[PATCH] net/pcap: support MTU set

[Ido Goshen]

This test https://doc.dpdk.org/dts/test_plans/jumboframes_test_plan.html#test-case-jumbo-frames-with-no-jumbo-frame-support fails for pcap pmd 
Jumbo packet is unexpectedly received and transmitted

----------------------------------------------------------------------------------------
without patch:

root at u18c_3nbp:/home/cgs/workspace/master/jumbo# ./dpdk-testpmd --no-huge -m1024 -l 0-2  --vdev='net_pcap0,rx_pcap=rx_pcap=jumbo_9000.pcap,tx_pcap=file_tx.pcap' -- --no-flush-rx --total-num-mbufs=2048 -i
...
testpmd> start
...
testpmd> show port stats 0

  ######################## NIC statistics for port 0  ########################
  RX-packets: 1          RX-missed: 0          RX-bytes:  8996
  RX-errors: 0
  RX-nombuf:  0         
  TX-packets: 1          TX-errors: 0          TX-bytes:  8996

  Throughput (since last show)
  Rx-pps:            0          Rx-bps:            0
  Tx-pps:            0          Tx-bps:            0
  ############################################################################

----------------------------------------------------------------------------------------
While with the patch it will fail unless --max-pkt-len is used to support jumbo

root at u18c_3nbp:/home/cgs/workspace/master/jumbo# ./dpdk-testpmd-patch --no-huge -m1024 -l 0-2  --vdev='net_pcap0,rx_pcap=rx_pcap=jumbo_9000.pcap,tx_pcap=file_tx.pcap' -- --no-flush-rx --total-num-mbufs=2048 -i
...
testpmd> start
...
testpmd> show port stats 0

  ######################## NIC statistics for port 0  ########################
  RX-packets: 0          RX-missed: 0          RX-bytes:  0
  RX-errors: 1
  RX-nombuf:  0         
  TX-packets: 0          TX-errors: 0          TX-bytes:  0

  Throughput (since last show)
  Rx-pps:            0          Rx-bps:            0
  Tx-pps:            0          Tx-bps:            0
  ############################################################################

root at u18c_3nbp:/home/cgs/workspace/master/jumbo# ./dpdk-testpmd-patch --no-huge -m1024 -l 0-2  --vdev='net_pcap0,rx_pcap=rx_pcap=jumbo_9000.pcap,tx_pcap=file_tx.pcap' -- --no-flush-rx --total-num-mbufs=2048 -i --max-pkt-len 9400
...
testpmd> start
...
testpmd> show port stats 0

  ######################## NIC statistics for port 0  ########################
  RX-packets: 1          RX-missed: 0          RX-bytes:  8996
  RX-errors: 0
  RX-nombuf:  0         
  TX-packets: 1          TX-errors: 0          TX-bytes:  8996

  Throughput (since last show)
  Rx-pps:            0          Rx-bps:            0
  Tx-pps:            0          Tx-bps:            0
  ############################################################################

> -----Original Message-----
> From: Ido Goshen
> Sent: Thursday, 17 March 2022 21:12
> To: Stephen Hemminger <stephen at networkplumber.org>
> Cc: Ferruh Yigit <ferruh.yigit at intel.com>; dev at dpdk.org
> Subject: RE: [PATCH] net/pcap: support MTU set
> 
> As far as I can see the initial device MTU is derived from port *RX* configuration
> in struct rte_eth_rxmode https://doc.dpdk.org/api-
> 21.11/structrte__eth__rxmode.html
> Couple of real NICs I've tested (ixgbe, i40e based) don't allow oversized, tests
> details can be seen in https://bugs.dpdk.org/show_bug.cgi?id=961
> 
> > -----Original Message-----
> > From: Stephen Hemminger <stephen at networkplumber.org>
> > Sent: Thursday, 17 March 2022 20:21
> > To: Ido Goshen <Ido at cgstowernetworks.com>
> > Cc: Ferruh Yigit <ferruh.yigit at intel.com>; dev at dpdk.org
> > Subject: Re: [PATCH] net/pcap: support MTU set
> >
> > On Thu, 17 Mar 2022 19:43:47 +0200
> > ido g <ido at cgstowernetworks.com> wrote:
> >
> > > +		if (unlikely(header.caplen > dev->data->mtu)) {
> > > +			pcap_q->rx_stat.err_pkts++;
> > > +			rte_pktmbuf_free(mbuf);
> > > +			break;
> > > +		}
> >
> > MTU should only be enforced on transmit.
> > Other real network devices allow oversized packets.
> >
> > Since the pcap file is something user provides, if you don't want that
> > then use something to filter the file.