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

["Zhang, Qi Z" <qi.z.zhang@intel.com>]

> -----Original Message-----
> From: Xing, Beilei
> Sent: Thursday, October 25, 2018 2:41 AM
> To: Zhang, Qi Z <qi.z.zhang@intel.com>
> Cc: dev@dpdk.org; stable@dpdk.org
> Subject: [PATCH] net/i40e: fix Rx instability with vector mode
> 
> 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>
> ---
>  doc/guides/nics/i40e.rst     |  7 +++++++
>  drivers/net/i40e/i40e_rxtx.c | 18 +++++++++++++++++-
>  2 files changed, 24 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..aa46d83 100644
> --- a/drivers/net/i40e/i40e_rxtx.c
> +++ b/drivers/net/i40e/i40e_rxtx.c
> @@ -1735,10 +1735,17 @@ 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.
> +		 */
> +		if ((rxq->nb_rx_desc & (rxq->nb_rx_desc - 1)))
> +			ad->rx_vec_allowed = false;
> +		else
> +			ad->rx_vec_allowed = true;
>  		i40e_set_rx_function(dev);
>  		return 0;
>  	}
> @@ -1811,6 +1818,15 @@ 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 ((nb_desc & (nb_desc - 1)))
> +		ad->rx_vec_allowed = false;
> +	else
> +		ad->rx_vec_allowed = true;

rx_vec_allowed will be overwritten unexpectedly for multi-queues case

We may only to overwrite with below condition check.

If (first_queue || 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