Re: [dpdk-dev] [PATCH v2 01/22] ethdev: fix MTU size exceeds max rx packet length

Wed, 13 Jan 2021 02:26:07 -0800 

On 1/6/2021 3:36 AM, Yang, SteveX wrote:

Hi Lijun,

Thanks for your review. Please check my comments inline.
Regards,
Steve Yang.


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Subject: Re: [PATCH v2 01/22] ethdev: fix MTU size exceeds max rx packet
length



在 2020/12/17 17:22, Steve Yang 写道:

If max rx packet length is smaller then MTU + Ether overhead, that
will drop all MTU size packets.

Update the MTU size according to the max rx packet and Ether overhead.

Fixes: 59d0ecdbf0e1 ("ethdev: MTU accessors")

Signed-off-by: Steve Yang <stevex.y...@intel.com>
---
   lib/librte_ethdev/rte_ethdev.c | 21 ++++++++++++++++++---
   1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/lib/librte_ethdev/rte_ethdev.c
b/lib/librte_ethdev/rte_ethdev.c index 17ddacc78d..ff6a1e675f 100644
--- a/lib/librte_ethdev/rte_ethdev.c
+++ b/lib/librte_ethdev/rte_ethdev.c
@@ -1292,6 +1292,7 @@ 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;

@@ -1323,6 +1324,15 @@ rte_eth_dev_configure(uint16_t port_id,

uint16_t nb_rx_q, uint16_t nb_tx_q,

   if (ret != 0)
   goto rollback;

+/* Get the real Ethernet overhead length */
+if (dev_info.max_mtu &&
+    dev_info.max_mtu != UINT16_MAX &&
+    dev_info.max_rx_pktlen &&
+    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;

+
   /* 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.
@@ -1410,13 +1420,18 @@ 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 */
+dev->data->mtu = dev->data->dev_conf.rxmode.max_rx_pkt_len -
+overhead_len;
+

Hi
    I think the dev->data->mtu should be updated after configured success. So
the update in this position seems unreasonable.


Good suggestion, I've checked some PMDs, and the corresponding assignments are 
existed within their 'dev_configure_ops'.
For example: [bnxt_ethdev.c # 1100]
```
if (rx_offloads & DEV_RX_OFFLOAD_JUMBO_FRAME) {
eth_dev->data->mtu =
eth_dev->data->dev_conf.rxmode.max_rx_pkt_len -
RTE_ETHER_HDR_LEN - RTE_ETHER_CRC_LEN - VLAN_TAG_SIZE *
BNXT_NUM_VLANS;
bnxt_mtu_set_op(eth_dev, eth_dev->data->mtu);
}
```
Hence, I will move this 'mtu' assignment to line after 
'(*dev->dev_ops->dev_configure)(dev)'.


There is a 'rollback' already for the similar reason.
What do you think to store the old MTU and restore it in the rollback if needed? So you don't need to change where MTU set. 


Actually, it doesn't matter if it is the JUMBO frame, the 'mtu' must keep pace 
with 'max_rx_pkt_len' anytime.
E.g.: if 'mtu' is 1500, and 'max_rx_pkt_len' is set to 1510 by command line, 
that 'mtu' must be reduced to '1510 - 18 = 1492' in 'dev_configure' phase, even 
though it is not a Jumbo frame.



According to the API definition:
 max_rx_pkt_len;  /**< Only used if JUMBO_FRAME enabled. */
The concern was removing this check from the ethdev may break some PMDs that does not follow the API and use the 'max_rx_pkt_len' even if JUMBO frame offload set.
For this release, we can afford to break the PMDs implementing wrong and fix them after testing. 


'max_rx_pkt_len' is only used when jumbo_frame enabled, as follows:
struct rte_eth_rxmode {
.....
uint32_t max_rx_pkt_len;  /**< Only used if JUMBO_FRAME
enabled. */
/** Maximum allowed size of LRO aggregated packet. */
....};

So If DEV_RX_OFFLOAD_JUMBO_FRAME is set to rxmode.offload, driver
should configure mtu to hardware according to 'max_rx_pkt_len' and update
dev->data->mtu. This seems more reasonable. And some PMD drivers are
already doing this.




In addition, validity check for 'max_rx_pkt_len' in
rte_eth_dev_configure API may be error. It should be greater than
'RTE_ETHER_MTU + overhead_len'.
Because driver must enable DEV_RX_OFFLOAD_JUMBO_FRAME when user
set mtu
with greater than 1500 by 'rte_eth_dev_set_mtu' API.



I'm not sure if it is following validity check you mentioned.

+if (pktlen < RTE_ETHER_MIN_MTU + overhead_len ||
+pktlen > RTE_ETHER_MTU + overhead_len)

If yes, no issue here, because the 'rte_eth_dev_configure' is only used for 
initializing device,
and just gives out an initial 'max_rx_pkt_len' value by default. Once user 
tries to change mtu via 'set mtu',
the 'max_rx_pkt_len' will be updated to expected value in the 'mtu_set' ops.
Another aspect, that is the 'disable DEV_RX_OFFLOAD_JUMBO' branch, 
'max_rx_pkt_len' must be limited to
effective max value for non-Jumbo frame.
In the 'disable DEV_RX_OFFLOAD_JUMBO' branch, 'max_rx_pkt_len' should be invalid according API doc as mentioned above, I guess Lijun refers to this. 

Overall what about updating as following, in 'rte_eth_dev_configure()':

if (offloads & DEV_RX_OFFLOAD_JUMBO_FRAME) {
  // max_rx_pkt_len checks
  old_mtu = mtu
  mtu = max_rx_pkt_len - overhead_len
}

...

rollback:
 mtu = old_mtu;



What do you think?

Thanks
Lijun Ou


   /*
    * If LRO is enabled, check that the maximum aggregated packet
    * size is supported by the configured device.

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