4. ABI Versioning

This document details the mechanics of ABI version management in DPDK.

4.1. What is a library’s soname?

System libraries usually adopt the familiar major and minor version naming convention, where major versions (e.g. librte_eal 21.x, 22.x) are presumed to be ABI incompatible with each other and minor versions (e.g. librte_eal 21.1, 21.2) are presumed to be ABI compatible. A library’s soname. is typically used to provide backward compatibility information about a given library, describing the lowest common denominator ABI supported by the library. The soname or logical name for the library, is typically comprised of the library’s name and major version e.g. librte_eal.so.21.

During an application’s build process, a library’s soname is noted as a runtime dependency of the application. This information is then used by the dynamic linker when resolving the applications dependencies at runtime, to load a library supporting the correct ABI version. The library loaded at runtime therefore, may be a minor revision supporting the same major ABI version (e.g. librte_eal.21.2), as the library used to link the application (e.g librte_eal.21.0).

4.2. Major ABI versions

An ABI version change to a given library, especially in core libraries such as librte_mbuf, may cause an implicit ripple effect on the ABI of it’s consuming libraries, causing ABI breakages. There may however be no explicit reason to bump a dependent library’s ABI version, as there may have been no obvious change to the dependent library’s API, even though the library’s ABI compatibility will have been broken.

This interdependence of DPDK libraries, means that ABI versioning of libraries is more manageable at a project level, with all project libraries sharing a single ABI version. In addition, the need to maintain a stable ABI for some number of releases as described in the section ABI Policy, means that ABI version increments need to carefully planned and managed at a project level.

Major ABI versions are therefore declared typically aligned with an LTS release and is then supported some number of subsequent releases, shared across all libraries. This means that a single project level ABI version, reflected in all individual library’s soname, library filenames and associated version maps persists over multiple releases.

$ head ./build/lib/acl_exports.map
DPDK_21 {
       global:
...

$ head ./build/lib/eal_exports.map
DPDK_21 {
       global:
...

When an ABI change is made between major ABI versions to a given library, a new section is added to that library’s version map describing the impending new ABI version, as described in the section Examples of ABI Macro use. The library’s soname and filename however do not change, e.g. libacl.so.21, as ABI compatibility with the last major ABI version continues to be preserved for that library.

$ head ./build/lib/acl_exports.map
DPDK_21 {
       global:
...

DPDK_22 {
       global:

} DPDK_21;
...

$ head ./build/lib/eal_exports.map
DPDK_21 {
       global:
...

However when a new ABI version is declared, for example DPDK 22, old deprecated functions may be safely removed at this point and the entire old major ABI version removed, see the section Deprecating an entire ABI version on how this may be done.

$ head ./build/lib/acl_exports.map
DPDK_22 {
       global:
...

$ head ./build/lib/eal_exports.map
DPDK_22 {
       global:
...

At the same time, the major ABI version is changed atomically across all libraries by incrementing the major version in the ABI_VERSION file. This is done globally for all libraries.

4.2.1. Minor ABI versions

Each non-LTS release will also increment minor ABI version, to permit multiple DPDK versions being installed alongside each other. Both stable and experimental ABI’s are versioned using the global version file that is updated at the start of each release cycle, and are managed at the project level.

4.3. Versioning Macros

When a symbol is exported from a library to provide an API, it also provides a calling convention (ABI) that is embodied in its name, return type and arguments. Occasionally that function may need to change to accommodate new functionality or behavior. When that occurs, it is may be required to allow for backward compatibility for a time with older binaries that are dynamically linked to the DPDK.

To support backward compatibility the eal_export.h header file provides macros to use when updating exported functions. These macros allow multiple versions of a symbol to exist in a shared library so that older binaries need not be immediately recompiled.

The macros are:

RTE_VERSION_SYMBOL(ver, type, name, args): Creates a symbol version table entry binding symbol <name>@DPDK_<ver> to the internal function name <name>_v<ver>.
RTE_DEFAULT_SYMBOL(ver, type, name, args): Creates a symbol version entry instructing the linker to bind references to symbol <name> to the internal symbol <name>_v<ver>.
RTE_VERSION_EXPERIMENTAL_SYMBOL(type, name, args): Similar to RTE_VERSION_SYMBOL but for experimental API symbols. The macro is used when a symbol matures to become part of the stable ABI, to provide an alias to experimental until the next major ABI version.

4.3.1. Examples of ABI Macro use

4.3.1.1. Updating a public API

Assume we have a function as follows

/*
 * Create an acl context object for apps to
 * manipulate
 */
RTE_EXPORT_SYMBOL(rte_acl_create)
int
rte_acl_create(struct rte_acl_param *param)
{
       ...
}

Assume that struct rte_acl_ctx is a private structure, and that a developer wishes to enhance the acl api so that a debugging flag can be enabled on a per-context basis. This requires an addition to the structure (which, being private, is safe), but it also requires modifying the code as follows

/*
 * Create an acl context object for apps to
 * manipulate
 */
RTE_EXPORT_SYMBOL(rte_acl_create)
int
rte_acl_create(struct rte_acl_param *param, int debug)
{
       ...
}

Note also that, being a public function, the header file prototype must also be changed, as must all the call sites, to reflect the new ABI footprint. We will maintain previous ABI versions that are accessible only to previously compiled binaries.

The addition of a parameter to the function is ABI breaking as the function is public, and existing application may use it in its current form. However, the compatibility macros in DPDK allow a developer to use symbol versioning so that multiple functions can be mapped to the same public symbol based on when an application was linked to it.

We need to specify in the code which function maps to the rte_acl_create symbol at which versions. First, at the site of the initial symbol definition, we wrap the function with RTE_VERSION_SYMBOL, passing the current ABI version, the function return type, the function name and its arguments.

-RTE_EXPORT_SYMBOL(rte_acl_create)
-int
-rte_acl_create(struct rte_acl_param *param)
+RTE_VERSION_SYMBOL(21, int, rte_acl_create, (struct rte_acl_param *param))
{
       size_t sz;
       struct rte_acl_ctx *ctx;
       ...

The macro instructs the linker to create a new symbol rte_acl_create@DPDK_21, which matches the symbol created in older builds, but now points to the above newly named function rte_acl_create_v21. We have now mapped the original rte_acl_create symbol to the original function (but with a new name).

Please see the section Enabling versioning macros to enable this macro in the meson/ninja build.

Next, we need to create the new version of the symbol. We create a new function name and implement it appropriately, then wrap it in a call to RTE_DEFAULT_SYMBOL.

RTE_DEFAULT_SYMBOL(22, int, rte_acl_create, (struct rte_acl_param *param, int debug))
{
     int ret = rte_acl_create_v21(param);

     if (debug) {
     ...
     }

     return ret;
}

The macro instructs the linker to create the new default symbol rte_acl_create@DPDK_22, which points to the function named rte_acl_create_v22 (declared by the macro).

And that’s it. On the next shared library rebuild, there will be two versions of rte_acl_create, an old DPDK_21 version, used by previously built applications, and a new DPDK_22 version, used by newly built applications.

Note

Before you leave, please take care reviewing the sections on enabling versioning macros, and ABI deprecation.

4.3.1.2. Enabling versioning macros

Finally, we need to indicate to the meson/ninja build system to enable versioning macros when building the library or driver. In the libraries or driver where we have added symbol versioning, in the meson.build file we add the following

use_function_versioning = true

at the start of the head of the file. This will indicate to the tool-chain to enable the function version macros when building.

4.3.1.3. Aliasing experimental symbols

In situations in which an experimental symbol has been stable for some time, and it becomes a candidate for promotion to the stable ABI. At this time, when promoting the symbol, the maintainer may choose to provide an alias to the experimental symbol version, so as not to break consuming applications. This alias is then dropped in the next major ABI version.

The process to provide an alias to experimental is similar to that, of symbol versioning described above. Assume we have an experimental function rte_acl_create as follows:

#include <rte_compat.h>

/*
 * Create an acl context object for apps to
 * manipulate
 */
RTE_EXPORT_EXPERIMENTAL_SYMBOL(rte_acl_create)
__rte_experimental
int
rte_acl_create(struct rte_acl_param *param)
{
...
}

When we promote the symbol to the stable ABI, we simply strip the __rte_experimental annotation from the function.

/*
 * Create an acl context object for apps to
 * manipulate
 */
RTE_EXPORT_SYMBOL(rte_acl_create)
int
rte_acl_create(struct rte_acl_param *param)
{
       ...
}

Although there are strictly no guarantees or commitments associated with experimental symbols, a maintainer may wish to offer an alias to experimental. The process to add an alias to experimental, is similar to the symbol versioning process. Assuming we have an experimental symbol as before, we now add the symbol to both the EXPERIMENTAL and DPDK_22 version nodes.

#include <rte_compat.h>;

/*
 * Create an acl context object for apps to
 * manipulate
 */
RTE_DEFAULT_SYMBOL(22, int, rte_acl_create, (struct rte_acl_param *param))
{
...
}

RTE_VERSION_EXPERIMENTAL_SYMBOL(int, rte_acl_create, (struct rte_acl_param *param))
{
   return rte_acl_create(param);
}

4.3.1.4. Deprecating part of a public API

Lets assume that you’ve done the above updates, and in preparation for the next major ABI version you decide you would like to retire the old version of the function. After having gone through the ABI deprecation announcement process, removal is easy.

Next remove the corresponding versioned export.

-RTE_VERSION_SYMBOL(21, int, rte_acl_create, (struct rte_acl_param *param))

Note that the internal function definition must also be removed, but it is used in our example by the newer version v22, so we leave it in place and declare it as static. This is a coding style choice.

4.3.1.5. Deprecating an entire ABI version

While removing a symbol from an ABI may be useful, it is more practical to remove an entire version node at once, as is typically done at the declaration of a major ABI version. If a version node completely specifies an API, then removing part of it, typically makes it incomplete. In those cases it is better to remove the entire node.

Any uses of RTE_DEFAULT_SYMBOL that pointed to the old node should be updated to point to the new version node in any header files for all affected symbols.

-RTE_DEFAULT_SYMBOL(21, int, rte_acl_create, (struct rte_acl_param *param, int debug))
+RTE_DEFAULT_SYMBOL(22, int, rte_acl_create, (struct rte_acl_param *param, int debug))

Lastly, any RTE_VERSION_SYMBOL macros that point to the old version nodes should be removed, taking care to preserve any code that is shared with the new version node.

4.4. Running the ABI Validator

The devtools directory in the DPDK source tree contains a utility program, check-abi.sh, for validating the DPDK ABI based on the libabigail abidiff utility.

The syntax of the check-abi.sh utility is:

devtools/check-abi.sh <refdir> <newdir>

Where <refdir> specifies the directory housing the reference build of DPDK, and <newdir> specifies the DPDK build directory to check the ABI of.

The ABI compatibility is automatically verified when using a build script from devtools, if the variable DPDK_ABI_REF_VERSION is set with a tag, as described in ABI check recommendations.