[libvirt] [PATCH] util: Fix configure.ac check for DEVLINK_CMD_ESWITCH_GET

[John Ferlan]

Rather than checking for whether the devlink.h on the system has
multiple symbols, let's only check for whether the command we want
is defined.

Turns out the mechanism of providing multiple definitions to check via
AC_CHECK_DECLS in order to determine whether HAVE_DECL_DEVLINK should
be set resulted in a false positive since as long as one of the defs
was true, then the HAVE_DECL_DEVLINK got defined.

This is further complicated by a change between kernel 4.8 and 4.11
where the originally defined name DEVLINK_CMD_ESWITCH_MODE_GET was
changed to DEVLINK_CMD_ESWITCH_GET with the comment/caveat that
the old format is obsolete should never be used. Therefore, even
though some kernels will have a couple of the same symbols that
were added at the same time (DEVLINK_ATTR_ESWITCH_MODE and
DEVLINK_ESWITCH_MODE_SWITCHDEV), they won't have the newer name
and thus cause a build failure.

So even though multiple DEVLINK_CMD_SWITCH* symbols are used to
determine whether the set HAVE_DECL_DEVLINK, this should cover
those kernels before 4.11 with the old definition.

Signed-off-by: John Ferlan <jferlan at redhat.com>
---

 I'd push this as a build breaker, but I don't want to need to go through
 the trouble again if i got this wrong, so hopefully someone who's seeing
 this can try out the patch...  It's also present on a couple of the CI
 infrastructure environments (f23, centos-7):

  https://ci.centos.org/view/libvirt/job/libvirt-master-build/

 configure.ac | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index c9509c7f9..73ae7fdd5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -630,12 +630,16 @@ fi
 dnl
 dnl check for kernel headers required by devlink
 dnl
+dnl kernel 4.8 introduced DEVLINK_CMD_ESWITCH_MODE_GET, but that was
+dnl later changed in kernel 4.11 to be just DEVLINK_CMD_ESWITCH_GET, so
+dnl for "completeness" let's allow HAVE_DECL_DEVLINK to be define if
+dnl either is defined.
 if test "$with_linux" = "yes"; then
     AC_CHECK_HEADERS([linux/devlink.h])
-    AC_CHECK_DECLS([DEVLINK_GENL_VERSION, DEVLINK_GENL_NAME, DEVLINK_ATTR_MAX, DEVLINK_CMD_ESWITCH_GET, DEVLINK_ATTR_BUS_NAME, DEVLINK_ATTR_DEV_NAME, DEVLINK_ATTR_ESWITCH_MODE, DEVLINK_ESWITCH_MODE_SWITCHDEV],
+    AC_CHECK_DECLS([DEVLINK_CMD_ESWITCH_GET, DEVLINK_CMD_ESWITCH_MODE_GET],
                    [AC_DEFINE([HAVE_DECL_DEVLINK],
                               [1],
-                              [whether devlink declarations are available])],
+                              [whether devlink CMD_ESWITCH_GET is available])],
                    [],
                    [[#include <linux/devlink.h>]])
 fi
-- 
2.13.5