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 ./lib/acl/version.map
DPDK_21 {
global:
...
$ head ./lib/eal/version.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 ./lib/acl/version.map
DPDK_21 {
global:
...
DPDK_22 {
global:
} DPDK_21;
...
$ head ./lib/eal/version.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 ./lib/acl/version.map
DPDK_22 {
global:
...
$ head ./lib/eal/version.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 rte_function_versioning.h
header file provides macros to use when updating exported functions. These
macros are used in conjunction with the 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 symbolb@DPDK_n
to the internal functionbe
.BIND_DEFAULT_SYMBOL(b, e, n)
: Creates a symbol version entry instructing the linker to bind references to symbolb
to the internal symbolbe
.MAP_STATIC_SYMBOL(f, p)
: Declare the prototypef
, and map it to the fully qualified functionp
, 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 symbolbe
to signal that it is being used as an implementation of a particular version of symbolb
.VERSION_SYMBOL_EXPERIMENTAL(b, e)
: Creates a symbol version table entry binding versioned symbolb@EXPERIMENTAL
to the internal functionbe
. 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
*/
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_21 {
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_21 {
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_22 {
global:
rte_acl_create;
} DPDK_21;
The addition of the new block tells the linker that a new version node
DPDK_22
is available, which contains the symbol rte_acl_create, and inherits
the symbols from the DPDK_21 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_v21(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 21.
Immediately after the function, we add the VERSION_SYMBOL macro.
#include <rte_function_versioning.h>
...
VERSION_SYMBOL(rte_acl_create, _v21, 21);
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_21
, 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 v22
version of the symbol. We create a new
function name, with the v22
suffix, and implement it appropriately.
struct rte_acl_ctx * __vsym
rte_acl_create_v22(const struct rte_acl_param *param, int debug);
{
struct rte_acl_ctx *ctx = rte_acl_create_v21(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_22
. 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, _v22, 22);
The macro instructs the linker to create the new default symbol
rte_acl_create@DPDK_22
, 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_v21(const struct rte_acl_param *param);
struct rte_acl_ctx * __vsym
rte_acl_create_v22(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_21 version, used by previously built applications, and a new DPDK_22 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_v22
is
defined, we add this
struct rte_acl_ctx * __vsym
rte_acl_create_v22(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_v22);
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_v22
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.
4.3.1.4. 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_experimental
struct rte_acl_ctx *
rte_acl_create(const struct rte_acl_param *param)
{
...
}
In the map file, experimental symbols are listed as part of the EXPERIMENTAL
version node.
DPDK_21 {
global:
...
local: *;
};
EXPERIMENTAL {
global:
rte_acl_create;
};
When we promote the symbol to the stable ABI, we simply strip the
__rte_experimental
annotation from the function and move the symbol from the
EXPERIMENTAL
node, to the node of the next major ABI version as follow.
/*
* Create an acl context object for apps to
* manipulate
*/
struct rte_acl_ctx *
rte_acl_create(const struct rte_acl_param *param)
{
...
}
We then update the map file, adding the symbol rte_acl_create
to the DPDK_22
version node.
DPDK_21 {
global:
...
local: *;
};
DPDK_22 {
global:
rte_acl_create;
} DPDK_21;
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>;
#include <rte_function_versioning.h>
/*
* Create an acl context object for apps to
* manipulate
*/
struct rte_acl_ctx *
rte_acl_create(const struct rte_acl_param *param)
{
...
}
__rte_experimental
struct rte_acl_ctx *
rte_acl_create_e(const struct rte_acl_param *param)
{
return rte_acl_create(param);
}
VERSION_SYMBOL_EXPERIMENTAL(rte_acl_create, _e);
struct rte_acl_ctx *
rte_acl_create_v22(const struct rte_acl_param *param)
{
return rte_acl_create(param);
}
BIND_DEFAULT_SYMBOL(rte_acl_create, _v22, 22);
In the map file, we map the symbol to both the EXPERIMENTAL
and DPDK_22
version nodes.
DPDK_21 {
global:
...
local: *;
};
DPDK_22 {
global:
rte_acl_create;
} DPDK_21;
EXPERIMENTAL {
global:
rte_acl_create;
};
Note
Please note, similar to symbol versioning, when aliasing to experimental you will also need to take care of mapping static symbols.
4.3.1.5. 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_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: *;
};
DPDK_22 {
global:
rte_acl_create;
} DPDK_21;
Next remove the corresponding versioned export.
-VERSION_SYMBOL(rte_acl_create, _v21, 21);
Note that the internal function definition could also be removed, but its 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.
4.3.1.6. 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_22 {
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, _v21, 21);
+BIND_DEFAULT_SYMBOL(rte_acl_create, _v22, 22);
Lastly, any 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.