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
25 An ABI (Application Binary Interface) is the set of runtime interfaces exposed
26 by a library. It is similar to an API (Application Programming Interface) but
27 is the result of compilation. It is also effectively cloned when applications
28 link to dynamic libraries. That is to say when an application is compiled to
29 link against dynamic libraries, it is assumed that the ABI remains constant
30 between the time the application is compiled/linked, and the time that it runs.
31 Therefore, in the case of dynamic linking, it is critical that an ABI is
32 preserved, or (when modified), done in such a way that the application is unable
33 to behave improperly or in an unexpected fashion.
38 ABI versions are set at the time of major release labeling, and the ABI may
39 change multiple times, without warning, between the last release label and the
40 HEAD label of the git tree.
42 ABI versions, once released, are available until such time as their
43 deprecation has been noted in the Release Notes for at least one major release
44 cycle. For example consider the case where the ABI for DPDK 2.0 has been
45 shipped and then a decision is made to modify it during the development of
46 DPDK 2.1. The decision will be recorded in the Release Notes for the DPDK 2.1
47 release and the modification will be made available in the DPDK 2.2 release.
49 ABI versions may be deprecated in whole or in part as needed by a given
52 Some ABI changes may be too significant to reasonably maintain multiple
53 versions. In those cases ABI's may be updated without backward compatibility
54 being provided. The requirements for doing so are:
56 #. At least 3 acknowledgments of the need to do so must be made on the
57 dpdk.org mailing list.
59 #. The changes (including an alternative map file) must be gated with
60 the ``RTE_NEXT_ABI`` option, and provided with a deprecation notice at the
62 It will become the default ABI in the next release.
64 #. A full deprecation cycle, as explained above, must be made to offer
65 downstream consumers sufficient warning of the change.
67 #. At the beginning of the next release cycle, every ``RTE_NEXT_ABI``
68 conditions will be removed, the ``LIBABIVER`` variable in the makefile(s)
69 where the ABI is changed will be incremented, and the map files will
72 Note that the above process for ABI deprecation should not be undertaken
73 lightly. ABI stability is extremely important for downstream consumers of the
74 DPDK, especially when distributed in shared object form. Every effort should
75 be made to preserve the ABI whenever possible. The ABI should only be changed
76 for significant reasons, such as performance enhancements. ABI breakage due to
77 changes such as reorganizing public structure fields for aesthetic or
78 readability purposes should be avoided.
80 Examples of Deprecation Notices
81 -------------------------------
83 The following are some examples of ABI deprecation notices which would be
84 added to the Release Notes:
86 * The Macro ``#RTE_FOO`` is deprecated and will be removed with version 2.0,
87 to be replaced with the inline function ``rte_foo()``.
89 * The function ``rte_mbuf_grok()`` has been updated to include a new parameter
90 in version 2.0. Backwards compatibility will be maintained for this function
91 until the release of version 2.1
93 * The members of ``struct rte_foo`` have been reorganized in release 2.0 for
94 performance reasons. Existing binary applications will have backwards
95 compatibility in release 2.0, while newly built binaries will need to
96 reference the new structure variant ``struct rte_foo2``. Compatibility will
97 be removed in release 2.2, and all applications will require updating and
98 rebuilding to the new structure at that time, which will be renamed to the
99 original ``struct rte_foo``.
101 * Significant ABI changes are planned for the ``librte_dostuff`` library. The
102 upcoming release 2.0 will not contain these changes, but release 2.1 will,
103 and no backwards compatibility is planned due to the extensive nature of
104 these changes. Binaries using this library built prior to version 2.1 will
105 require updating and recompilation.
110 When a symbol is exported from a library to provide an API, it also provides a
111 calling convention (ABI) that is embodied in its name, return type and
112 arguments. Occasionally that function may need to change to accommodate new
113 functionality or behavior. When that occurs, it is desirable to allow for
114 backward compatibility for a time with older binaries that are dynamically
117 To support backward compatibility the ``lib/librte_compat/rte_compat.h``
118 header file provides macros to use when updating exported functions. These
119 macros are used in conjunction with the ``rte_<library>_version.map`` file for
120 a given library to allow multiple versions of a symbol to exist in a shared
121 library so that older binaries need not be immediately recompiled.
123 The macros exported are:
125 * ``VERSION_SYMBOL(b, e, n)``: Creates a symbol version table entry binding
126 versioned symbol ``b@DPDK_n`` to the internal function ``b_e``.
128 * ``BIND_DEFAULT_SYMBOL(b, e, n)``: Creates a symbol version entry instructing
129 the linker to bind references to symbol ``b`` to the internal symbol
132 * ``MAP_STATIC_SYMBOL(f, p)``: Declare the prototype ``f``, and map it to the
133 fully qualified function ``p``, so that if a symbol becomes versioned, it
134 can still be mapped back to the public symbol name.
136 Examples of ABI Macro use
137 -------------------------
139 Updating a public API
140 ~~~~~~~~~~~~~~~~~~~~~
142 Assume we have a function as follows
147 * Create an acl context object for apps to
151 rte_acl_create(const struct rte_acl_param *param)
157 Assume that struct rte_acl_ctx is a private structure, and that a developer
158 wishes to enhance the acl api so that a debugging flag can be enabled on a
159 per-context basis. This requires an addition to the structure (which, being
160 private, is safe), but it also requires modifying the code as follows
165 * Create an acl context object for apps to
169 rte_acl_create(const struct rte_acl_param *param, int debug)
175 Note also that, being a public function, the header file prototype must also be
176 changed, as must all the call sites, to reflect the new ABI footprint. We will
177 maintain previous ABI versions that are accessible only to previously compiled
180 The addition of a parameter to the function is ABI breaking as the function is
181 public, and existing application may use it in its current form. However, the
182 compatibility macros in DPDK allow a developer to use symbol versioning so that
183 multiple functions can be mapped to the same public symbol based on when an
184 application was linked to it. To see how this is done, we start with the
185 requisite libraries version map file. Initially the version map file for the
186 acl library looks like this
196 rte_acl_classify_alg;
197 rte_acl_classify_scalar;
200 rte_acl_find_existing;
202 rte_acl_ipv4vlan_add_rules;
203 rte_acl_ipv4vlan_build;
207 rte_acl_set_ctx_classify;
212 This file needs to be modified as follows
222 rte_acl_classify_alg;
223 rte_acl_classify_scalar;
226 rte_acl_find_existing;
228 rte_acl_ipv4vlan_add_rules;
229 rte_acl_ipv4vlan_build;
233 rte_acl_set_ctx_classify;
244 The addition of the new block tells the linker that a new version node is
245 available (DPDK_2.1), which contains the symbol rte_acl_create, and inherits the
246 symbols from the DPDK_2.0 node. This list is directly translated into a list of
247 exported symbols when DPDK is compiled as a shared library
249 Next, we need to specify in the code which function map to the rte_acl_create
250 symbol at which versions. First, at the site of the initial symbol definition,
251 we need to update the function so that it is uniquely named, and not in conflict
252 with the public symbol name
257 -rte_acl_create(const struct rte_acl_param *param)
258 +rte_acl_create_v20(const struct rte_acl_param *param)
261 struct rte_acl_ctx *ctx;
264 Note that the base name of the symbol was kept intact, as this is conducive to
265 the macros used for versioning symbols. That is our next step, mapping this new
266 symbol name to the initial symbol name at version node 2.0. Immediately after
267 the function, we add this line of code
271 VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
273 Remembering to also add the rte_compat.h header to the requisite c file where
274 these changes are being made. The above macro instructs the linker to create a
275 new symbol ``rte_acl_create@DPDK_2.0``, which matches the symbol created in older
276 builds, but now points to the above newly named function. We have now mapped
277 the original rte_acl_create symbol to the original function (but with a new
280 Next, we need to create the 2.1 version of the symbol. We create a new function
281 name, with a different suffix, and implement it appropriately
286 rte_acl_create_v21(const struct rte_acl_param *param, int debug);
288 struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
295 This code serves as our new API call. Its the same as our old call, but adds
296 the new parameter in place. Next we need to map this function to the symbol
297 ``rte_acl_create@DPDK_2.1``. To do this, we modify the public prototype of the call
298 in the header file, adding the macro there to inform all including applications,
299 that on re-link, the default rte_acl_create symbol should point to this
300 function. Note that we could do this by simply naming the function above
301 rte_acl_create, and the linker would chose the most recent version tag to apply
302 in the version script, but we can also do this in the header file
307 -rte_acl_create(const struct rte_acl_param *param);
308 +rte_acl_create(const struct rte_acl_param *param, int debug);
309 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
311 The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
312 header, to link to the rte_acl_create_v21 function and apply the DPDK_2.1
313 version node to it. This method is more explicit and flexible than just
314 re-implementing the exact symbol name, and allows for other features (such as
315 linking to the old symbol version by default, when the new ABI is to be opt-in
318 One last thing we need to do. Note that we've taken what was a public symbol,
319 and duplicated it into two uniquely and differently named symbols. We've then
320 mapped each of those back to the public symbol ``rte_acl_create`` with different
321 version tags. This only applies to dynamic linking, as static linking has no
322 notion of versioning. That leaves this code in a position of no longer having a
323 symbol simply named ``rte_acl_create`` and a static build will fail on that
326 To correct this, we can simply map a function of our choosing back to the public
327 symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro. Generally the
328 assumption is that the most recent version of the symbol is the one you want to
329 map. So, back in the C file where, immediately after ``rte_acl_create_v21`` is
334 struct rte_acl_create_v21(const struct rte_acl_param *param, int debug)
338 MAP_STATIC_SYMBOL(struct rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
340 That tells the compiler that, when building a static library, any calls to the
341 symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
343 That's it, on the next shared library rebuild, there will be two versions of
344 rte_acl_create, an old DPDK_2.0 version, used by previously built applications,
345 and a new DPDK_2.1 version, used by future built applications.
348 Deprecating part of a public API
349 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
351 Lets assume that you've done the above update, and after a few releases have
352 passed you decide you would like to retire the old version of the function.
353 After having gone through the ABI deprecation announcement process, removal is
354 easy. Start by removing the symbol from the requisite version map file:
364 rte_acl_classify_alg;
365 rte_acl_classify_scalar;
368 rte_acl_find_existing;
370 rte_acl_ipv4vlan_add_rules;
371 rte_acl_ipv4vlan_build;
375 rte_acl_set_ctx_classify;
386 Next remove the corresponding versioned export.
390 -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
393 Note that the internal function definition could also be removed, but its used
394 in our example by the newer version _v21, so we leave it in place. This is a
397 Lastly, we need to bump the LIBABIVER number for this library in the Makefile to
398 indicate to applications doing dynamic linking that this is a later, and
399 possibly incompatible library version:
406 Deprecating an entire ABI version
407 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
409 While removing a symbol from and ABI may be useful, it is often more practical
410 to remove an entire version node at once. If a version node completely
411 specifies an API, then removing part of it, typically makes it incomplete. In
412 those cases it is better to remove the entire node
414 To do this, start by modifying the version map file, such that all symbols from
415 the node to be removed are merged into the next node in the map
417 In the case of our map above, it would transform to look as follows
427 rte_acl_classify_alg;
428 rte_acl_classify_scalar;
431 rte_acl_find_existing;
433 rte_acl_ipv4vlan_add_rules;
434 rte_acl_ipv4vlan_build;
438 rte_acl_set_ctx_classify;
443 Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
444 updated to point to the new version node in any header files for all affected
449 -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
450 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
452 Lastly, any VERSION_SYMBOL macros that point to the old version node should be
453 removed, taking care to keep, where need old code in place to support newer
454 versions of the symbol.
456 Running the ABI Validator
457 -------------------------
459 The ``scripts`` directory in the DPDK source tree contains a utility program,
460 ``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
462 <http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
464 This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
465 utilities which can be installed via a package manager. For example::
467 sudo yum install abi-compliance-checker
468 sudo yum install abi-dumper
470 The syntax of the ``validate-abi.sh`` utility is::
472 ./scripts/validate-abi.sh <REV1> <REV2> <TARGET>
474 Where ``REV1`` and ``REV2`` are valid gitrevisions(7)
475 https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
476 on the local repo and target is the usual DPDK compilation target.
480 # Check between the previous and latest commit:
481 ./scripts/validate-abi.sh HEAD~1 HEAD x86_64-native-linuxapp-gcc
483 # Check between two tags:
484 ./scripts/validate-abi.sh v2.0.0 v2.1.0 x86_64-native-linuxapp-gcc
486 # Check between git master and local topic-branch "vhost-hacking":
487 ./scripts/validate-abi.sh master vhost-hacking x86_64-native-linuxapp-gcc
489 After the validation script completes (it can take a while since it need to
490 compile both tags) it will create compatibility reports in the
491 ``./compat_report`` directory. Listed incompatibilities can be found as
494 grep -lr Incompatible compat_reports/