1 .. SPDX-License-Identifier: BSD-3-Clause
2 Copyright 2018 The DPDK contributors
9 This document details the mechanics of ABI version management in DPDK.
13 What is a library's soname?
14 ---------------------------
16 System libraries usually adopt the familiar major and minor version naming
17 convention, where major versions (e.g. ``librte_eal 20.x, 21.x``) are presumed
18 to be ABI incompatible with each other and minor versions (e.g. ``librte_eal
19 20.1, 20.2``) are presumed to be ABI compatible. A library's `soname
20 <https://en.wikipedia.org/wiki/Soname>`_. is typically used to provide backward
21 compatibility information about a given library, describing the lowest common
22 denominator ABI supported by the library. The soname or logical name for the
23 library, is typically comprised of the library's name and major version e.g.
26 During an application's build process, a library's soname is noted as a runtime
27 dependency of the application. This information is then used by the `dynamic
28 linker <https://en.wikipedia.org/wiki/Dynamic_linker>`_ when resolving the
29 applications dependencies at runtime, to load a library supporting the correct
30 ABI version. The library loaded at runtime therefore, may be a minor revision
31 supporting the same major ABI version (e.g. ``librte_eal.20.2``), as the library
32 used to link the application (e.g ``librte_eal.20.0``).
34 .. _major_abi_versions:
39 An ABI version change to a given library, especially in core libraries such as
40 ``librte_mbuf``, may cause an implicit ripple effect on the ABI of it's
41 consuming libraries, causing ABI breakages. There may however be no explicit
42 reason to bump a dependent library's ABI version, as there may have been no
43 obvious change to the dependent library's API, even though the library's ABI
44 compatibility will have been broken.
46 This interdependence of DPDK libraries, means that ABI versioning of libraries
47 is more manageable at a project level, with all project libraries sharing a
48 **single ABI version**. In addition, the need to maintain a stable ABI for some
49 number of releases as described in the section :doc:`abi_policy`, means
50 that ABI version increments need to carefully planned and managed at a project
53 Major ABI versions are therefore declared typically aligned with an LTS release
54 and is then supported some number of subsequent releases, shared across all
55 libraries. This means that a single project level ABI version, reflected in all
56 individual library's soname, library filenames and associated version maps
57 persists over multiple releases.
61 $ head ./lib/librte_acl/rte_acl_version.map
66 $ head ./lib/librte_eal/rte_eal_version.map
71 When an ABI change is made between major ABI versions to a given library, a new
72 section is added to that library's version map describing the impending new ABI
73 version, as described in the section :ref:`example_abi_macro_usage`. The
74 library's soname and filename however do not change, e.g. ``libacl.so.20``, as
75 ABI compatibility with the last major ABI version continues to be preserved for
80 $ head ./lib/librte_acl/rte_acl_version.map
91 $ head ./lib/librte_eal/rte_eal_version.map
96 However when a new ABI version is declared, for example DPDK ``21``, old
97 depreciated functions may be safely removed at this point and the entire old
98 major ABI version removed, see the section :ref:`deprecating_entire_abi` on
103 $ head ./lib/librte_acl/rte_acl_version.map
108 $ head ./lib/librte_eal/rte_eal_version.map
113 At the same time, the major ABI version is changed atomically across all
114 libraries by incrementing the major version in the ABI_VERSION file. This is
115 done globally for all libraries that declare a stable ABI. For libraries marked
116 as EXPERIMENTAL, their major ABI version is always set to 0.
121 Each non-LTS release will also increment minor ABI version, to permit multiple
122 DPDK versions being installed alongside each other. Both stable and
123 experimental ABI's are versioned using the global version file that is updated
124 at the start of each release cycle, and are managed at the project level.
129 When a symbol is exported from a library to provide an API, it also provides a
130 calling convention (ABI) that is embodied in its name, return type and
131 arguments. Occasionally that function may need to change to accommodate new
132 functionality or behavior. When that occurs, it is may be required to allow for
133 backward compatibility for a time with older binaries that are dynamically
136 To support backward compatibility the ``rte_function_versioning.h``
137 header file provides macros to use when updating exported functions. These
138 macros are used in conjunction with the ``rte_<library>_version.map`` file for
139 a given library to allow multiple versions of a symbol to exist in a shared
140 library so that older binaries need not be immediately recompiled.
142 The macros exported are:
144 * ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
145 versioned symbol ``b@DPDK_n`` to the internal function ``be``.
147 * ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
148 the linker to bind references to symbol ``b`` to the internal symbol
151 * ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the
152 fully qualified function ``p``, so that if a symbol becomes versioned, it
153 can still be mapped back to the public symbol name.
155 * ``__vsym``: Annotation to be used in a declaration of the internal symbol
156 ``be`` to signal that it is being used as an implementation of a particular
157 version of symbol ``b``.
159 .. _example_abi_macro_usage:
161 Examples of ABI Macro use
162 ~~~~~~~~~~~~~~~~~~~~~~~~~
164 Updating a public API
165 _____________________
167 Assume we have a function as follows
172 * Create an acl context object for apps to
176 rte_acl_create(const struct rte_acl_param *param)
182 Assume that struct rte_acl_ctx is a private structure, and that a developer
183 wishes to enhance the acl api so that a debugging flag can be enabled on a
184 per-context basis. This requires an addition to the structure (which, being
185 private, is safe), but it also requires modifying the code as follows
190 * Create an acl context object for apps to
194 rte_acl_create(const struct rte_acl_param *param, int debug)
200 Note also that, being a public function, the header file prototype must also be
201 changed, as must all the call sites, to reflect the new ABI footprint. We will
202 maintain previous ABI versions that are accessible only to previously compiled
205 The addition of a parameter to the function is ABI breaking as the function is
206 public, and existing application may use it in its current form. However, the
207 compatibility macros in DPDK allow a developer to use symbol versioning so that
208 multiple functions can be mapped to the same public symbol based on when an
209 application was linked to it. To see how this is done, we start with the
210 requisite libraries version map file. Initially the version map file for the acl
211 library looks like this
221 rte_acl_classify_alg;
222 rte_acl_classify_scalar;
225 rte_acl_find_existing;
227 rte_acl_ipv4vlan_add_rules;
228 rte_acl_ipv4vlan_build;
232 rte_acl_set_ctx_classify;
237 This file needs to be modified as follows
247 rte_acl_classify_alg;
248 rte_acl_classify_scalar;
251 rte_acl_find_existing;
253 rte_acl_ipv4vlan_add_rules;
254 rte_acl_ipv4vlan_build;
258 rte_acl_set_ctx_classify;
269 The addition of the new block tells the linker that a new version node
270 ``DPDK_21`` is available, which contains the symbol rte_acl_create, and inherits
271 the symbols from the DPDK_20 node. This list is directly translated into a
272 list of exported symbols when DPDK is compiled as a shared library.
274 Next, we need to specify in the code which function maps to the rte_acl_create
275 symbol at which versions. First, at the site of the initial symbol definition,
276 we need to update the function so that it is uniquely named, and not in conflict
277 with the public symbol name
281 -struct rte_acl_ctx *
282 -rte_acl_create(const struct rte_acl_param *param)
283 +struct rte_acl_ctx * __vsym
284 +rte_acl_create_v20(const struct rte_acl_param *param)
287 struct rte_acl_ctx *ctx;
290 Note that the base name of the symbol was kept intact, as this is conducive to
291 the macros used for versioning symbols and we have annotated the function as
292 ``__vsym``, an implementation of a versioned symbol . That is our next step,
293 mapping this new symbol name to the initial symbol name at version node 20.
294 Immediately after the function, we add the VERSION_SYMBOL macro.
298 #include <rte_function_versioning.h>
301 VERSION_SYMBOL(rte_acl_create, _v20, 20);
303 Remembering to also add the rte_function_versioning.h header to the requisite c
304 file where these changes are being made. The macro instructs the linker to
305 create a new symbol ``rte_acl_create@DPDK_20``, which matches the symbol created
306 in older builds, but now points to the above newly named function. We have now
307 mapped the original rte_acl_create symbol to the original function (but with a
310 Please see the section :ref:`Enabling versioning macros
311 <enabling_versioning_macros>` to enable this macro in the meson/ninja build.
312 Next, we need to create the new ``v21`` version of the symbol. We create a new
313 function name, with the ``v21`` suffix, and implement it appropriately.
317 struct rte_acl_ctx * __vsym
318 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
320 struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
327 This code serves as our new API call. Its the same as our old call, but adds the
328 new parameter in place. Next we need to map this function to the new default
329 symbol ``rte_acl_create@DPDK_21``. To do this, immediately after the function,
330 we add the BIND_DEFAULT_SYMBOL macro.
334 #include <rte_function_versioning.h>
337 BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 21);
339 The macro instructs the linker to create the new default symbol
340 ``rte_acl_create@DPDK_21``, which points to the above newly named function.
342 We finally modify the prototype of the call in the public header file,
343 such that it contains both versions of the symbol and the public API.
348 rte_acl_create(const struct rte_acl_param *param);
350 struct rte_acl_ctx * __vsym
351 rte_acl_create_v20(const struct rte_acl_param *param);
353 struct rte_acl_ctx * __vsym
354 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
357 And that's it, on the next shared library rebuild, there will be two versions of
358 rte_acl_create, an old DPDK_20 version, used by previously built applications,
359 and a new DPDK_21 version, used by future built applications.
363 **Before you leave**, please take care reviewing the sections on
364 :ref:`mapping static symbols <mapping_static_symbols>`,
365 :ref:`enabling versioning macros <enabling_versioning_macros>`,
366 and :ref:`ABI deprecation <abi_deprecation>`.
369 .. _mapping_static_symbols:
371 Mapping static symbols
372 ______________________
374 Now we've taken what was a public symbol, and duplicated it into two uniquely
375 and differently named symbols. We've then mapped each of those back to the
376 public symbol ``rte_acl_create`` with different version tags. This only applies
377 to dynamic linking, as static linking has no notion of versioning. That leaves
378 this code in a position of no longer having a symbol simply named
379 ``rte_acl_create`` and a static build will fail on that missing symbol.
381 To correct this, we can simply map a function of our choosing back to the public
382 symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro. Generally the
383 assumption is that the most recent version of the symbol is the one you want to
384 map. So, back in the C file where, immediately after ``rte_acl_create_v21`` is
390 struct rte_acl_ctx * __vsym
391 rte_acl_create_v21(const struct rte_acl_param *param, int debug)
395 MAP_STATIC_SYMBOL(struct rte_acl_ctx *rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
397 That tells the compiler that, when building a static library, any calls to the
398 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
401 .. _enabling_versioning_macros:
403 Enabling versioning macros
404 __________________________
406 Finally, we need to indicate to the :doc:`meson/ninja build system
407 <../prog_guide/build-sdk-meson>` to enable versioning macros when building the
408 library or driver. In the libraries or driver where we have added symbol
409 versioning, in the ``meson.build`` file we add the following
413 use_function_versioning = true
415 at the start of the head of the file. This will indicate to the tool-chain to
416 enable the function version macros when building. There is no corresponding
417 directive required for the ``make`` build system.
421 Deprecating part of a public API
422 ________________________________
424 Lets assume that you've done the above updates, and in preparation for the next
425 major ABI version you decide you would like to retire the old version of the
426 function. After having gone through the ABI deprecation announcement process,
427 removal is easy. Start by removing the symbol from the requisite version map
438 rte_acl_classify_alg;
439 rte_acl_classify_scalar;
442 rte_acl_find_existing;
444 rte_acl_ipv4vlan_add_rules;
445 rte_acl_ipv4vlan_build;
449 rte_acl_set_ctx_classify;
460 Next remove the corresponding versioned export.
464 -VERSION_SYMBOL(rte_acl_create, _v20, 20);
467 Note that the internal function definition could also be removed, but its used
468 in our example by the newer version ``v21``, so we leave it in place and declare
469 it as static. This is a coding style choice.
471 .. _deprecating_entire_abi:
473 Deprecating an entire ABI version
474 _________________________________
476 While removing a symbol from an ABI may be useful, it is more practical to
477 remove an entire version node at once, as is typically done at the declaration
478 of a major ABI version. If a version node completely specifies an API, then
479 removing part of it, typically makes it incomplete. In those cases it is better
480 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, 20);
518 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 21);
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.
525 Running the ABI Validator
526 -------------------------
528 The ``devtools`` directory in the DPDK source tree contains a utility program,
529 ``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
531 <http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
533 This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
534 utilities which can be installed via a package manager. For example::
536 sudo yum install abi-compliance-checker
537 sudo yum install abi-dumper
539 The syntax of the ``validate-abi.sh`` utility is::
541 ./devtools/validate-abi.sh <REV1> <REV2>
543 Where ``REV1`` and ``REV2`` are valid gitrevisions(7)
544 https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
549 # Check between the previous and latest commit:
550 ./devtools/validate-abi.sh HEAD~1 HEAD
552 # Check on a specific compilation target:
553 ./devtools/validate-abi.sh -t x86_64-native-linux-gcc HEAD~1 HEAD
555 # Check between two tags:
556 ./devtools/validate-abi.sh v2.0.0 v2.1.0
558 # Check between git master and local topic-branch "vhost-hacking":
559 ./devtools/validate-abi.sh master vhost-hacking
561 After the validation script completes (it can take a while since it need to
562 compile both tags) it will create compatibility reports in the
563 ``./abi-check/compat_report`` directory. Listed incompatibilities can be found
566 grep -lr Incompatible abi-check/compat_reports/