versioning.rst revision 6b3e017e
1Managing ABI updates
2====================
3
4Description
5-----------
6
7This document details some methods for handling ABI management in the DPDK.
8Note this document is not exhaustive, in that C library versioning is flexible
9allowing multiple methods to achieve various goals, but it will provide the user
10with some introductory methods
11
12General Guidelines
13------------------
14
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
20   LIBABIVER macro
21
22What is an ABI
23--------------
24
25An ABI (Application Binary Interface) is the set of runtime interfaces exposed
26by a library. It is similar to an API (Application Programming Interface) but
27is the result of compilation.  It is also effectively cloned when applications
28link to dynamic libraries.  That is to say when an application is compiled to
29link against dynamic libraries, it is assumed that the ABI remains constant
30between the time the application is compiled/linked, and the time that it runs.
31Therefore, in the case of dynamic linking, it is critical that an ABI is
32preserved, or (when modified), done in such a way that the application is unable
33to behave improperly or in an unexpected fashion.
34
35The DPDK ABI policy
36-------------------
37
38ABI versions are set at the time of major release labeling, and the ABI may
39change multiple times, without warning, between the last release label and the
40HEAD label of the git tree.
41
42ABI versions, once released, are available until such time as their
43deprecation has been noted in the Release Notes for at least one major release
44cycle. For example consider the case where the ABI for DPDK 2.0 has been
45shipped and then a decision is made to modify it during the development of
46DPDK 2.1. The decision will be recorded in the Release Notes for the DPDK 2.1
47release and the modification will be made available in the DPDK 2.2 release.
48
49ABI versions may be deprecated in whole or in part as needed by a given
50update.
51
52Some ABI changes may be too significant to reasonably maintain multiple
53versions. In those cases ABI's may be updated without backward compatibility
54being provided. The requirements for doing so are:
55
56#. At least 3 acknowledgments of the need to do so must be made on the
57   dpdk.org mailing list.
58
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
61   same time.
62   It will become the default ABI in the next release.
63
64#. A full deprecation cycle, as explained above, must be made to offer
65   downstream consumers sufficient warning of the change.
66
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
70   be updated.
71
72Note that the above process for ABI deprecation should not be undertaken
73lightly. ABI stability is extremely important for downstream consumers of the
74DPDK, especially when distributed in shared object form. Every effort should
75be made to preserve the ABI whenever possible. The ABI should only be changed
76for significant reasons, such as performance enhancements. ABI breakage due to
77changes such as reorganizing public structure fields for aesthetic or
78readability purposes should be avoided.
79
80Examples of Deprecation Notices
81-------------------------------
82
83The following are some examples of ABI deprecation notices which would be
84added to the Release Notes:
85
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()``.
88
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
92
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``.
100
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.
106
107Versioning Macros
108-----------------
109
110When a symbol is exported from a library to provide an API, it also provides a
111calling convention (ABI) that is embodied in its name, return type and
112arguments. Occasionally that function may need to change to accommodate new
113functionality or behavior. When that occurs, it is desirable to allow for
114backward compatibility for a time with older binaries that are dynamically
115linked to the DPDK.
116
117To support backward compatibility the ``lib/librte_compat/rte_compat.h``
118header file provides macros to use when updating exported functions. These
119macros are used in conjunction with the ``rte_<library>_version.map`` file for
120a given library to allow multiple versions of a symbol to exist in a shared
121library so that older binaries need not be immediately recompiled.
122
123The macros exported are:
124
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``.
127
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
130  ``b_e``.
131
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.
135
136Examples of ABI Macro use
137-------------------------
138
139Updating a public API
140~~~~~~~~~~~~~~~~~~~~~
141
142Assume we have a function as follows
143
144.. code-block:: c
145
146 /*
147  * Create an acl context object for apps to
148  * manipulate
149  */
150 struct rte_acl_ctx *
151 rte_acl_create(const struct rte_acl_param *param)
152 {
153        ...
154 }
155
156
157Assume that struct rte_acl_ctx is a private structure, and that a developer
158wishes to enhance the acl api so that a debugging flag can be enabled on a
159per-context basis.  This requires an addition to the structure (which, being
160private, is safe), but it also requires modifying the code as follows
161
162.. code-block:: c
163
164 /*
165  * Create an acl context object for apps to
166  * manipulate
167  */
168 struct rte_acl_ctx *
169 rte_acl_create(const struct rte_acl_param *param, int debug)
170 {
171        ...
172 }
173
174
175Note also that, being a public function, the header file prototype must also be
176changed, as must all the call sites, to reflect the new ABI footprint.  We will
177maintain previous ABI versions that are accessible only to previously compiled
178binaries
179
180The addition of a parameter to the function is ABI breaking as the function is
181public, and existing application may use it in its current form.  However, the
182compatibility macros in DPDK allow a developer to use symbol versioning so that
183multiple functions can be mapped to the same public symbol based on when an
184application was linked to it.  To see how this is done, we start with the
185requisite libraries version map file.  Initially the version map file for the
186acl library looks like this
187
188.. code-block:: none
189
190   DPDK_2.0 {
191        global:
192
193        rte_acl_add_rules;
194        rte_acl_build;
195        rte_acl_classify;
196        rte_acl_classify_alg;
197        rte_acl_classify_scalar;
198        rte_acl_create;
199        rte_acl_dump;
200        rte_acl_find_existing;
201        rte_acl_free;
202        rte_acl_ipv4vlan_add_rules;
203        rte_acl_ipv4vlan_build;
204        rte_acl_list_dump;
205        rte_acl_reset;
206        rte_acl_reset_rules;
207        rte_acl_set_ctx_classify;
208
209        local: *;
210   };
211
212This file needs to be modified as follows
213
214.. code-block:: none
215
216   DPDK_2.0 {
217        global:
218
219        rte_acl_add_rules;
220        rte_acl_build;
221        rte_acl_classify;
222        rte_acl_classify_alg;
223        rte_acl_classify_scalar;
224        rte_acl_create;
225        rte_acl_dump;
226        rte_acl_find_existing;
227        rte_acl_free;
228        rte_acl_ipv4vlan_add_rules;
229        rte_acl_ipv4vlan_build;
230        rte_acl_list_dump;
231        rte_acl_reset;
232        rte_acl_reset_rules;
233        rte_acl_set_ctx_classify;
234
235        local: *;
236   };
237
238   DPDK_2.1 {
239        global:
240        rte_acl_create;
241
242   } DPDK_2.0;
243
244The addition of the new block tells the linker that a new version node is
245available (DPDK_2.1), which contains the symbol rte_acl_create, and inherits the
246symbols from the DPDK_2.0 node.  This list is directly translated into a list of
247exported symbols when DPDK is compiled as a shared library
248
249Next, we need to specify in the code which function map to the rte_acl_create
250symbol at which versions.  First, at the site of the initial symbol definition,
251we need to update the function so that it is uniquely named, and not in conflict
252with the public symbol name
253
254.. code-block:: c
255
256  struct rte_acl_ctx *
257 -rte_acl_create(const struct rte_acl_param *param)
258 +rte_acl_create_v20(const struct rte_acl_param *param)
259 {
260        size_t sz;
261        struct rte_acl_ctx *ctx;
262        ...
263
264Note that the base name of the symbol was kept intact, as this is conducive to
265the macros used for versioning symbols.  That is our next step, mapping this new
266symbol name to the initial symbol name at version node 2.0.  Immediately after
267the function, we add this line of code
268
269.. code-block:: c
270
271   VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
272
273Remembering to also add the rte_compat.h header to the requisite c file where
274these changes are being made.  The above macro instructs the linker to create a
275new symbol ``rte_acl_create@DPDK_2.0``, which matches the symbol created in older
276builds, but now points to the above newly named function.  We have now mapped
277the original rte_acl_create symbol to the original function (but with a new
278name)
279
280Next, we need to create the 2.1 version of the symbol.  We create a new function
281name, with a different suffix, and  implement it appropriately
282
283.. code-block:: c
284
285   struct rte_acl_ctx *
286   rte_acl_create_v21(const struct rte_acl_param *param, int debug);
287   {
288        struct rte_acl_ctx *ctx = rte_acl_create_v20(param);
289
290        ctx->debug = debug;
291
292        return ctx;
293   }
294
295This code serves as our new API call.  Its the same as our old call, but adds
296the 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
298in the header file, adding the macro there to inform all including applications,
299that on re-link, the default rte_acl_create symbol should point to this
300function.  Note that we could do this by simply naming the function above
301rte_acl_create, and the linker would chose the most recent version tag to apply
302in the version script, but we can also do this in the header file
303
304.. code-block:: c
305
306   struct rte_acl_ctx *
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);
310
311The BIND_DEFAULT_SYMBOL macro explicitly tells applications that include this
312header, to link to the rte_acl_create_v21 function and apply the DPDK_2.1
313version node to it.  This method is more explicit and flexible than just
314re-implementing the exact symbol name, and allows for other features (such as
315linking to the old symbol version by default, when the new ABI is to be opt-in
316for a period.
317
318One last thing we need to do.  Note that we've taken what was a public symbol,
319and duplicated it into two uniquely and differently named symbols.  We've then
320mapped each of those back to the public symbol ``rte_acl_create`` with different
321version tags.  This only applies to dynamic linking, as static linking has no
322notion of versioning.  That leaves this code in a position of no longer having a
323symbol simply named ``rte_acl_create`` and a static build will fail on that
324missing symbol.
325
326To correct this, we can simply map a function of our choosing back to the public
327symbol in the static build with the ``MAP_STATIC_SYMBOL`` macro.  Generally the
328assumption is that the most recent version of the symbol is the one you want to
329map.  So, back in the C file where, immediately after ``rte_acl_create_v21`` is
330defined, we add this
331
332.. code-block:: c
333
334   struct rte_acl_ctx *
335   rte_acl_create_v21(const struct rte_acl_param *param, int debug)
336   {
337        ...
338   }
339   MAP_STATIC_SYMBOL(struct rte_acl_ctx *rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);
340
341That tells the compiler that, when building a static library, any calls to the
342symbol ``rte_acl_create`` should be linked to ``rte_acl_create_v21``
343
344That's it, on the next shared library rebuild, there will be two versions of
345rte_acl_create, an old DPDK_2.0 version, used by previously built applications,
346and a new DPDK_2.1 version, used by future built applications.
347
348
349Deprecating part of a public API
350~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
351
352Lets assume that you've done the above update, and after a few releases have
353passed you decide you would like to retire the old version of the function.
354After having gone through the ABI deprecation announcement process, removal is
355easy.  Start by removing the symbol from the requisite version map file:
356
357.. code-block:: none
358
359   DPDK_2.0 {
360        global:
361
362        rte_acl_add_rules;
363        rte_acl_build;
364        rte_acl_classify;
365        rte_acl_classify_alg;
366        rte_acl_classify_scalar;
367        rte_acl_dump;
368 -      rte_acl_create
369        rte_acl_find_existing;
370        rte_acl_free;
371        rte_acl_ipv4vlan_add_rules;
372        rte_acl_ipv4vlan_build;
373        rte_acl_list_dump;
374        rte_acl_reset;
375        rte_acl_reset_rules;
376        rte_acl_set_ctx_classify;
377
378        local: *;
379   };
380
381   DPDK_2.1 {
382        global:
383        rte_acl_create;
384   } DPDK_2.0;
385
386
387Next remove the corresponding versioned export.
388
389.. code-block:: c
390
391 -VERSION_SYMBOL(rte_acl_create, _v20, 2.0);
392
393
394Note that the internal function definition could also be removed, but its used
395in our example by the newer version _v21, so we leave it in place.  This is a
396coding style choice.
397
398Lastly, we need to bump the LIBABIVER number for this library in the Makefile to
399indicate to applications doing dynamic linking that this is a later, and
400possibly incompatible library version:
401
402.. code-block:: c
403
404   -LIBABIVER := 1
405   +LIBABIVER := 2
406
407Deprecating an entire ABI version
408~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
409
410While removing a symbol from and ABI may be useful, it is often more practical
411to remove an entire version node at once.  If a version node completely
412specifies an API, then removing part of it, typically makes it incomplete.  In
413those cases it is better to remove the entire node
414
415To do this, start by modifying the version map file, such that all symbols from
416the node to be removed are merged into the next node in the map
417
418In the case of our map above, it would transform to look as follows
419
420.. code-block:: none
421
422   DPDK_2.1 {
423        global:
424
425        rte_acl_add_rules;
426        rte_acl_build;
427        rte_acl_classify;
428        rte_acl_classify_alg;
429        rte_acl_classify_scalar;
430        rte_acl_dump;
431        rte_acl_create
432        rte_acl_find_existing;
433        rte_acl_free;
434        rte_acl_ipv4vlan_add_rules;
435        rte_acl_ipv4vlan_build;
436        rte_acl_list_dump;
437        rte_acl_reset;
438        rte_acl_reset_rules;
439        rte_acl_set_ctx_classify;
440
441        local: *;
442 };
443
444Then any uses of BIND_DEFAULT_SYMBOL that pointed to the old node should be
445updated to point to the new version node in any header files for all affected
446symbols.
447
448.. code-block:: c
449
450 -BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 2.0);
451 +BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 2.1);
452
453Lastly, any VERSION_SYMBOL macros that point to the old version node should be
454removed, taking care to keep, where need old code in place to support newer
455versions of the symbol.
456
457Running the ABI Validator
458-------------------------
459
460The ``scripts`` directory in the DPDK source tree contains a utility program,
461``validate-abi.sh``, for validating the DPDK ABI based on the Linux `ABI
462Compliance Checker
463<http://ispras.linuxbase.org/index.php/ABI_compliance_checker>`_.
464
465This has a dependency on the ``abi-compliance-checker`` and ``and abi-dumper``
466utilities which can be installed via a package manager. For example::
467
468   sudo yum install abi-compliance-checker
469   sudo yum install abi-dumper
470
471The syntax of the ``validate-abi.sh`` utility is::
472
473   ./scripts/validate-abi.sh <REV1> <REV2> <TARGET>
474
475Where ``REV1`` and ``REV2`` are valid gitrevisions(7)
476https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
477on the local repo and target is the usual DPDK compilation target.
478
479For example::
480
481   # Check between the previous and latest commit:
482   ./scripts/validate-abi.sh HEAD~1 HEAD x86_64-native-linuxapp-gcc
483
484   # Check between two tags:
485   ./scripts/validate-abi.sh v2.0.0 v2.1.0 x86_64-native-linuxapp-gcc
486
487   # Check between git master and local topic-branch "vhost-hacking":
488   ./scripts/validate-abi.sh master vhost-hacking x86_64-native-linuxapp-gcc
489
490After the validation script completes (it can take a while since it need to
491compile both tags) it will create compatibility reports in the
492``./compat_report`` directory. Listed incompatibilities can be found as
493follows::
494
495  grep -lr Incompatible compat_reports/
496