doc: mention mlx5 MPRQ incompatibility with external buffers

Multi-Packet Rx queue uses PMD-managed buffers to store packets.
These buffers are externally attached to user mbufs.
This conflicts with the feature that allows using user-managed
externally attached buffers in an application.
Add the corresponding limitation to MLX5 documentation that MPRQ
and external data buffers cannot be used together.

Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>

diff --git a/doc/guides/nics/mlx5.rst b/doc/guides/nics/mlx5.rst
index 3c3687a..4805d08 100644 (file)
--- a/doc/guides/nics/mlx5.rst
+++ b/doc/guides/nics/mlx5.rst
@@ -261,6 +261,9 @@ Limitations
   Rx mbufs must be freed before the device is closed. Otherwise, the mempool of
   the external buffers will be freed by PMD and the application which still
   holds the external buffers may be corrupted.
+  User-managed mempools with external pinned data buffers
+  cannot be used in conjunction with MPRQ
+  since packets may be already attached to PMD-managed external buffers.
 
 - If Multi-Packet Rx queue is configured (``mprq_en``) and Rx CQE compression is
   enabled (``rxq_cqe_comp_en``) at the same time, RSS hash result is not fully