doc: add mlx5 MPRQ limitation with multi-process

MPRQ cannot be used in multi-process applications because of
externally attached MPRQ buffers. A callback is registered by
a primary process to free MPRQ buffers once they are no longer
needed. But this information is shared among all the processes.
The virtual address of the mlx5_mprq_buf_free_cb function is
different in a secondary process, which leads to a segmentation
fault. Document that MPRQ is not supported in a multi-process
app, since there is no way to find out if this is the one.

Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>

diff --git a/doc/guides/nics/mlx5.rst b/doc/guides/nics/mlx5.rst
index daa7f2a..c3cc0c0 100644 (file)
--- a/doc/guides/nics/mlx5.rst
+++ b/doc/guides/nics/mlx5.rst
@@ -138,6 +138,9 @@ Limitations
 - For secondary process:
 
   - Forked secondary process not supported.
+  - MPRQ is not supported. Callback to free externally attached MPRQ buffer is set
+    in a primary process, but has a different virtual address in a secondary process.
+    Calling a function at the wrong address leads to a segmentation fault.
   - External memory unregistered in EAL memseg list cannot be used for DMA
     unless such memory has been registered by ``mlx5_mr_update_ext_mp()`` in
     primary process and remapped to the same virtual address in secondary