RE: [PATCH] app/test-pmd: fix L4 checksum with padding data

["Deng, KaiwenX" <kaiwenx.deng@intel.com>]

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit@amd.com>
> Sent: Wednesday, November 15, 2023 3:10 AM
> To: Deng, KaiwenX <kaiwenx.deng@intel.com>; dev@dpdk.org
> Cc: stable@dpdk.org; Yang, Qiming <qiming.yang@intel.com>; Zhou, YidingX
> <yidingx.zhou@intel.com>; Singh, Aman Deep <aman.deep.singh@intel.com>;
> Zhang, Yuying <yuying.zhang@intel.com>; Matz, Olivier
> <olivier.matz@6wind.com>; De Lara Guarch, Pablo
> <pablo.de.lara.guarch@intel.com>
> Subject: Re: [PATCH] app/test-pmd: fix L4 checksum with padding data
> 
> On 11/14/2023 2:19 AM, Deng, KaiwenX wrote:
> >
> >
> >> -----Original Message-----
> >> From: Ferruh Yigit <ferruh.yigit@amd.com>
> >> Sent: Friday, November 3, 2023 12:03 PM
> >> To: Deng, KaiwenX <kaiwenx.deng@intel.com>; dev@dpdk.org
> >> Cc: stable@dpdk.org; Yang, Qiming <qiming.yang@intel.com>; Zhou,
> >> YidingX <yidingx.zhou@intel.com>; Singh, Aman Deep
> >> <aman.deep.singh@intel.com>; Zhang, Yuying <yuying.zhang@intel.com>;
> >> Matz, Olivier <olivier.matz@6wind.com>; De Lara Guarch, Pablo
> >> <pablo.de.lara.guarch@intel.com>
> >> Subject: Re: [PATCH] app/test-pmd: fix L4 checksum with padding data
> >>
> >> On 11/3/2023 2:49 AM, Deng, KaiwenX wrote:
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: Ferruh Yigit <ferruh.yigit@amd.com>
> >>>> Sent: Friday, November 3, 2023 3:20 AM
> >>>> To: Deng, KaiwenX <kaiwenx.deng@intel.com>; dev@dpdk.org
> >>>> Cc: stable@dpdk.org; Yang, Qiming <qiming.yang@intel.com>; Zhou,
> >>>> YidingX <yidingx.zhou@intel.com>; Singh, Aman Deep
> >>>> <aman.deep.singh@intel.com>; Zhang, Yuying
> >>>> <yuying.zhang@intel.com>; Matz, Olivier <olivier.matz@6wind.com>;
> >>>> De Lara Guarch, Pablo <pablo.de.lara.guarch@intel.com>
> >>>> Subject: Re: [PATCH] app/test-pmd: fix L4 checksum with padding
> >>>> data
> >>>>
> >>>> On 8/4/2023 9:28 AM, Kaiwen Deng wrote:
> >>>>> IEEE 802 packets may have a minimum size limit. The data fields
> >>>>> should be padded when necessary. In some cases, the padding data
> >>>>> is not
> >> zero.
> >>>>> Testpmd does not trim these IP packets to the true length of the
> >>>>> frame, so errors will occur when calculating TCP or UDP checksum.
> >>>>>
> >>>>
> >>>> Hi Kaiwen,
> >>>>
> >>>> I am trying to understand the problem, what is the testcase that
> >>>> has checksum error?
> >>>>
> >>>> Are the received mbuf data_len & pkt_len wrong? Instead of trying
> >>>> to fix the mbuf during forwarding, can we fix where packet generated?
> >>>>
> >>> Hi Ferruh,
> >>>
> >>> In effect, the packet is padded by the switch.
> >>> IEEE 802 packets may have a minimum size limit. The data fields
> >>> should be padded by switch when necessary. In some switches, the
> >>> padding data is
> >> not zero.
> >>>
> >>> Csumonly doesn't trim these packets to the true length of the frame.
> >>> In csumonly, the received mbuf data_len is the true length of the
> >>> packet plus
> >> the padding data len.
> >>> Therefore, padding data is included in the checksum calculation.
> >>> When the padding data is not zero, the checksum is wrong.
> >>>
> > Hi,
> > Sorry for late reply.
> > The minimum frame length specified by IEEE 802.3 is 64 bytes. In
> > practice, there are many packets less than 64 bytes that are padding through
> the switch.
> >
> > We found this issue because some customers found that their packets
> > could not calculate checksum correctly, they would send some packets
> > less than 64 bytes, but our app didn't strip the padding data for such
> packets.
> >
> 
> OK, so switch in between is padding packets to make them compatible with
> standard.
> 
> From DPDK application perspective received packet is 64 bytes, right?
> Problem happens because where verifies the checksum gets different
> checksum that expected, but this is because packet is modified in between by
> the networking setup.
> I am not sure about trying to fix this in the testpmd.
> 
> Why not send packets that are >= 64 bytes from sender side, or configure
> switch to not add padding or maybe use different switch?
> 
If we send a 40 bytes UDP packet, it will be padded to 64 bytes as it passes through 
the switch, whereas the Linux kernel stack strips out the padding data as it receives 
the packet. 
I think maybe DPDK applications should be aligned with the Linux kernel. 
Otherwise the csumonly application only supports packets above 64 bytes.
> 
> >>
> >> Thanks for clarification.
> >>
> >> Even some non-zero padding added, it will calculate the csum
> >> successfully, but I assume in this case csum becomes different than
> >> expected csum and test fails?
> >>
> >> In this case why not fix the generated packets, and make them
> >> compatible to minimum size requirement? What is generating packets?
> >>
> >>
> >>>>> This commit fixes this issue by triming IP packets to the true
> >>>>> length of the frame in testpmd.
> >>>>>
> >>>>> Fixes: 03d17e4d0179 ("app/testpmd: do not change IP addrs in
> >>>>> checksum
> >>>>> engine")
> >>>>> Cc: stable@dpdk.org
> >>>>>
> >>>>> Signed-off-by: Kaiwen Deng <kaiwenx.deng@intel.com>
> >>>>> ---
> >>>>>  app/test-pmd/csumonly.c | 32
> >> ++++++++++++++++++++++++++++++++
> >>>>>  1 file changed, 32 insertions(+)
> >>>>>
> >>>>> diff --git a/app/test-pmd/csumonly.c b/app/test-pmd/csumonly.c
> >>>>> index 7af635e3f7..58b72b714a 100644
> >>>>> --- a/app/test-pmd/csumonly.c
> >>>>> +++ b/app/test-pmd/csumonly.c
> >>>>> @@ -853,12 +853,14 @@ pkt_burst_checksum_forward(struct
> >>>> fwd_stream *fs)
> >>>>>  	uint16_t nb_rx;
> >>>>>  	uint16_t nb_prep;
> >>>>>  	uint16_t i;
> >>>>> +	uint16_t pad_len;
> >>>>>  	uint64_t rx_ol_flags, tx_ol_flags;
> >>>>>  	uint64_t tx_offloads;
> >>>>>  	uint32_t rx_bad_ip_csum;
> >>>>>  	uint32_t rx_bad_l4_csum;
> >>>>>  	uint32_t rx_bad_outer_l4_csum;
> >>>>>  	uint32_t rx_bad_outer_ip_csum;
> >>>>> +	uint32_t l3_off;
> >>>>>  	struct testpmd_offload_info info;
> >>>>>
> >>>>>  	/* receive a burst of packet */
> >>>>> @@ -980,6 +982,36 @@ pkt_burst_checksum_forward(struct
> >> fwd_stream
> >>>> *fs)
> >>>>>  			l3_hdr = (char *)l3_hdr + info.outer_l3_len +
> >>>> info.l2_len;
> >>>>>  		}
> >>>>>
> >>>>> +		if (info.is_tunnel) {
> >>>>> +			l3_off = info.outer_l2_len +
> >>>>> +					info.outer_l3_len +
> >>>>> +					info.l2_len;
> >>>>> +		} else {
> >>>>> +			l3_off = info.l2_len;
> >>>>> +		}
> >>>>> +		switch (info.ethertype) {
> >>>>> +		case _htons(RTE_ETHER_TYPE_IPV4):
> >>>>> +			pad_len = rte_pktmbuf_data_len(m) -
> >>>>> +					(l3_off +
> >>>>> +					rte_be_to_cpu_16(
> >>>>> +					((struct rte_ipv4_hdr *)l3_hdr)-
> >>>>> total_length));
> >>>>> +			break;
> >>>>> +		case _htons(RTE_ETHER_TYPE_IPV6):
> >>>>> +			pad_len = rte_pktmbuf_data_len(m) -
> >>>>> +					(l3_off +
> >>>>> +					rte_be_to_cpu_16(
> >>>>> +					((struct rte_ipv6_hdr *)l3_hdr)-
> >>>>> payload_len));
> >>>>> +			break;
> >>>>> +		default:
> >>>>> +			pad_len = 0;
> >>>>> +			break;
> >>>>> +		}
> >>>>> +
> >>>>> +		if (pad_len) {
> >>>>> +			rte_pktmbuf_data_len(m) =
> >>>> rte_pktmbuf_data_len(m) - pad_len;
> >>>>> +			rte_pktmbuf_pkt_len(m) = rte_pktmbuf_data_len(m);
> >>>>> +		}
> >>>>> +
> >>>>>  		/* step 2: depending on user command line configuration,
> >>>>>  		 * recompute checksum either in software or flag the
> >>>>>  		 * mbuf to offload the calculation to the NIC. If TSO
> >>>
> >