Re: [dpdk-dev] [PATCH v3 01/22] ethdev: fix MTU size exceeds max rx packet length

[oulijun <oulijun@huawei.com>]

Hi Steve
This is a very good job! But I have some question and suggestions.
Please check it.

在 2021/1/14 17:45, Steve Yang 写道:
> Ethdev is using default Ethernet overhead to decide if provided
> 'max_rx_pkt_len' value is bigger than max (non jumbo) MTU value,
> and limits it to MAX if it is.
> 
> Since the application/driver used Ethernet overhead is different than
> the ethdev one, check result is wrong.
> 
> If the driver is using Ethernet overhead bigger than the default one,
> the provided 'max_rx_pkt_len' is trimmed down, and in the driver when
> correct Ethernet overhead is used to convert back, the resulting MTU is
> less than the intended one, causing some packets to be dropped.
> 
> Like,
> app     -> max_rx_pkt_len = 1500/*mtu*/ + 22/*overhead*/ = 1522
> ethdev  -> 1522 > 1518/*MAX*/; max_rx_pkt_len = 1518
> driver  -> MTU = 1518 - 22 = 1496
> Packets with size 1497-1500 are dropped although intention is to be able
> to send/receive them.
> 
> The fix is to make ethdev use the correct Ethernet overhead for port,
> instead of default one.
> 
> Fixes: 59d0ecdbf0e1 ("ethdev: MTU accessors")
> 
> Signed-off-by: Steve Yang <stevex.yang@intel.com>
> ---
>   lib/librte_ethdev/rte_ethdev.c | 27 ++++++++++++++++++++++++---
>   1 file changed, 24 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/librte_ethdev/rte_ethdev.c b/lib/librte_ethdev/rte_ethdev.c
> index 17ddacc78d..19ca4c4512 100644
> --- a/lib/librte_ethdev/rte_ethdev.c
> +++ b/lib/librte_ethdev/rte_ethdev.c
> @@ -1292,8 +1292,10 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t nb_tx_q,
>   	struct rte_eth_dev *dev;
>   	struct rte_eth_dev_info dev_info;
>   	struct rte_eth_conf orig_conf;
> +	uint16_t overhead_len;
>   	int diag;
>   	int ret;
> +	uint16_t old_mtu;
>   
>   	RTE_ETH_VALID_PORTID_OR_ERR_RET(port_id, -ENODEV);
>   
> @@ -1319,10 +1321,20 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t nb_tx_q,
>   		memcpy(&dev->data->dev_conf, dev_conf,
>   		       sizeof(dev->data->dev_conf));
>   
> +	/* Backup mtu for rollback */
> +	old_mtu = dev->data->mtu;
> +
>   	ret = rte_eth_dev_info_get(port_id, &dev_info);
>   	if (ret != 0)
>   		goto rollback;
>   
> +	/* Get the real Ethernet overhead length */
> +	if (dev_info.max_mtu != UINT16_MAX &&
> +	    dev_info.max_rx_pktlen > dev_info.max_mtu)
> +		overhead_len = dev_info.max_rx_pktlen - dev_info.max_mtu;
> +	else
> +		overhead_len = RTE_ETHER_HDR_LEN + RTE_ETHER_CRC_LEN;
> +
The Ethernet frame header length supported by each NIC may be different, 
which cause the difference of allowed packets size and drop you say.
The diference of Ethernet frame header length have an impact for the 
maximum Ethernet frame length, which is a boundary value
of enabling jumbo frame through the ' max_rx_pkt_len '.
However, we need to do the same thing you do above to get the 
overhead_len every time, which will
cause a lot of duplicate code in the framework and app. For examples, 
parsing and processing for '--max-pkt-len=xxx' parameter,
  and " cmd_config_max_pkt_len_parsed " in testpmd, and here modifying 
here dev_configure API.
It's a little redundant and troublesome.

Maybe, it is necessary for driver to directly report the supported 
maximum Ethernet frame length by rte_dev_info_get API.
As following:
struct rte_eth_dev_info {
	xxxx
	/**
	 * The maximum Ethernet frame length supported by each
	 * driver varies with the Ethernet header length.
	 */
	uint16_t eth_max_len;
	xxxx
}

int
rte_eth_dev_info_get(uint16_t port_id, struct rte_eth_dev_info *dev_info)
{
     xxx
	dev_info->min_mtu = RTE_ETHER_MIN_MTU;
	dev_info->max_mtu = UINT16_MAX;
	dev_info->eth_max_len = RTE_ETHER_MAX_LEN;
     xxx
}

And then:
xxx_devv_infos_get(struct rte_eth_dev *eth_dev, struct rte_eth_dev_info 
*info)
{
	xxx
	info->eth_max_len = xxx_ETHER_MAX _LEN;
	xxx
}
What do you think?
>   	/* If number of queues specified by application for both Rx and Tx is
>   	 * zero, use driver preferred values. This cannot be done individually
>   	 * as it is valid for either Tx or Rx (but not both) to be zero.
> @@ -1410,11 +1422,18 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t nb_tx_q,
>   			goto rollback;
>   		}
>   	} else {
> -		if (dev_conf->rxmode.max_rx_pkt_len < RTE_ETHER_MIN_LEN ||
> -			dev_conf->rxmode.max_rx_pkt_len > RTE_ETHER_MAX_LEN)
> +		uint16_t pktlen = dev_conf->rxmode.max_rx_pkt_len;
> +		if (pktlen < RTE_ETHER_MIN_MTU + overhead_len ||
> +		    pktlen > RTE_ETHER_MTU + overhead_len)
>   			/* Use default value */
>   			dev->data->dev_conf.rxmode.max_rx_pkt_len =
> -							RTE_ETHER_MAX_LEN;
> +						RTE_ETHER_MTU + overhead_len;
> +	}
> +
> +	/* Scale the MTU size to adapt max_rx_pkt_len */
> +	if (dev_conf->rxmode.offloads & DEV_RX_OFFLOAD_JUMBO_FRAME) {
> +		dev->data->mtu = dev->data->dev_conf.rxmode.max_rx_pkt_len -
> +				overhead_len;
>   	}
Now that we update mtu here when jumbo frame enabled. It is necessary to 
check validity of max_rx_pkt_len.
As following:
if (dev_conf->rxmode.offloads & DEV_RX_OFFLOAD_JUMBO_FRAME) {
	if (dev_conf->rxmode.max_rx_pkt_len > dev_info.max_rx_pktlen ||
		dev_conf->rxmode.max_rx_pkt_len <=  RTE_ETHER_MTU + overhead_len) {
			xxxx
		}
} else {
		xxx
}

If it does not thing above, which will cause an unreasonable case.
Like,
dev_conf->rxmode.offloads & DEV_RX_OFFLOAD_JUMBO_FRAME == 1
dev_conf->rxmode.max_rx_pkt_len = 1500
overhead_len = 26
dev->data->mtu = dev_conf->rxmode.max_rx_pkt_len - overhead_len = 1500 - 
26 = 1474

In fact, DEV_RX_OFFLOAD_JUMBO_FRAME is set to rxmode.offloads when mtu > 
1500.
>   
>   	/*
> @@ -1549,6 +1568,8 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t nb_tx_q,
>   	eth_dev_tx_queue_config(dev, 0);
>   rollback:
>   	memcpy(&dev->data->dev_conf, &orig_conf, sizeof(dev->data->dev_conf));
> +	if (old_mtu != dev->data->mtu)
> +		dev->data->mtu = old_mtu;
>   
>   	rte_ethdev_trace_configure(port_id, nb_rx_q, nb_tx_q, dev_conf, ret);
>   	return ret;
>