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 #. The libraries marked in experimental state may change without constraint.
17 #. The addition of symbols is generally not problematic
18 #. The modification of symbols can generally be managed with versioning
19 #. The removal of symbols generally is an ABI break and requires bumping of the
21 #. Updates to the minimum hardware requirements, which drop support for hardware which
22 was previously supported, should be treated as an ABI change.
27 An ABI (Application Binary Interface) is the set of runtime interfaces exposed
28 by a library. It is similar to an API (Application Programming Interface) but
29 is the result of compilation. It is also effectively cloned when applications
30 link to dynamic libraries. That is to say when an application is compiled to
31 link against dynamic libraries, it is assumed that the ABI remains constant
32 between the time the application is compiled/linked, and the time that it runs.
33 Therefore, in the case of dynamic linking, it is critical that an ABI is
34 preserved, or (when modified), done in such a way that the application is unable
35 to behave improperly or in an unexpected fashion.
40 ABI versions are set at the time of major release labeling, and the ABI may
41 change multiple times, without warning, between the last release label and the
42 HEAD label of the git tree.
44 ABI versions, once released, are available until such time as their
45 deprecation has been noted in the Release Notes for at least one major release
46 cycle. For example consider the case where the ABI for DPDK 2.0 has been
47 shipped and then a decision is made to modify it during the development of
48 DPDK 2.1. The decision will be recorded in the Release Notes for the DPDK 2.1
49 release and the modification will be made available in the DPDK 2.2 release.
51 ABI versions may be deprecated in whole or in part as needed by a given
54 Some ABI changes may be too significant to reasonably maintain multiple
55 versions. In those cases ABI's may be updated without backward compatibility
56 being provided. The requirements for doing so are:
58 #. At least 3 acknowledgments of the need to do so must be made on the
59 dpdk.org mailing list.
61 #. The changes (including an alternative map file) must be gated with
62 the ``RTE_NEXT_ABI`` option, and provided with a deprecation notice at the
64 It will become the default ABI in the next release.
66 #. A full deprecation cycle, as explained above, must be made to offer
67 downstream consumers sufficient warning of the change.
69 #. At the beginning of the next release cycle, every ``RTE_NEXT_ABI``
70 conditions will be removed, the ``LIBABIVER`` variable in the makefile(s)
71 where the ABI is changed will be incremented, and the map files will
74 Note that the above process for ABI deprecation should not be undertaken
75 lightly. ABI stability is extremely important for downstream consumers of the
76 DPDK, especially when distributed in shared object form. Every effort should
77 be made to preserve the ABI whenever possible. The ABI should only be changed
78 for significant reasons, such as performance enhancements. ABI breakage due to
79 changes such as reorganizing public structure fields for aesthetic or
80 readability purposes should be avoided.
84 Updates to the minimum hardware requirements, which drop support for hardware
85 which was previously supported, should be treated as an ABI change, and
86 follow the relevant deprecation policy procedures as above: 3 acks and
87 announcement at least one release in advance.
89 Examples of Deprecation Notices
90 -------------------------------
92 The following are some examples of ABI deprecation notices which would be
93 added to the Release Notes:
95 * The Macro ``#RTE_FOO`` is deprecated and will be removed with version 2.0,
96 to be replaced with the inline function ``rte_foo()``.
98 * The function ``rte_mbuf_grok()`` has been updated to include a new parameter
99 in version 2.0. Backwards compatibility will be maintained for this function
100 until the release of version 2.1
102 * The members of ``struct rte_foo`` have been reorganized in release 2.0 for
103 performance reasons. Existing binary applications will have backwards
104 compatibility in release 2.0, while newly built binaries will need to
105 reference the new structure variant ``struct rte_foo2``. Compatibility will
106 be removed in release 2.2, and all applications will require updating and
107 rebuilding to the new structure at that time, which will be renamed to the
108 original ``struct rte_foo``.
110 * Significant ABI changes are planned for the ``librte_dostuff`` library. The
111 upcoming release 2.0 will not contain these changes, but release 2.1 will,
112 and no backwards compatibility is planned due to the extensive nature of
113 these changes. Binaries using this library built prior to version 2.1 will
114 require updating and recompilation.
119 When a symbol is exported from a library to provide an API, it also provides a
120 calling convention (ABI) that is embodied in its name, return type and
121 arguments. Occasionally that function may need to change to accommodate new
122 functionality or behavior. When that occurs, it is desirable to allow for
123 backward compatibility for a time with older binaries that are dynamically
126 To support backward compatibility the ``lib/librte_compat/rte_compat.h``
127 header file provides macros to use when updating exported functions. These
128 macros are used in conjunction with the ``rte_<library>_version.map`` file for
129 a given library to allow multiple versions of a symbol to exist in a shared
130 library so that older binaries need not be immediately recompiled.
132 The macros exported are:
134 * ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
135 versioned symbol ``b@DPDK_n`` to the internal function ``b_e``.
137 * ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
138 the linker to bind references to symbol ``b`` to the internal symbol
141 * ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the
142 fully qualified function ``p``, so that if a symbol becomes versioned, it
143 can still be mapped back to the public symbol name.
145 Setting a Major ABI version
146 ---------------------------
148 Downstreams might want to provide different DPDK releases at the same time to
149 support multiple consumers of DPDK linked against older and newer sonames.
151 Also due to the interdependencies that DPDK libraries can have applications
152 might end up with an executable space in which multiple versions of a library
155 Think of LibA that got an ABI bump and LibB that did not get an ABI bump but is
162 \-> LibB.new -> LibA.new
164 That is a conflict which can be avoided by setting ``CONFIG_RTE_MAJOR_ABI``.
165 If set, the value of ``CONFIG_RTE_MAJOR_ABI`` overwrites all - otherwise per
166 library - versions defined in the libraries ``LIBABIVER``.
167 An example might be ``CONFIG_RTE_MAJOR_ABI=16.11`` which will make all libraries
168 ``librte<?>.so.16.11`` instead of ``librte<?>.so.<LIBABIVER>``.
170 Examples of ABI Macro use
171 -------------------------
173 Updating a public API
174 ~~~~~~~~~~~~~~~~~~~~~
176 Assume we have a function as follows
181 * Create an acl context object for apps to
185 rte_acl_create(const struct rte_acl_param *param)
191 Assume that struct rte_acl_ctx is a private structure, and that a developer
192 wishes to enhance the acl api so that a debugging flag can be enabled on a
193 per-context basis. This requires an addition to the structure (which, being
194 private, is safe), but it also requires modifying the code as follows
199 * Create an acl context object for apps to
203 rte_acl_create(const struct rte_acl_param *param, int debug)
209 Note also that, being a public function, the header file prototype must also be
210 changed, as must all the call sites, to reflect the new ABI footprint. We will
211 maintain previous ABI versions that are accessible only to previously compiled
214 The addition of a parameter to the function is ABI breaking as the function is
215 public, and existing application may use it in its current form. However, the
216 compatibility macros in DPDK allow a developer to use symbol versioning so that
217 multiple functions can be mapped to the same public symbol based on when an
218 application was linked to it. To see how this is done, we start with the
219 requisite libraries version map file. Initially the version map file for the
220 acl library looks like this
230 rte_acl_classify_alg;
231 rte_acl_classify_scalar;
234 rte_acl_find_existing;
236 rte_acl_ipv4vlan_add_rules;
237 rte_acl_ipv4vlan_build;
241 rte_acl_set_ctx_classify;
246 This file needs to be modified as follows
256 rte_acl_classify_alg;
257 rte_acl_classify_scalar;
260 rte_acl_find_existing;
262 rte_acl_ipv4vlan_add_rules;
263 rte_acl_ipv4vlan_build;
267 rte_acl_set_ctx_classify;
278 The addition of the new block tells the linker that a new version node is
279 available (DPDK_2.1), which contains the symbol rte_acl_create, and inherits the
280 symbols from the DPDK_2.0 node. This list is directly translated into a list of
281 exported symbols when DPDK is compiled as a shared library
283 Next, we need to specify in the code which function map to the rte_acl_create
284 symbol at which versions. First, at the site of the initial symbol definition,
285 we need to update the function so that it is uniquely named, and not in conflict
286 with the public symbol name
291 -rte_acl_create(const struct rte_acl_param *param)
292 +rte_acl_create_v20(const struct rte_acl_param *param)
295 struct rte_acl_ctx *ctx;
298 Note that the base name of the symbol was kept intact, as this is conducive to
299 the macros used for versioning symbols. That is our next step, mapping this new
300 symbol name to the initial symbol name at version node 2.0. Immediately after
301 the function, we add this line of code
305 VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
307 Remembering to also add the rte_compat.h header to the requisite c file where
308 these changes are being made. The above macro instructs the linker to create a
309 new symbol ``rte_acl_create@DPDK_2.0``, which matches the symbol created in older
310 builds, but now points to the above newly named function. We have now mapped
311 the original rte_acl_create symbol to the original function (but with a new
314 Next, we need to create the 2.1 version of the symbol. We create a new function
315 name, with a different suffix, and implement it appropriately
320 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
322 struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
329 This code serves as our new API call. Its the same as our old call, but adds
330 the new parameter in place. Next we need to map this function to the symbol
331 ``rte_acl_create@DPDK_2.1``. To do this, we modify the public prototype of the call
332 in the header file, adding the macro there to inform all including applications,
333 that on re-link, the default rte_acl_create symbol should point to this
334 function. Note that we could do this by simply naming the function above
335 rte_acl_create, and the linker would chose the most recent version tag to apply
336 in the version script, but we can also do this in the header file
341 -rte_acl_create(const struct rte_acl_param *param);
342 +rte_acl_create(const struct rte_acl_param *param, int debug);
343 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
345 The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
346 header, to link to the rte_acl_create_v21 function and apply the DPDK_2.1
347 version node to it. This method is more explicit and flexible than just
348 re-implementing the exact symbol name, and allows for other features (such as
349 linking to the old symbol version by default, when the new ABI is to be opt-in
352 One last thing we need to do. Note that we've taken what was a public symbol,
353 and duplicated it into two uniquely and differently named symbols. We've then
354 mapped each of those back to the public symbol ``rte_acl_create`` with different
355 version tags. This only applies to dynamic linking, as static linking has no
356 notion of versioning. That leaves this code in a position of no longer having a
357 symbol simply named ``rte_acl_create`` and a static build will fail on that
360 To correct this, we can simply map a function of our choosing back to the public
361 symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro. Generally the
362 assumption is that the most recent version of the symbol is the one you want to
363 map. So, back in the C file where, immediately after ``rte_acl_create_v21`` is
369 rte_acl_create_v21(const struct rte_acl_param *param, int debug)
373 MAP_STATIC_SYMBOL(struct rte_acl_ctx *rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
375 That tells the compiler that, when building a static library, any calls to the
376 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
378 That's it, on the next shared library rebuild, there will be two versions of
379 rte_acl_create, an old DPDK_2.0 version, used by previously built applications,
380 and a new DPDK_2.1 version, used by future built applications.
383 Deprecating part of a public API
384 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
386 Lets assume that you've done the above update, and after a few releases have
387 passed you decide you would like to retire the old version of the function.
388 After having gone through the ABI deprecation announcement process, removal is
389 easy. Start by removing the symbol from the requisite version map file:
399 rte_acl_classify_alg;
400 rte_acl_classify_scalar;
403 rte_acl_find_existing;
405 rte_acl_ipv4vlan_add_rules;
406 rte_acl_ipv4vlan_build;
410 rte_acl_set_ctx_classify;
421 Next remove the corresponding versioned export.
425 -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
428 Note that the internal function definition could also be removed, but its used
429 in our example by the newer version _v21, so we leave it in place. This is a
432 Lastly, we need to bump the LIBABIVER number for this library in the Makefile to
433 indicate to applications doing dynamic linking that this is a later, and
434 possibly incompatible library version:
441 Deprecating an entire ABI version
442 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
444 While removing a symbol from and ABI may be useful, it is often more practical
445 to remove an entire version node at once. If a version node completely
446 specifies an API, then removing part of it, typically makes it incomplete. In
447 those cases it is better to remove the entire node
449 To do this, start by modifying the version map file, such that all symbols from
450 the node to be removed are merged into the next node in the map
452 In the case of our map above, it would transform to look as follows
462 rte_acl_classify_alg;
463 rte_acl_classify_scalar;
466 rte_acl_find_existing;
468 rte_acl_ipv4vlan_add_rules;
469 rte_acl_ipv4vlan_build;
473 rte_acl_set_ctx_classify;
478 Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
479 updated to point to the new version node in any header files for all affected
484 -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
485 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
487 Lastly, any VERSION_SYMBOL macros that point to the old version node should be
488 removed, taking care to keep, where need old code in place to support newer
489 versions of the symbol.
491 Running the ABI Validator
492 -------------------------
494 The ``devtools`` directory in the DPDK source tree contains a utility program,
495 ``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
497 <http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
499 This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
500 utilities which can be installed via a package manager. For example::
502 sudo yum install abi-compliance-checker
503 sudo yum install abi-dumper
505 The syntax of the ``validate-abi.sh`` utility is::
507 ./devtools/validate-abi.sh <REV1> <REV2> <TARGET>
509 Where ``REV1`` and ``REV2`` are valid gitrevisions(7)
510 https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
511 on the local repo and target is the usual DPDK compilation target.
515 # Check between the previous and latest commit:
516 ./devtools/validate-abi.sh HEAD~1 HEAD x86_64-native-linuxapp-gcc
518 # Check between two tags:
519 ./devtools/validate-abi.sh v2.0.0 v2.1.0 x86_64-native-linuxapp-gcc
521 # Check between git master and local topic-branch "vhost-hacking":
522 ./devtools/validate-abi.sh master vhost-hacking x86_64-native-linuxapp-gcc
524 After the validation script completes (it can take a while since it need to
525 compile both tags) it will create compatibility reports in the
526 ``./compat_report`` directory. Listed incompatibilities can be found as
529 grep -lr Incompatible compat_reports/