7 This document details some methods for handling ABI management in the DPDK.
8 Note this document is not exhaustive, in that C library versioning is flexible
9 allowing multiple methods to achieve various goals, but it will provide the user
10 with some introductory methods
15 #. Whenever possible, ABI should be preserved
16 #. Libraries or APIs marked in ``experimental`` state may change without constraint.
17 #. New APIs will be marked as ``experimental`` for at least one release to allow
18 any issues found by users of the new API to be fixed quickly
19 #. The addition of symbols is generally not problematic
20 #. The modification of symbols can generally be managed with versioning
21 #. The removal of symbols generally is an ABI break and requires bumping of the
23 #. Updates to the minimum hardware requirements, which drop support for hardware which
24 was previously supported, should be treated as an ABI change.
29 An ABI (Application Binary Interface) is the set of runtime interfaces exposed
30 by a library. It is similar to an API (Application Programming Interface) but
31 is the result of compilation. It is also effectively cloned when applications
32 link to dynamic libraries. That is to say when an application is compiled to
33 link against dynamic libraries, it is assumed that the ABI remains constant
34 between the time the application is compiled/linked, and the time that it runs.
35 Therefore, in the case of dynamic linking, it is critical that an ABI is
36 preserved, or (when modified), done in such a way that the application is unable
37 to behave improperly or in an unexpected fashion.
42 ABI versions are set at the time of major release labeling, and the ABI may
43 change multiple times, without warning, between the last release label and the
44 HEAD label of the git tree.
46 APIs marked as ``experimental`` are not considered part of the ABI and may
47 change without warning at any time. Since changes to APIs are most likely
48 immediately after their introduction, as users begin to take advantage of
49 those new APIs and start finding issues with them, new DPDK APIs will be
50 automatically marked as ``experimental`` to allow for a period of stabilization
51 before they become part of a tracked ABI.
53 Note that marking an API as experimental is a multi step process.
54 To mark an API as experimental, the symbols which are desired to be exported
55 must be placed in an EXPERIMENTAL version block in the corresponding libraries'
57 Secondly, the corresponding definitions of those exported functions, and
58 their forward declarations (in the development header files), must be marked
59 with the ``__rte_experimental`` tag (see ``rte_compat.h``).
60 The DPDK build makefiles perform a check to ensure that the map file and the
61 C code reflect the same list of symbols.
62 This check can be circumvented by defining ``ALLOW_EXPERIMENTAL_API``
63 during compilation in the corresponding library Makefile.
65 In addition to tagging the code with ``__rte_experimental``,
66 the doxygen markup must also contain the EXPERIMENTAL string,
67 and the MAINTAINERS file should note the EXPERIMENTAL libraries.
69 ABI versions, once released, are available until such time as their
70 deprecation has been noted in the Release Notes for at least one major release
71 cycle. For example consider the case where the ABI for DPDK 2.0 has been
72 shipped and then a decision is made to modify it during the development of
73 DPDK 2.1. The decision will be recorded in the Release Notes for the DPDK 2.1
74 release and the modification will be made available in the DPDK 2.2 release.
76 ABI versions may be deprecated in whole or in part as needed by a given
79 Some ABI changes may be too significant to reasonably maintain multiple
80 versions. In those cases ABI's may be updated without backward compatibility
81 being provided. The requirements for doing so are:
83 #. At least 3 acknowledgments of the need to do so must be made on the
84 dpdk.org mailing list.
86 - The acknowledgment of the maintainer of the component is mandatory, or if
87 no maintainer is available for the component, the tree/sub-tree maintainer
88 for that component must acknowledge the ABI change instead.
90 - It is also recommended that acknowledgments from different "areas of
91 interest" be sought for each deprecation, for example: from NIC vendors,
92 CPU vendors, end-users, etc.
94 #. The changes (including an alternative map file) must be gated with
95 the ``RTE_NEXT_ABI`` option, and provided with a deprecation notice at the
97 It will become the default ABI in the next release.
99 #. A full deprecation cycle, as explained above, must be made to offer
100 downstream consumers sufficient warning of the change.
102 #. At the beginning of the next release cycle, every ``RTE_NEXT_ABI``
103 conditions will be removed, the ``LIBABIVER`` variable in the makefile(s)
104 where the ABI is changed will be incremented, and the map files will
107 Note that the above process for ABI deprecation should not be undertaken
108 lightly. ABI stability is extremely important for downstream consumers of the
109 DPDK, especially when distributed in shared object form. Every effort should
110 be made to preserve the ABI whenever possible. The ABI should only be changed
111 for significant reasons, such as performance enhancements. ABI breakage due to
112 changes such as reorganizing public structure fields for aesthetic or
113 readability purposes should be avoided.
117 Updates to the minimum hardware requirements, which drop support for hardware
118 which was previously supported, should be treated as an ABI change, and
119 follow the relevant deprecation policy procedures as above: 3 acks and
120 announcement at least one release in advance.
122 Examples of Deprecation Notices
123 -------------------------------
125 The following are some examples of ABI deprecation notices which would be
126 added to the Release Notes:
128 * The Macro ``#RTE_FOO`` is deprecated and will be removed with version 2.0,
129 to be replaced with the inline function ``rte_foo()``.
131 * The function ``rte_mbuf_grok()`` has been updated to include a new parameter
132 in version 2.0. Backwards compatibility will be maintained for this function
133 until the release of version 2.1
135 * The members of ``struct rte_foo`` have been reorganized in release 2.0 for
136 performance reasons. Existing binary applications will have backwards
137 compatibility in release 2.0, while newly built binaries will need to
138 reference the new structure variant ``struct rte_foo2``. Compatibility will
139 be removed in release 2.2, and all applications will require updating and
140 rebuilding to the new structure at that time, which will be renamed to the
141 original ``struct rte_foo``.
143 * Significant ABI changes are planned for the ``librte_dostuff`` library. The
144 upcoming release 2.0 will not contain these changes, but release 2.1 will,
145 and no backwards compatibility is planned due to the extensive nature of
146 these changes. Binaries using this library built prior to version 2.1 will
147 require updating and recompilation.
152 When a symbol is exported from a library to provide an API, it also provides a
153 calling convention (ABI) that is embodied in its name, return type and
154 arguments. Occasionally that function may need to change to accommodate new
155 functionality or behavior. When that occurs, it is desirable to allow for
156 backward compatibility for a time with older binaries that are dynamically
159 To support backward compatibility the ``lib/librte_compat/rte_compat.h``
160 header file provides macros to use when updating exported functions. These
161 macros are used in conjunction with the ``rte_<library>_version.map`` file for
162 a given library to allow multiple versions of a symbol to exist in a shared
163 library so that older binaries need not be immediately recompiled.
165 The macros exported are:
167 * ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
168 versioned symbol ``b@DPDK_n`` to the internal function ``b_e``.
170 * ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
171 the linker to bind references to symbol ``b`` to the internal symbol
174 * ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the
175 fully qualified function ``p``, so that if a symbol becomes versioned, it
176 can still be mapped back to the public symbol name.
178 Setting a Major ABI version
179 ---------------------------
181 Downstreams might want to provide different DPDK releases at the same time to
182 support multiple consumers of DPDK linked against older and newer sonames.
184 Also due to the interdependencies that DPDK libraries can have applications
185 might end up with an executable space in which multiple versions of a library
188 Think of LibA that got an ABI bump and LibB that did not get an ABI bump but is
195 \-> LibB.new -> LibA.new
197 That is a conflict which can be avoided by setting ``CONFIG_RTE_MAJOR_ABI``.
198 If set, the value of ``CONFIG_RTE_MAJOR_ABI`` overwrites all - otherwise per
199 library - versions defined in the libraries ``LIBABIVER``.
200 An example might be ``CONFIG_RTE_MAJOR_ABI=16.11`` which will make all libraries
201 ``librte<?>.so.16.11`` instead of ``librte<?>.so.<LIBABIVER>``.
203 Examples of ABI Macro use
204 -------------------------
206 Updating a public API
207 ~~~~~~~~~~~~~~~~~~~~~
209 Assume we have a function as follows
214 * Create an acl context object for apps to
218 rte_acl_create(const struct rte_acl_param *param)
224 Assume that struct rte_acl_ctx is a private structure, and that a developer
225 wishes to enhance the acl api so that a debugging flag can be enabled on a
226 per-context basis. This requires an addition to the structure (which, being
227 private, is safe), but it also requires modifying the code as follows
232 * Create an acl context object for apps to
236 rte_acl_create(const struct rte_acl_param *param, int debug)
242 Note also that, being a public function, the header file prototype must also be
243 changed, as must all the call sites, to reflect the new ABI footprint. We will
244 maintain previous ABI versions that are accessible only to previously compiled
247 The addition of a parameter to the function is ABI breaking as the function is
248 public, and existing application may use it in its current form. However, the
249 compatibility macros in DPDK allow a developer to use symbol versioning so that
250 multiple functions can be mapped to the same public symbol based on when an
251 application was linked to it. To see how this is done, we start with the
252 requisite libraries version map file. Initially the version map file for the
253 acl library looks like this
263 rte_acl_classify_alg;
264 rte_acl_classify_scalar;
267 rte_acl_find_existing;
269 rte_acl_ipv4vlan_add_rules;
270 rte_acl_ipv4vlan_build;
274 rte_acl_set_ctx_classify;
279 This file needs to be modified as follows
289 rte_acl_classify_alg;
290 rte_acl_classify_scalar;
293 rte_acl_find_existing;
295 rte_acl_ipv4vlan_add_rules;
296 rte_acl_ipv4vlan_build;
300 rte_acl_set_ctx_classify;
311 The addition of the new block tells the linker that a new version node is
312 available (DPDK_2.1), which contains the symbol rte_acl_create, and inherits the
313 symbols from the DPDK_2.0 node. This list is directly translated into a list of
314 exported symbols when DPDK is compiled as a shared library
316 Next, we need to specify in the code which function map to the rte_acl_create
317 symbol at which versions. First, at the site of the initial symbol definition,
318 we need to update the function so that it is uniquely named, and not in conflict
319 with the public symbol name
324 -rte_acl_create(const struct rte_acl_param *param)
325 +rte_acl_create_v20(const struct rte_acl_param *param)
328 struct rte_acl_ctx *ctx;
331 Note that the base name of the symbol was kept intact, as this is conducive to
332 the macros used for versioning symbols. That is our next step, mapping this new
333 symbol name to the initial symbol name at version node 2.0. Immediately after
334 the function, we add this line of code
338 VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
340 Remembering to also add the rte_compat.h header to the requisite c file where
341 these changes are being made. The above macro instructs the linker to create a
342 new symbol ``rte_acl_create@DPDK_2.0``, which matches the symbol created in older
343 builds, but now points to the above newly named function. We have now mapped
344 the original rte_acl_create symbol to the original function (but with a new
347 Next, we need to create the 2.1 version of the symbol. We create a new function
348 name, with a different suffix, and implement it appropriately
353 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
355 struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
362 This code serves as our new API call. Its the same as our old call, but adds
363 the new parameter in place. Next we need to map this function to the symbol
364 ``rte_acl_create@DPDK_2.1``. To do this, we modify the public prototype of the call
365 in the header file, adding the macro there to inform all including applications,
366 that on re-link, the default rte_acl_create symbol should point to this
367 function. Note that we could do this by simply naming the function above
368 rte_acl_create, and the linker would chose the most recent version tag to apply
369 in the version script, but we can also do this in the header file
374 -rte_acl_create(const struct rte_acl_param *param);
375 +rte_acl_create(const struct rte_acl_param *param, int debug);
376 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
378 The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
379 header, to link to the rte_acl_create_v21 function and apply the DPDK_2.1
380 version node to it. This method is more explicit and flexible than just
381 re-implementing the exact symbol name, and allows for other features (such as
382 linking to the old symbol version by default, when the new ABI is to be opt-in
385 One last thing we need to do. Note that we've taken what was a public symbol,
386 and duplicated it into two uniquely and differently named symbols. We've then
387 mapped each of those back to the public symbol ``rte_acl_create`` with different
388 version tags. This only applies to dynamic linking, as static linking has no
389 notion of versioning. That leaves this code in a position of no longer having a
390 symbol simply named ``rte_acl_create`` and a static build will fail on that
393 To correct this, we can simply map a function of our choosing back to the public
394 symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro. Generally the
395 assumption is that the most recent version of the symbol is the one you want to
396 map. So, back in the C file where, immediately after ``rte_acl_create_v21`` is
402 rte_acl_create_v21(const struct rte_acl_param *param, int debug)
406 MAP_STATIC_SYMBOL(struct rte_acl_ctx *rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
408 That tells the compiler that, when building a static library, any calls to the
409 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
411 That's it, on the next shared library rebuild, there will be two versions of
412 rte_acl_create, an old DPDK_2.0 version, used by previously built applications,
413 and a new DPDK_2.1 version, used by future built applications.
416 Deprecating part of a public API
417 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
419 Lets assume that you've done the above update, and after a few releases have
420 passed you decide you would like to retire the old version of the function.
421 After having gone through the ABI deprecation announcement process, removal is
422 easy. Start by removing the symbol from the requisite version map file:
432 rte_acl_classify_alg;
433 rte_acl_classify_scalar;
436 rte_acl_find_existing;
438 rte_acl_ipv4vlan_add_rules;
439 rte_acl_ipv4vlan_build;
443 rte_acl_set_ctx_classify;
454 Next remove the corresponding versioned export.
458 -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
461 Note that the internal function definition could also be removed, but its used
462 in our example by the newer version _v21, so we leave it in place. This is a
465 Lastly, we need to bump the LIBABIVER number for this library in the Makefile to
466 indicate to applications doing dynamic linking that this is a later, and
467 possibly incompatible library version:
474 Deprecating an entire ABI version
475 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
477 While removing a symbol from and ABI may be useful, it is often more practical
478 to remove an entire version node at once. If a version node completely
479 specifies an API, then removing part of it, typically makes it incomplete. In
480 those cases it is better to remove the entire node
482 To do this, start by modifying the version map file, such that all symbols from
483 the node to be removed are merged into the next node in the map
485 In the case of our map above, it would transform to look as follows
495 rte_acl_classify_alg;
496 rte_acl_classify_scalar;
499 rte_acl_find_existing;
501 rte_acl_ipv4vlan_add_rules;
502 rte_acl_ipv4vlan_build;
506 rte_acl_set_ctx_classify;
511 Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
512 updated to point to the new version node in any header files for all affected
517 -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
518 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
520 Lastly, any VERSION_SYMBOL macros that point to the old version node should be
521 removed, taking care to keep, where need old code in place to support newer
522 versions of the symbol.
524 Running the ABI Validator
525 -------------------------
527 The ``devtools`` directory in the DPDK source tree contains a utility program,
528 ``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
530 <http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
532 This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
533 utilities which can be installed via a package manager. For example::
535 sudo yum install abi-compliance-checker
536 sudo yum install abi-dumper
538 The syntax of the ``validate-abi.sh`` utility is::
540 ./devtools/validate-abi.sh <REV1> <REV2> <TARGET>
542 Where ``REV1`` and ``REV2`` are valid gitrevisions(7)
543 https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
544 on the local repo and target is the usual DPDK compilation target.
548 # Check between the previous and latest commit:
549 ./devtools/validate-abi.sh HEAD~1 HEAD x86_64-native-linuxapp-gcc
551 # Check between two tags:
552 ./devtools/validate-abi.sh v2.0.0 v2.1.0 x86_64-native-linuxapp-gcc
554 # Check between git master and local topic-branch "vhost-hacking":
555 ./devtools/validate-abi.sh master vhost-hacking x86_64-native-linuxapp-gcc
557 After the validation script completes (it can take a while since it need to
558 compile both tags) it will create compatibility reports in the
559 ``./compat_report`` directory. Listed incompatibilities can be found as
562 grep -lr Incompatible compat_reports/