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 addition of symbols is generally not problematic
17 #. The modification of symbols can generally be managed with versioning
18 #. The removal of symbols generally is an ABI break and requires bumping of the
24 An ABI (Application Binary Interface) is the set of runtime interfaces exposed
25 by a library. It is similar to an API (Application Programming Interface) but
26 is the result of compilation. It is also effectively cloned when applications
27 link to dynamic libraries. That is to say when an application is compiled to
28 link against dynamic libraries, it is assumed that the ABI remains constant
29 between the time the application is compiled/linked, and the time that it runs.
30 Therefore, in the case of dynamic linking, it is critical that an ABI is
31 preserved, or (when modified), done in such a way that the application is unable
32 to behave improperly or in an unexpected fashion.
37 ABI versions are set at the time of major release labeling, and the ABI may
38 change multiple times, without warning, between the last release label and the
39 HEAD label of the git tree.
41 ABI versions, once released, are available until such time as their
42 deprecation has been noted in the Release Notes for at least one major release
43 cycle. For example consider the case where the ABI for DPDK 2.0 has been
44 shipped and then a decision is made to modify it during the development of
45 DPDK 2.1. The decision will be recorded in the Release Notes for the DPDK 2.1
46 release and the modification will be made available in the DPDK 2.2 release.
48 ABI versions may be deprecated in whole or in part as needed by a given
51 Some ABI changes may be too significant to reasonably maintain multiple
52 versions. In those cases ABI's may be updated without backward compatibility
53 being provided. The requirements for doing so are:
55 #. At least 3 acknowledgments of the need to do so must be made on the
56 dpdk.org mailing list.
58 #. The changes (including an alternative map file) must be gated with
59 the ``RTE_NEXT_ABI`` option, and provided with a deprecation notice at the
61 It will become the default ABI in the next release.
63 #. A full deprecation cycle, as explained above, must be made to offer
64 downstream consumers sufficient warning of the change.
66 #. At the beginning of the next release cycle, every ``RTE_NEXT_ABI``
67 conditions will be removed, the ``LIBABIVER`` variable in the makefile(s)
68 where the ABI is changed will be incremented, and the map files will
71 Note that the above process for ABI deprecation should not be undertaken
72 lightly. ABI stability is extremely important for downstream consumers of the
73 DPDK, especially when distributed in shared object form. Every effort should
74 be made to preserve the ABI whenever possible. The ABI should only be changed
75 for significant reasons, such as performance enhancements. ABI breakage due to
76 changes such as reorganizing public structure fields for aesthetic or
77 readability purposes should be avoided.
79 Examples of Deprecation Notices
80 -------------------------------
82 The following are some examples of ABI deprecation notices which would be
83 added to the Release Notes:
85 * The Macro ``#RTE_FOO`` is deprecated and will be removed with version 2.0,
86 to be replaced with the inline function ``rte_foo()``.
88 * The function ``rte_mbuf_grok()`` has been updated to include a new parameter
89 in version 2.0. Backwards compatibility will be maintained for this function
90 until the release of version 2.1
92 * The members of ``struct rte_foo`` have been reorganized in release 2.0 for
93 performance reasons. Existing binary applications will have backwards
94 compatibility in release 2.0, while newly built binaries will need to
95 reference the new structure variant ``struct rte_foo2``. Compatibility will
96 be removed in release 2.2, and all applications will require updating and
97 rebuilding to the new structure at that time, which will be renamed to the
98 original ``struct rte_foo``.
100 * Significant ABI changes are planned for the ``librte_dostuff`` library. The
101 upcoming release 2.0 will not contain these changes, but release 2.1 will,
102 and no backwards compatibility is planned due to the extensive nature of
103 these changes. Binaries using this library built prior to version 2.1 will
104 require updating and recompilation.
109 When a symbol is exported from a library to provide an API, it also provides a
110 calling convention (ABI) that is embodied in its name, return type and
111 arguments. Occasionally that function may need to change to accommodate new
112 functionality or behavior. When that occurs, it is desirable to allow for
113 backward compatibility for a time with older binaries that are dynamically
116 To support backward compatibility the ``lib/librte_compat/rte_compat.h``
117 header file provides macros to use when updating exported functions. These
118 macros are used in conjunction with the ``rte_<library>_version.map`` file for
119 a given library to allow multiple versions of a symbol to exist in a shared
120 library so that older binaries need not be immediately recompiled.
122 The macros exported are:
124 * ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
125 versioned symbol ``b@DPDK_n`` to the internal function ``b_e``.
127 * ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
128 the linker to bind references to symbol ``b`` to the internal symbol
131 * ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the
132 fully qualified function ``p``, so that if a symbol becomes versioned, it
133 can still be mapped back to the public symbol name.
135 Examples of ABI Macro use
136 -------------------------
138 Updating a public API
139 ~~~~~~~~~~~~~~~~~~~~~
141 Assume we have a function as follows
146 * Create an acl context object for apps to
150 rte_acl_create(const struct rte_acl_param *param)
156 Assume that struct rte_acl_ctx is a private structure, and that a developer
157 wishes to enhance the acl api so that a debugging flag can be enabled on a
158 per-context basis. This requires an addition to the structure (which, being
159 private, is safe), but it also requires modifying the code as follows
164 * Create an acl context object for apps to
168 rte_acl_create(const struct rte_acl_param *param, int debug)
174 Note also that, being a public function, the header file prototype must also be
175 changed, as must all the call sites, to reflect the new ABI footprint. We will
176 maintain previous ABI versions that are accessible only to previously compiled
179 The addition of a parameter to the function is ABI breaking as the function is
180 public, and existing application may use it in its current form. However, the
181 compatibility macros in DPDK allow a developer to use symbol versioning so that
182 multiple functions can be mapped to the same public symbol based on when an
183 application was linked to it. To see how this is done, we start with the
184 requisite libraries version map file. Initially the version map file for the
185 acl library looks like this
195 rte_acl_classify_alg;
196 rte_acl_classify_scalar;
199 rte_acl_find_existing;
201 rte_acl_ipv4vlan_add_rules;
202 rte_acl_ipv4vlan_build;
206 rte_acl_set_ctx_classify;
211 This file needs to be modified as follows
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;
243 The addition of the new block tells the linker that a new version node is
244 available (DPDK_2.1), which contains the symbol rte_acl_create, and inherits the
245 symbols from the DPDK_2.0 node. This list is directly translated into a list of
246 exported symbols when DPDK is compiled as a shared library
248 Next, we need to specify in the code which function map to the rte_acl_create
249 symbol at which versions. First, at the site of the initial symbol definition,
250 we need to update the function so that it is uniquely named, and not in conflict
251 with the public symbol name
256 -rte_acl_create(const struct rte_acl_param *param)
257 +rte_acl_create_v20(const struct rte_acl_param *param)
260 struct rte_acl_ctx *ctx;
263 Note that the base name of the symbol was kept intact, as this is condusive to
264 the macros used for versioning symbols. That is our next step, mapping this new
265 symbol name to the initial symbol name at version node 2.0. Immediately after
266 the function, we add this line of code
270 VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
272 Remembering to also add the rte_compat.h header to the requisite c file where
273 these changes are being made. The above macro instructs the linker to create a
274 new symbol ``rte_acl_create@DPDK_2.0``, which matches the symbol created in older
275 builds, but now points to the above newly named function. We have now mapped
276 the original rte_acl_create symbol to the original function (but with a new
279 Next, we need to create the 2.1 version of the symbol. We create a new function
280 name, with a different suffix, and implement it appropriately
285 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
287 struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
294 This code serves as our new API call. Its the same as our old call, but adds
295 the new parameter in place. Next we need to map this function to the symbol
296 ``rte_acl_create@DPDK_2.1``. To do this, we modify the public prototype of the call
297 in the header file, adding the macro there to inform all including applications,
298 that on re-link, the default rte_acl_create symbol should point to this
299 function. Note that we could do this by simply naming the function above
300 rte_acl_create, and the linker would chose the most recent version tag to apply
301 in the version script, but we can also do this in the header file
306 -rte_acl_create(const struct rte_acl_param *param);
307 +rte_acl_create(const struct rte_acl_param *param, int debug);
308 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
310 The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
311 header, to link to the rte_acl_create_v21 function and apply the DPDK_2.1
312 version node to it. This method is more explicit and flexible than just
313 re-implementing the exact symbol name, and allows for other features (such as
314 linking to the old symbol version by default, when the new ABI is to be opt-in
317 One last thing we need to do. Note that we've taken what was a public symbol,
318 and duplicated it into two uniquely and differently named symbols. We've then
319 mapped each of those back to the public symbol ``rte_acl_create`` with different
320 version tags. This only applies to dynamic linking, as static linking has no
321 notion of versioning. That leaves this code in a position of no longer having a
322 symbol simply named ``rte_acl_create`` and a static build will fail on that
325 To correct this, we can simply map a function of our choosing back to the public
326 symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro. Generally the
327 assumption is that the most recent version of the symbol is the one you want to
328 map. So, back in the C file where, immediately after ``rte_acl_create_v21`` is
333 struct rte_acl_create_v21(const struct rte_acl_param *param, int debug)
337 MAP_STATIC_SYMBOL(struct rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
339 That tells the compiler that, when building a static library, any calls to the
340 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
342 That's it, on the next shared library rebuild, there will be two versions of
343 rte_acl_create, an old DPDK_2.0 version, used by previously built applications,
344 and a new DPDK_2.1 version, used by future built applications.
347 Deprecating part of a public API
348 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
350 Lets assume that you've done the above update, and after a few releases have
351 passed you decide you would like to retire the old version of the function.
352 After having gone through the ABI deprecation announcement process, removal is
353 easy. Start by removing the symbol from the requisite version map file:
363 rte_acl_classify_alg;
364 rte_acl_classify_scalar;
367 rte_acl_find_existing;
369 rte_acl_ipv4vlan_add_rules;
370 rte_acl_ipv4vlan_build;
374 rte_acl_set_ctx_classify;
385 Next remove the corresponding versioned export
388 -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
391 Note that the internal function definition could also be removed, but its used
392 in our example by the newer version _v21, so we leave it in place. This is a
395 Lastly, we need to bump the LIBABIVER number for this library in the Makefile to
396 indicate to applications doing dynamic linking that this is a later, and
397 possibly incompatible library version:
404 Deprecating an entire ABI version
405 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
407 While removing a symbol from and ABI may be useful, it is often more practical
408 to remove an entire version node at once. If a version node completely
409 specifies an API, then removing part of it, typically makes it incomplete. In
410 those cases it is better to remove the entire node
412 To do this, start by modifying the version map file, such that all symbols from
413 the node to be removed are merged into the next node in the map
415 In the case of our map above, it would transform to look as follows
425 rte_acl_classify_alg;
426 rte_acl_classify_scalar;
429 rte_acl_find_existing;
431 rte_acl_ipv4vlan_add_rules;
432 rte_acl_ipv4vlan_build;
436 rte_acl_set_ctx_classify;
441 Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
442 updated to point to the new version node in any header files for all affected
447 -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
448 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
450 Lastly, any VERSION_SYMBOL macros that point to the old version node should be
451 removed, taking care to keep, where need old code in place to support newer
452 versions of the symbol.
454 Running the ABI Validator
455 -------------------------
457 The ``scripts`` directory in the DPDK source tree contains a utility program,
458 ``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
460 <http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
462 This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
463 utilities which can be installed via a package manager. For example::
465 sudo yum install abi-compliance-checker
466 sudo yum install abi-dumper
468 The syntax of the ``validate-abi.sh`` utility is::
470 ./scripts/validate-abi.sh <TAG1> <TAG2> <TARGET>
472 Where ``TAG1`` and ``TAG2`` are valid git tags on the local repo and target is
473 the usual DPDK compilation target.
475 For example to test the current committed HEAD against a previous release tag
476 we could add a temporary tag and run the utility as follows::
479 ./scripts/validate-abi.sh v2.0.0 MY_TEMP_TAG x86_64-native-linuxapp-gcc
481 After the validation script completes (it can take a while since it need to
482 compile both tags) it will create compatibility reports in the
483 ``./compat_report`` directory. Listed incompatibilities can be found as
486 grep -lr Incompatible compat_reports/