[PATCH 1/1] net/tap: add a check that Rx/Tx have the same num of queues

[Nobuhiro MIKI]

Users can create the desired number of RxQ and TxQ in DPDK. For
example, if the number of RxQ = 2 and the number of TxQ = 5,
a total of 8 file descriptors will be created for a tap device,
including RxQ, TxQ, and one for keepalive [1]. The RxQ and TxQ
with the same ID are paired by dup(2) [2].

In this scenario, Kernel will have 3 RxQ where packets are
incoming but not read. The reason for this is that there are only
2 RxQ that are polled by DPDK, while there are 5 queues in the
kernel. This patch add a checking if DPDK has appropriate numbers
of queues to avoid unexpected packet drop.

I had first discussed this issue in OVS [3], but changed my mind
that a fix in DPDK would be more appropriate.

[1]: https://github.com/DPDK/dpdk/blob/0c6e27549c/drivers/net/tap/rte_eth_tap.c#L1967-L1973
[2]: https://github.com/DPDK/dpdk/blob/0c6e27549c/drivers/net/tap/rte_eth_tap.c#L1465-L1466
[3]: https://mail.openvswitch.org/pipermail/ovs-dev/2021-November/389690.html

Signed-off-by: Nobuhiro MIKI <nmiki at yahoo-corp.jp>
---
 drivers/net/tap/rte_eth_tap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/tap/rte_eth_tap.c b/drivers/net/tap/rte_eth_tap.c
index f1b48cae82..f0090a604d 100644
--- a/drivers/net/tap/rte_eth_tap.c
+++ b/drivers/net/tap/rte_eth_tap.c
@@ -940,6 +940,14 @@ tap_dev_configure(struct rte_eth_dev *dev)
 			RTE_PMD_TAP_MAX_QUEUES);
 		return -1;
 	}
+	if (dev->data->nb_rx_queues != dev->data->nb_tx_queues) {
+		TAP_LOG(ERR,
+			"%s: number of rx queues %d must be equal to number of tx queues %d",
+			dev->device->name,
+			dev->data->nb_rx_queues,
+			dev->data->nb_tx_queues);
+		return -1;
+	}
 
 	TAP_LOG(INFO, "%s: %s: TX configured queues number: %u",
 		dev->device->name, pmd->name, dev->data->nb_tx_queues);
-- 
2.24.4