exported functions. These
211 macros are used in conjunction with the ``rte_<library>_version.map`` file for
212 a given library to allow multiple versions of a symbol to exist in a shared
213 library so that older binaries need not be immediately recompiled.
215 The macros exported are:
217 * ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
218 versioned symbol ``b@DPDK_n`` to the internal function ``b_e``.
220 * ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
221 the linker to bind references to symbol ``b`` to the internal symbol
224 * ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the
225 fully qualified function ``p``, so that if a symbol becomes versioned, it
226 can still be mapped back to the public symbol name.
228 Examples of ABI Macro use
229 ^^^^^^^^^^^^^^^^^^^^^^^^^
231 Updating a public API
232 _____________________
234 Assume we have a function as follows
239 * Create an acl context object for apps to
243 rte_acl_create(const struct rte_acl_param *param)
249 Assume that struct rte_acl_ctx is a private structure, and that a developer
250 wishes to enhance the acl api so that a debugging flag can be enabled on a
251 per-context basis. This requires an addition to the structure (which, being
252 private, is safe), but it also requires modifying the code as follows
257 * Create an acl context object for apps to
261 rte_acl_create(const struct rte_acl_param *param, int debug)
267 Note also that, being a public function, the header file prototype must also be
268 changed, as must all the call sites, to reflect the new ABI footprint. We will
269 maintain previous ABI versions that are accessible only to previously compiled
272 The addition of a parameter to the function is ABI breaking as the function is
273 public, and existing application may use it in its current form. However, the
274 compatibility macros in DPDK allow a developer to use symbol versioning so that
275 multiple functions can be mapped to the same public symbol based on when an
276 application was linked to it. To see how this is done, we start with the
277 requisite libraries version map file. Initially the version map file for the
278 acl library looks like this
288 rte_acl_classify_alg;
289 rte_acl_classify_scalar;
292 rte_acl_find_existing;
294 rte_acl_ipv4vlan_add_rules;
295 rte_acl_ipv4vlan_build;
299 rte_acl_set_ctx_classify;
304 This file needs to be modified as follows
314 rte_acl_classify_alg;
315 rte_acl_classify_scalar;
318 rte_acl_find_existing;
320 rte_acl_ipv4vlan_add_rules;
321 rte_acl_ipv4vlan_build;
325 rte_acl_set_ctx_classify;
336 The addition of the new block tells the linker that a new version node is
337 available (DPDK_2.1), which contains the symbol rte_acl_create, and inherits the
338 symbols from the DPDK_2.0 node. This list is directly translated into a list of
339 exported symbols when DPDK is compiled as a shared library
341 Next, we need to specify in the code which function map to the rte_acl_create
342 symbol at which versions. First, at the site of the initial symbol definition,
343 we need to update the function so that it is uniquely named, and not in conflict
344 with the public symbol name
349 -rte_acl_create(const struct rte_acl_param *param)
350 +rte_acl_create_v20(const struct rte_acl_param *param)
353 struct rte_acl_ctx *ctx;
356 Note that the base name of the symbol was kept intact, as this is conducive to
357 the macros used for versioning symbols. That is our next step, mapping this new
358 symbol name to the initial symbol name at version node 2.0. Immediately after
359 the function, we add this line of code
363 VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
365 Remembering to also add the rte_compat.h header to the requisite c file where
366 these changes are being made. The above macro instructs the linker to create a
367 new symbol ``rte_acl_create@DPDK_2.0``, which matches the symbol created in older
368 builds, but now points to the above newly named function. We have now mapped
369 the original rte_acl_create symbol to the original function (but with a new
372 Next, we need to create the 2.1 version of the symbol. We create a new function
373 name, with a different suffix, and implement it appropriately
378 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
380 struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
387 This code serves as our new API call. Its the same as our old call, but adds
388 the new parameter in place. Next we need to map this function to the symbol
389 ``rte_acl_create@DPDK_2.1``. To do this, we modify the public prototype of the call
390 in the header file, adding the macro there to inform all including applications,
391 that on re-link, the default rte_acl_create symbol should point to this
392 function. Note that we could do this by simply naming the function above
393 rte_acl_create, and the linker would chose the most recent version tag to apply
394 in the version script, but we can also do this in the header file
399 -rte_acl_create(const struct rte_acl_param *param);
400 +rte_acl_create(const struct rte_acl_param *param, int debug);
401 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
403 The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
404 header, to link to the rte_acl_create_v21 function and apply the DPDK_2.1
405 version node to it. This method is more explicit and flexible than just
406 re-implementing the exact symbol name, and allows for other features (such as
407 linking to the old symbol version by default, when the new ABI is to be opt-in
410 One last thing we need to do. Note that we've taken what was a public symbol,
411 and duplicated it into two uniquely and differently named symbols. We've then
412 mapped each of those back to the public symbol ``rte_acl_create`` with different
413 version tags. This only applies to dynamic linking, as static linking has no
414 notion of versioning. That leaves this code in a position of no longer having a
415 symbol simply named ``rte_acl_create`` and a static build will fail on that
418 To correct this, we can simply map a function of our choosing back to the public
419 symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro. Generally the
420 assumption is that the most recent version of the symbol is the one you want to
421 map. So, back in the C file where, immediately after ``rte_acl_create_v21`` is
427 rte_acl_create_v21(const struct rte_acl_param *param, int debug)
431 MAP_STATIC_SYMBOL(struct rte_acl_ctx *rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
433 That tells the compiler that, when building a static library, any calls to the
434 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
436 That's it, on the next shared library rebuild, there will be two versions of
437 rte_acl_create, an old DPDK_2.0 version, used by previously built applications,
438 and a new DPDK_2.1 version, used by future built applications.
441 Deprecating part of a public API
442 ________________________________
444 Lets assume that you've done the above update, and after a few releases have
445 passed you decide you would like to retire the old version of the function.
446 After having gone through the ABI deprecation announcement process, removal is
447 easy. Start by removing the symbol from the requisite version map file:
457 rte_acl_classify_alg;
458 rte_acl_classify_scalar;
461 rte_acl_find_existing;
463 rte_acl_ipv4vlan_add_rules;
464 rte_acl_ipv4vlan_build;
468 rte_acl_set_ctx_classify;
479 Next remove the corresponding versioned export.
483 -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
486 Note that the internal function definition could also be removed, but its used
487 in our example by the newer version _v21, so we leave it in place. This is a
490 Lastly, we need to bump the LIBABIVER number for this library in the Makefile to
491 indicate to applications doing dynamic linking that this is a later, and
492 possibly incompatible library version:
499 Deprecating an entire ABI version
500 _________________________________
502 While removing a symbol from and ABI may be useful, it is often more practical
503 to remove an entire version node at once. If a version node completely
504 specifies an API, then removing part of it, typically makes it incomplete. In
505 those cases it is better to remove the entire node
507 To do this, start by modifying the version map file, such that all symbols from
508 the node to be removed are merged into the next node in the map
510 In the case of our map above, it would transform to look as follows
520 rte_acl_classify_alg;
521 rte_acl_classify_scalar;
524 rte_acl_find_existing;
526 rte_acl_ipv4vlan_add_rules;
527 rte_acl_ipv4vlan_build;
531 rte_acl_set_ctx_classify;
536 Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
537 updated to point to the new version node in any header files for all affected
542 -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
543 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
545 Lastly, any VERSION_SYMBOL macros that point to the old version node should be
546 removed, taking care to keep, where need old code in place to support newer
547 versions of the symbol.
550 Running the ABI Validator
551 -------------------------
553 The ``devtools`` directory in the DPDK source tree contains a utility program,
554 ``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
556 <http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
558 This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
559 utilities which can be installed via a package manager. For example::
561 sudo yum install abi-compliance-checker
562 sudo yum install abi-dumper
564 The syntax of the ``validate-abi.sh`` utility is::
566 ./devtools/validate-abi.sh <REV1> <REV2>
568 Where ``REV1`` and ``REV2`` are valid gitrevisions(7)
569 https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
574 # Check between the previous and latest commit:
575 ./devtools/validate-abi.sh HEAD~1 HEAD
577 # Check on a specific compilation target:
578 ./devtools/validate-abi.sh -t x86_64-native-linux-gcc HEAD~1 HEAD
580 # Check between two tags:
581 ./devtools/validate-abi.sh v2.0.0 v2.1.0
583 # Check between git master and local topic-branch "vhost-hacking":
584 ./devtools/validate-abi.sh master vhost-hacking
586 After the validation script completes (it can take a while since it need to
587 compile both tags) it will create compatibility reports in the
588 ``./abi-check/compat_report`` directory. Listed incompatibilities can be found
591 grep -lr Incompatible abi-check/compat_reports/