doc: update libxdp usage in af_xdp guide

When libxdp is used, the LIBXDP_OBJECT_PATH environment variable must be
set to the location of where libxdp placed its bpf object files. This is
usually in /usr/local/lib/bpf or /usr/local/lib64/bpf. Failure to do so
will result in the PMD not initialising correctly as the bpf program is
not found. Document this requirement.

Also, mention that the following logs which are generated on application
launch can be ignored:

  libbpf: elf: skipping unrecognized data section(7) .xdp_run_config
  libbpf: elf: skipping unrecognized data section(8) xdp_metadata

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>

diff --git a/doc/guides/nics/af_xdp.rst b/doc/guides/nics/af_xdp.rst
index 3d8b70e3f8ce82659560aae13e3e8630f1681fbf..ee1e146a9e82fef5ab7a1467a92c4b0ed25b4fff 100644 (file)
--- a/doc/guides/nics/af_xdp.rst
+++ b/doc/guides/nics/af_xdp.rst
@@ -44,6 +44,9 @@ This is a Linux-specific PMD, thus the following prerequisites apply:
 
 *  A Linux Kernel (version > v4.18) with XDP sockets configuration enabled;
 *  Both libxdp >=v1.2.2 and libbpf libraries installed, or, libbpf <=v0.6.0
+*  If using libxdp, it requires an environment variable called
+   LIBXDP_OBJECT_PATH to be set to the location of where libxdp placed its bpf
+   object files. This is usually in /usr/local/lib/bpf or /usr/local/lib64/bpf.
 *  A Kernel bound interface to attach to;
 *  For need_wakeup feature, it requires kernel version later than v5.3-rc1;
 *  For PMD zero copy, it requires kernel version later than v5.4-rc1;
@@ -150,4 +153,16 @@ Limitations
   as this is the maximum number of fds that can be sent through the IPC APIs as
   defined by RTE_MP_MAX_FD_NUM.
 
+- **libxdp**
+
+  When using the default program (ie. when the vdev arg 'xdp_prog' is not used),
+  the following logs will appear when an application is launched:
+
+  .. code-block:: console
+
+    libbpf: elf: skipping unrecognized data section(7) .xdp_run_config
+    libbpf: elf: skipping unrecognized data section(8) xdp_metadata
+
+  These logs are not errors and can be ignored.
+
   [1] https://lwn.net/Articles/837010/