Re: [dpdk-dev] [PATCH v3 1/3] ethdev: fix MTU doesn't update when jumbo frame disabled

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 1/25/2021 9:49 AM, Yang, SteveX wrote:
> Hi Huisong,
> 
> Thanks for your review.
> 
> The validity of the pair <DEV_RX_OFFLOAD_JUMBO_FRAME, max_rx_pkt_len> should be 
> checked from application layer (e.g.: testpmd),
> 
> and the RTE layer should keep open enough to adapt the high-layer requirement.
> 
> I’m not sure if exists some applications/NICs that treat ‘packet size < 1500’ as 
> JUMBO_FRAME. If so, that also can work as expect with current code.
> 
> @Yigit, Ferruh <mailto:ferruh.yigit@intel.com>, please correct me if something 
> understand wrong.
> 

Hi Huisong,

Agree that there is a grey area in the API, the question is if 'JUMBO_FRAME' is 
set, can application set the 'max_rx_pkt_len' less than "RTE_ETHER_MTU + 
overhead_len". Lijun (cc'ed) has the same concern.

The API documentation, and checks in the 'rte_eth_dev_configure()' enables 
setting this for a long time, I am reluctant to add this limitation now.
Although agree that application should set 'JUMBO_FRAME' properly based on 
requested 'MTU' value.


> BTW, there perhaps are some confused problems about jumbo frame and 
> max_rx_pkt_len, and Ferruh has scheduled to re-factor this part at release 21.11.
> 
> If you’re interesting about it, please refer to following link: [RFC,v2] doc: 
> announce max Rx packet len field deprecation - Patchwork (dpdk.org) 
> <http://patches.dpdk.org/patch/84522/>
> 
> Thanks & Regards,
> 
> Steve Yang.
> 
> *From:* Huisong Li <lihuisong@huawei.com>
> *Sent:* Monday, January 25, 2021 3:12 PM
> *To:* Yang, SteveX <stevex.yang@intel.com>; dev@dpdk.org
> *Cc:* Lu, Wenzhuo <wenzhuo.lu@intel.com>; Li, Xiaoyun <xiaoyun.li@intel.com>; 
> Iremonger, Bernard <bernard.iremonger@intel.com>; thomas@monjalon.net; Yigit, 
> Ferruh <ferruh.yigit@intel.com>; andrew.rybchenko@oktetlabs.ru; Yang, Qiming 
> <qiming.yang@intel.com>; oulijun@huawei.com; huangdaode@huawei.com
> *Subject:* Re: [dpdk-dev] [PATCH v3 1/3] ethdev: fix MTU doesn't update when 
> jumbo frame disabled
> 
> Hi Steve,
> 
> In the current modification, the MTU is updated based on 'max_rx_pkt_len' 
> regardless of whether jumbo frame is enabled.
> 
> Now, MTU is correct when jumbo frmae is disabled. However, when jumbo frame is 
> enabled, the MTU value may be inconsistent with
> 
> the definition of the enabled jumbo frame. Like:
> 
> 1/ DEV_RX_OFFLOAD_JUMBO_FRAME is set;
> 
> 2/ max_rx_pkt_len = 1200
> 
> 3/ dev->data->mtu = 1200 - overhead_len(18) = 1182
> 
> In rte_eth_dev_configure API, the check for 'max_rx_pkt_len' is as follows:
> 
> if (dev_conf->rxmode.offloads & DEV_RX_OFFLOAD_JUMBO_FRAME) {  //jumbo frame enabled
>          if (dev_conf->rxmode.max_rx_pkt_len > dev_info.max_rx_pktlen) {
>              xxxx
>              goto rollback;
>          } else if (*dev_conf->rxmode.max_rx_pkt_len < RTE_ETHER_MIN_LEN*) {
>              xxxx
>              goto rollback;
>          }
> } else { //jumbo frame disabled
> 
>          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_MTU + overhead_len;
> 
> }
> 
> Since the applicatin sets DEV_RX_OFFLOAD_JUMBO_FRAME to enable jumbo frame, and 
> the framework API needs to update
> 
> the MTU based on 'max_rx_pkt_len', but the framework API uses 
> *RTE_ETHER_MIN_LEN(64)* to verify the boundary value of
> 
> 'max_rx_pkt_len', instead of "RTE_ETHER_MTU + overhead_len".  As far as I know, 
> if the applicatin sets DEV_RX_OFFLOAD_JUMBO_FRAME
> 
> and 'max_rx_pkt_len' is 1200, the framework API or driver should return a 
> failure. As mentioned in this patch set, the jumbo frame
> 
> offload is set only when 'max_rx_pkt_len' requested is greater than 
> "RTE_ETHER_MTU + eth_overhead" in testpmd.
> 
> I really don't understand it.  How do you understand this behavior?
> 
> Thanks.
> 
> 在 2021/1/22 17:01, Steve Yang 写道:
> 
>     The MTU value should be updated to 'max_rx_pkt_len - overhead'
> 
>     no matter if the JUMBO FRAME offload enabled. If not update this MTU,
> 
>     use will get the wrong MTU info via some command.
> 
>     E.g.: 'show port info all' in testpmd tool.
> 
>     Actually, the 'max_rx_pkt_len' has been used for other purposes in many
> 
>     places now, even though the 'max_rx_pkt_len' is expected 'Only used if
> 
>     JUMBO_FRAME enabled'.
> 
>     For examples,
> 
>     'max_rx_pkt_len' perhaps can be used as the 'rx_ctx.rxmax' in i40e.
> 
>     Fixes: bf0f90d92d30 ("ethdev: fix max Rx packet length check")
> 
>     Signed-off-by: Steve Yang<stevex.yang@intel.com>  <mailto:stevex.yang@intel.com>
> 
>     ---
> 
>       lib/librte_ethdev/rte_ethdev.c | 8 ++++----
> 
>       1 file changed, 4 insertions(+), 4 deletions(-)
> 
>     diff --git a/lib/librte_ethdev/rte_ethdev.c b/lib/librte_ethdev/rte_ethdev.c
> 
>     index daf5f24f7e..42857e3b67 100644
> 
>     --- a/lib/librte_ethdev/rte_ethdev.c
> 
>     +++ b/lib/librte_ethdev/rte_ethdev.c
> 
>     @@ -1421,10 +1421,6 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t nb_tx_q,
> 
>                              ret = -EINVAL;
> 
>                              goto rollback;
> 
>                      }
> 
>     -
> 
>     -               /* Scale the MTU size to adapt max_rx_pkt_len */
> 
>     -               dev->data->mtu = dev->data->dev_conf.rxmode.max_rx_pkt_len -
> 
>     -                              overhead_len;
> 
>              } else {
> 
>                      uint16_t pktlen = dev_conf->rxmode.max_rx_pkt_len;
> 
>                      if (pktlen < RTE_ETHER_MIN_MTU + overhead_len ||
> 
>     @@ -1434,6 +1430,10 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t nb_tx_q,
> 
>                                                     RTE_ETHER_MTU + overhead_len;
> 
>              }
> 
>       
> 
>     +       /* Scale the MTU size to adapt max_rx_pkt_len */
> 
>     +       dev->data->mtu = dev->data->dev_conf.rxmode.max_rx_pkt_len -
> 
>     +                              overhead_len;
> 
>     +
> 
>              /*
> 
>               * If LRO is enabled, check that the maximum aggregated packet
> 
>               * size is supported by the configured device.
>