[dpdk-dev] [PATCH v2] net/i40e: fix Rx instability with vector mode

[Beilei Xing <beilei.xing@intel.com>]

Previously, there is instability during vector Rx if descriptor
number is not power of 2, e.g. process hang and some Rx packets
are unexpectedly empty. That's because vector Rx mode assumes Rx
descriptor number is power of 2 when doing bit mask.
This patch allows vector mode only when the number of Rx descriptor
is power of 2.

Fixes: 8e109464c022 ("i40e: allow vector Rx and Tx usage")
Fixes: a3c83a2527e1 ("net/i40e: enable runtime queue setup")
Cc: stable@dpdk.org

Signed-off-by: Beilei Xing <beilei.xing@intel.com>
---

v2 changes:
 - rx_vec_allowed is global configuration, avoid overwrite.

 doc/guides/nics/i40e.rst     |  7 +++++++
 drivers/net/i40e/i40e_rxtx.c | 13 ++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/doc/guides/nics/i40e.rst b/doc/guides/nics/i40e.rst
index ab3928a..bfacbd1 100644
--- a/doc/guides/nics/i40e.rst
+++ b/doc/guides/nics/i40e.rst
@@ -172,6 +172,13 @@ Runtime Config Options
 
   -w 84:00.0,use-latest-supported-vec=1
 
+Vector RX Pre-conditions
+~~~~~~~~~~~~~~~~~~~~~~~~
+For Vector RX it is assumed that the number of descriptor rings will be a power
+of 2. With this pre-condition, the ring pointer can easily scroll back to the
+head after hitting the tail without a conditional check. In addition Vector RX
+can use this assumption to do a bit mask using ``ring_size - 1``.
+
 Driver compilation and testing
 ------------------------------
 
diff --git a/drivers/net/i40e/i40e_rxtx.c b/drivers/net/i40e/i40e_rxtx.c
index a827456..f6dcca0 100644
--- a/drivers/net/i40e/i40e_rxtx.c
+++ b/drivers/net/i40e/i40e_rxtx.c
@@ -1735,10 +1735,14 @@ i40e_dev_rx_queue_setup_runtime(struct rte_eth_dev *dev,
 		 * i40e_set_rx_function.
 		 */
 		ad->rx_bulk_alloc_allowed = true;
-		ad->rx_vec_allowed = true;
 		dev->data->scattered_rx = use_scattered_rx;
 		if (use_def_burst_func)
 			ad->rx_bulk_alloc_allowed = false;
+		/**
+		 * Vector mode is allowed only when number of Rx queue
+		 * descriptor is a power of 2.
+		 */
+		ad->rx_vec_allowed = !(rxq->nb_rx_desc & (rxq->nb_rx_desc - 1));
 		i40e_set_rx_function(dev);
 		return 0;
 	}
@@ -1811,6 +1815,13 @@ i40e_dev_rx_queue_setup(struct rte_eth_dev *dev,
 		return -EINVAL;
 	}
 
+	/**
+	 * Vector mode is allowed only when number of Rx queue
+	 * descriptor is a power of 2.
+	 */
+	if (ad->rx_vec_allowed)
+		ad->rx_vec_allowed = !(nb_desc & (nb_desc - 1));
+
 	/* Free memory if needed */
 	if (dev->data->rx_queues[queue_idx]) {
 		i40e_dev_rx_queue_release(dev->data->rx_queues[queue_idx]);
-- 
2.5.5