[PATCH v2 1/1] net/tap: add a check that Rx/Tx have the same num of queues

[Ferruh Yigit]

On 1/12/2022 7:23 AM, Nobuhiro MIKI wrote:
> 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. The RxQ and TxQ
> with the same ID are paired by dup(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 Kernel.
> This patch add a checking if DPDK has appropriate numbers of
> queues to avoid unexpected packet drop.
> 
> Signed-off-by: Nobuhiro MIKI <nmiki at yahoo-corp.jp>


It makes sense to add this check, since the driver logic seem already has
the assumption that Rx & Tx queue numbers will be same.

But can you please update the tap documentation for this limitation/restriction?
'doc/guides/nics/tap.rst'.


> ---
> v2: fix commit message
> 
> I had first discussed this issue in OVS [1], but changed my mind
> that a fix in DPDK would be more appropriate.
> [1]: https://mail.openvswitch.org/pipermail/ovs-dev/2021-November/389690.html
> ---
>   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 5bb472f1a6..02eb311e09 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);