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 20.x, 21.x) are presumed to be ABI incompatible with each other and minor versions (e.g. librte_eal 20.1, 20.2) 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.20.

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.20.2), as the library used to link the application (e.g librte_eal.20.0).

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 ./lib/librte_acl/rte_acl_version.map
DPDK_20 {
       global:
...

$ head ./lib/librte_eal/rte_eal_version.map
DPDK_20 {
       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.20, as ABI compatibility with the last major ABI version continues to be preserved for that library.

$ head ./lib/librte_acl/rte_acl_version.map
DPDK_20 {
       global:
...

DPDK_21 {
       global:

} DPDK_20;
...

$ head ./lib/librte_eal/rte_eal_version.map
DPDK_20 {
       global:
...

However when a new ABI version is declared, for example DPDK 21, old depreciated 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 ./lib/librte_acl/rte_acl_version.map
DPDK_21 {
       global:
...

$ head ./lib/librte_eal/rte_eal_version.map
DPDK_21 {
       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 that declare a stable ABI. For libraries marked as EXPERIMENTAL, their major ABI version is always set to 0.

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 rte_function_versioning.h header file provides macros to use when updating exported functions. These macros are used in conjunction with the rte_<library>_version.map file for a given library to allow multiple versions of a symbol to exist in a shared library so that older binaries need not be immediately recompiled.

The macros exported are:

  • VERSION_SYMBOL(b, e, n): Creates a symbol version table entry binding versioned symbol b@DPDK_n to the internal function be.
  • BIND_DEFAULT_SYMBOL(b, e, n): Creates a symbol version entry instructing the linker to bind references to symbol b to the internal symbol be.
  • MAP_STATIC_SYMBOL(f, p): Declare the prototype f, and map it to the fully qualified function p, so that if a symbol becomes versioned, it can still be mapped back to the public symbol name.
  • __vsym: Annotation to be used in a declaration of the internal symbol be to signal that it is being used as an implementation of a particular version of symbol b.

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
 */
struct rte_acl_ctx *
rte_acl_create(const 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
 */
struct rte_acl_ctx *
rte_acl_create(const 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. To see how this is done, we start with the requisite libraries version map file. Initially the version map file for the acl library looks like this

DPDK_20 {
     global:

     rte_acl_add_rules;
     rte_acl_build;
     rte_acl_classify;
     rte_acl_classify_alg;
     rte_acl_classify_scalar;
     rte_acl_create;
     rte_acl_dump;
     rte_acl_find_existing;
     rte_acl_free;
     rte_acl_ipv4vlan_add_rules;
     rte_acl_ipv4vlan_build;
     rte_acl_list_dump;
     rte_acl_reset;
     rte_acl_reset_rules;
     rte_acl_set_ctx_classify;

     local: *;
};

This file needs to be modified as follows

DPDK_20 {
     global:

     rte_acl_add_rules;
     rte_acl_build;
     rte_acl_classify;
     rte_acl_classify_alg;
     rte_acl_classify_scalar;
     rte_acl_create;
     rte_acl_dump;
     rte_acl_find_existing;
     rte_acl_free;
     rte_acl_ipv4vlan_add_rules;
     rte_acl_ipv4vlan_build;
     rte_acl_list_dump;
     rte_acl_reset;
     rte_acl_reset_rules;
     rte_acl_set_ctx_classify;

     local: *;
};

DPDK_21 {
     global:
     rte_acl_create;

} DPDK_20;

The addition of the new block tells the linker that a new version node DPDK_21 is available, which contains the symbol rte_acl_create, and inherits the symbols from the DPDK_20 node. This list is directly translated into a list of exported symbols when DPDK is compiled as a shared library.

Next, 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 need to update the function so that it is uniquely named, and not in conflict with the public symbol name

-struct rte_acl_ctx *
-rte_acl_create(const struct rte_acl_param *param)
+struct rte_acl_ctx * __vsym
+rte_acl_create_v20(const struct rte_acl_param *param)
{
       size_t sz;
       struct rte_acl_ctx *ctx;
       ...

Note that the base name of the symbol was kept intact, as this is conducive to the macros used for versioning symbols and we have annotated the function as __vsym, an implementation of a versioned symbol . That is our next step, mapping this new symbol name to the initial symbol name at version node 20. Immediately after the function, we add the VERSION_SYMBOL macro.

#include <rte_function_versioning.h>

...
VERSION_SYMBOL(rte_acl_create, _v20, 20);

Remembering to also add the rte_function_versioning.h header to the requisite c file where these changes are being made. The macro instructs the linker to create a new symbol rte_acl_create@DPDK_20, which matches the symbol created in older builds, but now points to the above newly named function. 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 v21 version of the symbol. We create a new function name, with the v21 suffix, and implement it appropriately.

struct rte_acl_ctx * __vsym
rte_acl_create_v21(const struct rte_acl_param *param, int debug);
{
     struct rte_acl_ctx *ctx = rte_acl_create_v20(param);

     ctx->debug = debug;

     return ctx;
}

This code serves as our new API call. Its the same as our old call, but adds the new parameter in place. Next we need to map this function to the new default symbol rte_acl_create@DPDK_21. To do this, immediately after the function, we add the BIND_DEFAULT_SYMBOL macro.

#include <rte_function_versioning.h>

...
BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 21);

The macro instructs the linker to create the new default symbol rte_acl_create@DPDK_21, which points to the above newly named function.

We finally modify the prototype of the call in the public header file, such that it contains both versions of the symbol and the public API.

struct rte_acl_ctx *
rte_acl_create(const struct rte_acl_param *param);

struct rte_acl_ctx * __vsym
rte_acl_create_v20(const struct rte_acl_param *param);

struct rte_acl_ctx * __vsym
rte_acl_create_v21(const struct rte_acl_param *param, int debug);

And that’s it, on the next shared library rebuild, there will be two versions of rte_acl_create, an old DPDK_20 version, used by previously built applications, and a new DPDK_21 version, used by future built applications.

Note

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

4.3.1.2. Mapping static symbols

Now we’ve taken what was a public symbol, and duplicated it into two uniquely and differently named symbols. We’ve then mapped each of those back to the public symbol rte_acl_create with different version tags. This only applies to dynamic linking, as static linking has no notion of versioning. That leaves this code in a position of no longer having a symbol simply named rte_acl_create and a static build will fail on that missing symbol.

To correct this, we can simply map a function of our choosing back to the public symbol in the static build with the MAP_STATIC_SYMBOL macro. Generally the assumption is that the most recent version of the symbol is the one you want to map. So, back in the C file where, immediately after rte_acl_create_v21 is defined, we add this

struct rte_acl_ctx * __vsym
rte_acl_create_v21(const struct rte_acl_param *param, int debug)
{
     ...
}
MAP_STATIC_SYMBOL(struct rte_acl_ctx *rte_acl_create(const struct rte_acl_param *param, int debug), rte_acl_create_v21);

That tells the compiler that, when building a static library, any calls to the symbol rte_acl_create should be linked to rte_acl_create_v21

4.3.1.3. 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. There is no corresponding directive required for the make build system.

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. Start by removing the symbol from the requisite version map file:

  DPDK_20 {
       global:

       rte_acl_add_rules;
       rte_acl_build;
       rte_acl_classify;
       rte_acl_classify_alg;
       rte_acl_classify_scalar;
       rte_acl_dump;
-      rte_acl_create
       rte_acl_find_existing;
       rte_acl_free;
       rte_acl_ipv4vlan_add_rules;
       rte_acl_ipv4vlan_build;
       rte_acl_list_dump;
       rte_acl_reset;
       rte_acl_reset_rules;
       rte_acl_set_ctx_classify;

       local: *;
  };

  DPDK_21 {
       global:
       rte_acl_create;
  } DPDK_20;

Next remove the corresponding versioned export.

-VERSION_SYMBOL(rte_acl_create, _v20, 20);

Note that the internal function definition could also be removed, but its used in our example by the newer version v21, 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.

To do this, start by modifying the version map file, such that all symbols from the node to be removed are merged into the next node in the map.

In the case of our map above, it would transform to look as follows

  DPDK_21 {
       global:

       rte_acl_add_rules;
       rte_acl_build;
       rte_acl_classify;
       rte_acl_classify_alg;
       rte_acl_classify_scalar;
       rte_acl_dump;
       rte_acl_create
       rte_acl_find_existing;
       rte_acl_free;
       rte_acl_ipv4vlan_add_rules;
       rte_acl_ipv4vlan_build;
       rte_acl_list_dump;
       rte_acl_reset;
       rte_acl_reset_rules;
       rte_acl_set_ctx_classify;

       local: *;
};

Then any uses of BIND_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.

-BIND_DEFAULT_SYMBOL(rte_acl_create, _v20, 20);
+BIND_DEFAULT_SYMBOL(rte_acl_create, _v21, 21);

Lastly, any VERSION_SYMBOL macros that point to the old version node should be removed, taking care to keep, where need old code in place to support newer versions of the symbol.

4.4. Running the ABI Validator

The devtools directory in the DPDK source tree contains a utility program, validate-abi.sh, for validating the DPDK ABI based on the Linux ABI Compliance Checker.

This has a dependency on the abi-compliance-checker and and abi-dumper utilities which can be installed via a package manager. For example:

sudo yum install abi-compliance-checker
sudo yum install abi-dumper

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

./devtools/validate-abi.sh <REV1> <REV2>

Where REV1 and REV2 are valid gitrevisions(7) https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html on the local repo.

For example:

# Check between the previous and latest commit:
./devtools/validate-abi.sh HEAD~1 HEAD

# Check on a specific compilation target:
./devtools/validate-abi.sh -t x86_64-native-linux-gcc HEAD~1 HEAD

# Check between two tags:
./devtools/validate-abi.sh v2.0.0 v2.1.0

# Check between git master and local topic-branch "vhost-hacking":
./devtools/validate-abi.sh master vhost-hacking

After the validation script completes (it can take a while since it need to compile both tags) it will create compatibility reports in the ./abi-check/compat_report directory. Listed incompatibilities can be found as follows:

grep -lr Incompatible abi-check/compat_reports/