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 individual library's soname, e.g.
115 ``libacl.so.21``. This is done by bumping the LIBABIVER number in the libraries
116 Makefile to indicate to dynamic linking applications that this is a later, and
117 possibly incompatible library version:
128 When a symbol is exported from a library to provide an API, it also provides a
129 calling convention (ABI) that is embodied in its name, return type and
130 arguments. Occasionally that function may need to change to accommodate new
131 functionality or behavior. When that occurs, it is may be required to allow for
132 backward compatibility for a time with older binaries that are dynamically
135 To support backward compatibility the ``rte_function_versioning.h``
136 header file provides macros to use when updating exported functions. These
137 macros are used in conjunction with the ``rte_<library>_version.map`` file for
138 a given library to allow multiple versions of a symbol to exist in a shared
139 library so that older binaries need not be immediately recompiled.
141 The macros exported are:
143 * ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
144 versioned symbol ``b@DPDK_n`` to the internal function ``be``.
146 * ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
147 the linker to bind references to symbol ``b`` to the internal symbol
150 * ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the
151 fully qualified function ``p``, so that if a symbol becomes versioned, it
152 can still be mapped back to the public symbol name.
154 * ``__vsym``: Annotation to be used in a declaration of the internal symbol
155 ``be`` to signal that it is being used as an implementation of a particular
156 version of symbol ``b``.
158 .. _example_abi_macro_usage:
160 Examples of ABI Macro use
161 ~~~~~~~~~~~~~~~~~~~~~~~~~
163 Updating a public API
164 _____________________
166 Assume we have a function as follows
171 * Create an acl context object for apps to
175 rte_acl_create(const struct rte_acl_param *param)
181 Assume that struct rte_acl_ctx is a private structure, and that a developer
182 wishes to enhance the acl api so that a debugging flag can be enabled on a
183 per-context basis. This requires an addition to the structure (which, being
184 private, is safe), but it also requires modifying the code as follows
189 * Create an acl context object for apps to
193 rte_acl_create(const struct rte_acl_param *param, int debug)
199 Note also that, being a public function, the header file prototype must also be
200 changed, as must all the call sites, to reflect the new ABI footprint. We will
201 maintain previous ABI versions that are accessible only to previously compiled
204 The addition of a parameter to the function is ABI breaking as the function is
205 public, and existing application may use it in its current form. However, the
206 compatibility macros in DPDK allow a developer to use symbol versioning so that
207 multiple functions can be mapped to the same public symbol based on when an
208 application was linked to it. To see how this is done, we start with the
209 requisite libraries version map file. Initially the version map file for the acl
210 library looks like this
220 rte_acl_classify_alg;
221 rte_acl_classify_scalar;
224 rte_acl_find_existing;
226 rte_acl_ipv4vlan_add_rules;
227 rte_acl_ipv4vlan_build;
231 rte_acl_set_ctx_classify;
236 This file needs to be modified as follows
246 rte_acl_classify_alg;
247 rte_acl_classify_scalar;
250 rte_acl_find_existing;
252 rte_acl_ipv4vlan_add_rules;
253 rte_acl_ipv4vlan_build;
257 rte_acl_set_ctx_classify;
268 The addition of the new block tells the linker that a new version node is
269 available (DPDK_21), which contains the symbol rte_acl_create, and inherits
270 the symbols from the DPDK_20 node. This list is directly translated into a
271 list of exported symbols when DPDK is compiled as a shared library
273 Next, we need to specify in the code which function map to the rte_acl_create
274 symbol at which versions. First, at the site of the initial symbol definition,
275 we need to update the function so that it is uniquely named, and not in conflict
276 with the public symbol name
280 -struct rte_acl_ctx *
281 -rte_acl_create(const struct rte_acl_param *param)
282 +struct rte_acl_ctx * __vsym
283 +rte_acl_create_v20(const struct rte_acl_param *param)
286 struct rte_acl_ctx *ctx;
289 Note that the base name of the symbol was kept intact, as this is conducive to
290 the macros used for versioning symbols and we have annotated the function as an
291 implementation of versioned symbol. That is our next step, mapping this new
292 symbol name to the initial symbol name at version node 20. Immediately after
293 the function, we add this line of code
297 VERSION_SYMBOL(rte_acl_create, _v20, 20);
299 Remembering to also add the rte_function_versioning.h header to the requisite c
300 file where these changes are being made. The above macro instructs the linker to
301 create a new symbol ``rte_acl_create@DPDK_20``, which matches the symbol created
302 in older builds, but now points to the above newly named function. We have now
303 mapped the original rte_acl_create symbol to the original function (but with a
306 Next, we need to create the 21 version of the symbol. We create a new function
307 name, with a different suffix, and implement it appropriately
311 struct rte_acl_ctx * __vsym
312 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
314 struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
321 This code serves as our new API call. Its the same as our old call, but adds the
322 new parameter in place. Next we need to map this function to the symbol
323 ``rte_acl_create@DPDK_21``. To do this, we modify the public prototype of the
324 call in the header file, adding the macro there to inform all including
325 applications, that on re-link, the default rte_acl_create symbol should point to
326 this function. Note that we could do this by simply naming the function above
327 rte_acl_create, and the linker would chose the most recent version tag to apply
328 in the version script, but we can also do this in the header file
333 -rte_acl_create(const struct rte_acl_param *param);
334 +rte_acl_create_v21(const struct rte_acl_param *param, int debug);
335 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 21);
337 The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
338 header, to link to the rte_acl_create_v21 function and apply the DPDK_21
339 version node to it. This method is more explicit and flexible than just
340 re-implementing the exact symbol name, and allows for other features (such as
341 linking to the old symbol version by default, when the new ABI is to be opt-in
344 One last thing we need to do. Note that we've taken what was a public symbol,
345 and duplicated it into two uniquely and differently named symbols. We've then
346 mapped each of those back to the public symbol ``rte_acl_create`` with different
347 version tags. This only applies to dynamic linking, as static linking has no
348 notion of versioning. That leaves this code in a position of no longer having a
349 symbol simply named ``rte_acl_create`` and a static build will fail on that
352 To correct this, we can simply map a function of our choosing back to the public
353 symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro. Generally the
354 assumption is that the most recent version of the symbol is the one you want to
355 map. So, back in the C file where, immediately after ``rte_acl_create_v21`` is
361 struct rte_acl_ctx * __vsym
362 rte_acl_create_v21(const struct rte_acl_param *param, int debug)
366 MAP_STATIC_SYMBOL(struct rte_acl_ctx *rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
368 That tells the compiler that, when building a static library, any calls to the
369 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
371 That's it, on the next shared library rebuild, there will be two versions of
372 rte_acl_create, an old DPDK_20 version, used by previously built applications,
373 and a new DPDK_21 version, used by future built applications.
376 Deprecating part of a public API
377 ________________________________
379 Lets assume that you've done the above update, and in preparation for the next
380 major ABI version you decide you would like to retire the old version of the
381 function. After having gone through the ABI deprecation announcement process,
382 removal is easy. Start by removing the symbol from the requisite version map
393 rte_acl_classify_alg;
394 rte_acl_classify_scalar;
397 rte_acl_find_existing;
399 rte_acl_ipv4vlan_add_rules;
400 rte_acl_ipv4vlan_build;
404 rte_acl_set_ctx_classify;
415 Next remove the corresponding versioned export.
419 -VERSION_SYMBOL(rte_acl_create, _v20, 20);
422 Note that the internal function definition could also be removed, but its used
423 in our example by the newer version v21, so we leave it in place and declare it
424 as static. This is a coding style choice.
426 .. _deprecating_entire_abi:
428 Deprecating an entire ABI version
429 _________________________________
431 While removing a symbol from an ABI may be useful, it is more practical to
432 remove an entire version node at once, as is typically done at the declaration
433 of a major ABI version. If a version node completely specifies an API, then
434 removing part of it, typically makes it incomplete. In those cases it is better
435 to remove the entire node.
437 To do this, start by modifying the version map file, such that all symbols from
438 the node to be removed are merged into the next node in the map.
440 In the case of our map above, it would transform to look as follows
450 rte_acl_classify_alg;
451 rte_acl_classify_scalar;
454 rte_acl_find_existing;
456 rte_acl_ipv4vlan_add_rules;
457 rte_acl_ipv4vlan_build;
461 rte_acl_set_ctx_classify;
466 Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
467 updated to point to the new version node in any header files for all affected
472 -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 20);
473 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 21);
475 Lastly, any VERSION_SYMBOL macros that point to the old version node should be
476 removed, taking care to keep, where need old code in place to support newer
477 versions of the symbol.
480 Running the ABI Validator
481 -------------------------
483 The ``devtools`` directory in the DPDK source tree contains a utility program,
484 ``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
486 <http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
488 This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
489 utilities which can be installed via a package manager. For example::
491 sudo yum install abi-compliance-checker
492 sudo yum install abi-dumper
494 The syntax of the ``validate-abi.sh`` utility is::
496 ./devtools/validate-abi.sh <REV1> <REV2>
498 Where ``REV1`` and ``REV2`` are valid gitrevisions(7)
499 https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
504 # Check between the previous and latest commit:
505 ./devtools/validate-abi.sh HEAD~1 HEAD
507 # Check on a specific compilation target:
508 ./devtools/validate-abi.sh -t x86_64-native-linux-gcc HEAD~1 HEAD
510 # Check between two tags:
511 ./devtools/validate-abi.sh v2.0.0 v2.1.0
513 # Check between git master and local topic-branch "vhost-hacking":
514 ./devtools/validate-abi.sh master vhost-hacking
516 After the validation script completes (it can take a while since it need to
517 compile both tags) it will create compatibility reports in the
518 ``./abi-check/compat_report`` directory. Listed incompatibilities can be found
521 grep -lr Incompatible abi-check/compat_reports/