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 #. A full deprecation cycle, as explained above, must be made to offer
59 downstream consumers sufficient warning of the change.
61 #. The ``LIBABIVER`` variable in the makefile(s) where the ABI changes are
62 incorporated must be incremented in parallel with the ABI changes
65 Note that the above process for ABI deprecation should not be undertaken
66 lightly. ABI stability is extremely important for downstream consumers of the
67 DPDK, especially when distributed in shared object form. Every effort should
68 be made to preserve the ABI whenever possible. The ABI should only be changed
69 for significant reasons, such as performance enhancements. ABI breakage due to
70 changes such as reorganizing public structure fields for aesthetic or
71 readability purposes should be avoided.
73 Examples of Deprecation Notices
74 -------------------------------
76 The following are some examples of ABI deprecation notices which would be
77 added to the Release Notes:
79 * The Macro ``#RTE_FOO`` is deprecated and will be removed with version 2.0,
80 to be replaced with the inline function ``rte_foo()``.
82 * The function ``rte_mbuf_grok()`` has been updated to include a new parameter
83 in version 2.0. Backwards compatibility will be maintained for this function
84 until the release of version 2.1
86 * The members of ``struct rte_foo`` have been reorganized in release 2.0 for
87 performance reasons. Existing binary applications will have backwards
88 compatibility in release 2.0, while newly built binaries will need to
89 reference the new structure variant ``struct rte_foo2``. Compatibility will
90 be removed in release 2.2, and all applications will require updating and
91 rebuilding to the new structure at that time, which will be renamed to the
92 original ``struct rte_foo``.
94 * Significant ABI changes are planned for the ``librte_dostuff`` library. The
95 upcoming release 2.0 will not contain these changes, but release 2.1 will,
96 and no backwards compatibility is planned due to the extensive nature of
97 these changes. Binaries using this library built prior to version 2.1 will
98 require updating and recompilation.
103 When a symbol is exported from a library to provide an API, it also provides a
104 calling convention (ABI) that is embodied in its name, return type and
105 arguments. Occasionally that function may need to change to accommodate new
106 functionality or behavior. When that occurs, it is desirable to allow for
107 backward compatibility for a time with older binaries that are dynamically
110 To support backward compatibility the ``lib/librte_compat/rte_compat.h``
111 header file provides macros to use when updating exported functions. These
112 macros are used in conjunction with the ``rte_<library>_version.map`` file for
113 a given library to allow multiple versions of a symbol to exist in a shared
114 library so that older binaries need not be immediately recompiled.
116 The macros exported are:
118 * ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
119 versioned symbol ``b@DPDK_n`` to the internal function ``b_e``.
121 * ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
122 the linker to bind references to symbol ``b`` to the internal symbol
125 * ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the fully
126 qualified function ``p``, so that if a symbol becomes versioned, it can still be
127 mapped back to the public symbol name.
129 Examples of ABI Macro use
130 -------------------------
132 Updating a public API
133 ~~~~~~~~~~~~~~~~~~~~~
135 Assume we have a function as follows
140 * Create an acl context object for apps to
144 rte_acl_create(const struct rte_acl_param *param)
150 Assume that struct rte_acl_ctx is a private structure, and that a developer
151 wishes to enhance the acl api so that a debugging flag can be enabled on a
152 per-context basis. This requires an addition to the structure (which, being
153 private, is safe), but it also requires modifying the code as follows
158 * Create an acl context object for apps to
162 rte_acl_create(const struct rte_acl_param *param, int debug)
168 Note also that, being a public function, the header file prototype must also be
169 changed, as must all the call sites, to reflect the new ABI footprint. We will
170 maintain previous ABI versions that are accessible only to previously compiled
173 The addition of a parameter to the function is ABI breaking as the function is
174 public, and existing application may use it in its current form. However, the
175 compatibility macros in DPDK allow a developer to use symbol versioning so that
176 multiple functions can be mapped to the same public symbol based on when an
177 application was linked to it. To see how this is done, we start with the
178 requisite libraries version map file. Initially the version map file for the
179 acl library looks like this
189 rte_acl_classify_alg;
190 rte_acl_classify_scalar;
193 rte_acl_find_existing;
195 rte_acl_ipv4vlan_add_rules;
196 rte_acl_ipv4vlan_build;
200 rte_acl_set_ctx_classify;
205 This file needs to be modified as follows
215 rte_acl_classify_alg;
216 rte_acl_classify_scalar;
219 rte_acl_find_existing;
221 rte_acl_ipv4vlan_add_rules;
222 rte_acl_ipv4vlan_build;
226 rte_acl_set_ctx_classify;
237 The addition of the new block tells the linker that a new version node is
238 available (DPDK_2.1), which contains the symbol rte_acl_create, and inherits the
239 symbols from the DPDK_2.0 node. This list is directly translated into a list of
240 exported symbols when DPDK is compiled as a shared library
242 Next, we need to specify in the code which function map to the rte_acl_create
243 symbol at which versions. First, at the site of the initial symbol definition,
244 we need to update the function so that it is uniquely named, and not in conflict
245 with the public symbol name
250 -rte_acl_create(const struct rte_acl_param *param)
251 +rte_acl_create_v20(const struct rte_acl_param *param)
254 struct rte_acl_ctx *ctx;
257 Note that the base name of the symbol was kept intact, as this is condusive to
258 the macros used for versioning symbols. That is our next step, mapping this new
259 symbol name to the initial symbol name at version node 2.0. Immediately after
260 the function, we add this line of code
264 VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
266 Remembering to also add the rte_compat.h header to the requisite c file where
267 these changes are being made. The above macro instructs the linker to create a
268 new symbol ``rte_acl_create@DPDK_2.0``, which matches the symbol created in older
269 builds, but now points to the above newly named function. We have now mapped
270 the original rte_acl_create symbol to the original function (but with a new
273 Next, we need to create the 2.1 version of the symbol. We create a new function
274 name, with a different suffix, and implement it appropriately
279 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
281 struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
288 This code serves as our new API call. Its the same as our old call, but adds
289 the new parameter in place. Next we need to map this function to the symbol
290 ``rte_acl_create@DPDK_2.1``. To do this, we modify the public prototype of the call
291 in the header file, adding the macro there to inform all including applications,
292 that on re-link, the default rte_acl_create symbol should point to this
293 function. Note that we could do this by simply naming the function above
294 rte_acl_create, and the linker would chose the most recent version tag to apply
295 in the version script, but we can also do this in the header file
300 -rte_acl_create(const struct rte_acl_param *param);
301 +rte_acl_create(const struct rte_acl_param *param, int debug);
302 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
304 The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
305 header, to link to the rte_acl_create_v21 function and apply the DPDK_2.1
306 version node to it. This method is more explicit and flexible than just
307 re-implementing the exact symbol name, and allows for other features (such as
308 linking to the old symbol version by default, when the new ABI is to be opt-in
311 One last thing we need to do. Note that we've taken what was a public symbol,
312 and duplicated it into two uniquely and differently named symbols. We've then
313 mapped each of those back to the public symbol ``rte_acl_create`` with different
314 version tags. This only applies to dynamic linking, as static linking has no
315 notion of versioning. That leaves this code in a position of no longer having a
316 symbol simply named ``rte_acl_create`` and a static build will fail on that
319 To correct this, we can simply map a function of our choosing back to the public
320 symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro. Generally the
321 assumption is that the most recent version of the symbol is the one you want to
322 map. So, back in the C file where, immediately after ``rte_acl_create_v21`` is
327 struct rte_acl_create_v21(const struct rte_acl_param *param, int debug)
331 MAP_STATIC_SYMBOL(struct rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
333 That tells the compiler that, when building a static library, any calls to the
334 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
336 That's it, on the next shared library rebuild, there will be two versions of
337 rte_acl_create, an old DPDK_2.0 version, used by previously built applications,
338 and a new DPDK_2.1 version, used by future built applications.
341 Deprecating part of a public API
342 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
344 Lets assume that you've done the above update, and after a few releases have
345 passed you decide you would like to retire the old version of the function.
346 After having gone through the ABI deprecation announcement process, removal is
347 easy. Start by removing the symbol from the requisite version map file:
357 rte_acl_classify_alg;
358 rte_acl_classify_scalar;
361 rte_acl_find_existing;
363 rte_acl_ipv4vlan_add_rules;
364 rte_acl_ipv4vlan_build;
368 rte_acl_set_ctx_classify;
379 Next remove the corresponding versioned export
382 -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
385 Note that the internal function definition could also be removed, but its used
386 in our example by the newer version _v21, so we leave it in place. This is a
389 Lastly, we need to bump the LIBABIVER number for this library in the Makefile to
390 indicate to applications doing dynamic linking that this is a later, and
391 possibly incompatible library version:
398 Deprecating an entire ABI version
399 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
401 While removing a symbol from and ABI may be useful, it is often more practical
402 to remove an entire version node at once. If a version node completely
403 specifies an API, then removing part of it, typically makes it incomplete. In
404 those cases it is better to remove the entire node
406 To do this, start by modifying the version map file, such that all symbols from
407 the node to be removed are merged into the next node in the map
409 In the case of our map above, it would transform to look as follows
419 rte_acl_classify_alg;
420 rte_acl_classify_scalar;
423 rte_acl_find_existing;
425 rte_acl_ipv4vlan_add_rules;
426 rte_acl_ipv4vlan_build;
430 rte_acl_set_ctx_classify;
435 Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
436 updated to point to the new version node in any header files for all affected
441 -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
442 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
444 Lastly, any VERSION_SYMBOL macros that point to the old version node should be
445 removed, taking care to keep, where need old code in place to support newer
446 versions of the symbol.
448 Running the ABI Validator
449 -------------------------
451 The ``scripts`` directory in the DPDK source tree contains a utility program,
452 ``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
454 <http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
456 This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
457 utilities which can be installed via a package manager. For example::
459 sudo yum install abi-compliance-checker
460 sudo yum install abi-dumper
462 The syntax of the ``validate-abi.sh`` utility is::
464 ./scripts/validate-abi.sh <TAG1> <TAG2> <TARGET>
466 Where ``TAG1`` and ``TAG2`` are valid git tags on the local repo and target is
467 the usual DPDK compilation target.
469 For example to test the current committed HEAD against a previous release tag
470 we could add a temporary tag and run the utility as follows::
473 ./scripts/validate-abi.sh v2.0.0 MY_TEMP_TAG x86_64-native-linuxapp-gcc
475 After the validation script completes (it can take a while since it need to
476 compile both tags) it will create compatibility reports in the
477 ``./compat_report`` directory. Listed incompatibilities can be found as
480 grep -lr Incompatible compat_reports/