Re: [dpdk-dev] [PATCH v2] app/testpmd: fix TX checksum calculation for tunnel

[Olivier Matz <olivier.matz@6wind.com>]

Hi Gregory,

Few comments below.

On Tue, Jul 27, 2021 at 04:07:57PM +0300, Gregory Etelson wrote:
> TX checksum of a tunnelled packet can be calculated for outer headers
> only or for both outer and inner parts. The calculation method is
> determined by application.
> If TX checksum calculation can be offloaded, hardware ignores
> existing checksum value and replaces it with an updated result.

This is not always true. Actually, the checksum value is optionally
set by software to the value that is expected by the hardware to offload
the checksum correctly. This is done through rte_eth_tx_prepare(), which
is called in csumonly test engine.

For instance, on an ixgbe NIC, it does:

  rte_eth_tx_prepare()
    eth_dev->tx_pkt_prepare()
      ixgbe_prep_pkts()
        rte_net_intel_cksum_flags_prepare()
          if packet is IP, set IP checksum to 0
          if packet is TCP or UDP, set L4 checksum to the phdr csum

This driver-specific rte_eth_tx_prepare() can indeed do nothing and let
the hardware ignore the checksum in the packet.

> If TX checksum is calculated by a software, existing value must be
> zeroed first.
> The testpmd checksum forwarding engine always zeroed inner checksums.
> If inner checksum calculation was offloaded, that header was left
> with 0 checksum value.
> Following outer software checksum calculation produced wrong value.
> The patch zeroes inner IPv4 checksum only before software calculation.

Sorry, I think I don't understand the issue. Are you trying to compute
the inner checksum by hardware and the outer checksum by software?

> Fixes: 51f694dd40f5 ("app/testpmd: rework checksum forward engine")

I'm not sure the problem origin is this commit (however, I may have
misunderstood your issue).

At the time this commit was done, it was required to set the TCP/UDP
checksum to the pseudo header checksum to offload an L4 checksum. See:
https://git.dpdk.org/dpdk/tree/lib/librte_mbuf/rte_mbuf.h?id=51f694dd40f5#n107

The introduction of rte_eth_tx_prepare() API removed this need, see:
https://git.dpdk.org/dpdk/commit/?id=6b520d54ebfe

Thanks,
Olivier

> Cc: stable@dpdk.org
> 
> Signed-off-by: Gregory Etelson <getelson@nvidia.com>
> ---
> v2: 
>  remove blank line between Fixes and Cc
>  explicitly compare with 0 value in `if ()` 
> ---
>  app/test-pmd/csumonly.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/app/test-pmd/csumonly.c b/app/test-pmd/csumonly.c
> index 0161f72175..bd5ad64a57 100644
> --- a/app/test-pmd/csumonly.c
> +++ b/app/test-pmd/csumonly.c
> @@ -480,17 +480,18 @@ process_inner_cksums(void *l3_hdr, const struct testpmd_offload_info *info,
>  
>  	if (info->ethertype == _htons(RTE_ETHER_TYPE_IPV4)) {
>  		ipv4_hdr = l3_hdr;
> -		ipv4_hdr->hdr_checksum = 0;
>  
>  		ol_flags |= PKT_TX_IPV4;
>  		if (info->l4_proto == IPPROTO_TCP && tso_segsz) {
>  			ol_flags |= PKT_TX_IP_CKSUM;
>  		} else {
> -			if (tx_offloads & DEV_TX_OFFLOAD_IPV4_CKSUM)
> +			if (tx_offloads & DEV_TX_OFFLOAD_IPV4_CKSUM) {
>  				ol_flags |= PKT_TX_IP_CKSUM;
> -			else
> +			} else if (ipv4_hdr->hdr_checksum != 0) {
> +				ipv4_hdr->hdr_checksum = 0;
>  				ipv4_hdr->hdr_checksum =
>  					rte_ipv4_cksum(ipv4_hdr);
> +			}
>  		}
>  	} else if (info->ethertype == _htons(RTE_ETHER_TYPE_IPV6))
>  		ol_flags |= PKT_TX_IPV6;
> @@ -501,10 +502,10 @@ process_inner_cksums(void *l3_hdr, const struct testpmd_offload_info *info,
>  		udp_hdr = (struct rte_udp_hdr *)((char *)l3_hdr + info->l3_len);
>  		/* do not recalculate udp cksum if it was 0 */
>  		if (udp_hdr->dgram_cksum != 0) {
> -			udp_hdr->dgram_cksum = 0;
> -			if (tx_offloads & DEV_TX_OFFLOAD_UDP_CKSUM)
> +			if (tx_offloads & DEV_TX_OFFLOAD_UDP_CKSUM) {
>  				ol_flags |= PKT_TX_UDP_CKSUM;
> -			else {
> +			} else {
> +				udp_hdr->dgram_cksum = 0;
>  				udp_hdr->dgram_cksum =
>  					get_udptcp_checksum(l3_hdr, udp_hdr,
>  						info->ethertype);
> @@ -514,12 +515,12 @@ process_inner_cksums(void *l3_hdr, const struct testpmd_offload_info *info,
>  			ol_flags |= PKT_TX_UDP_SEG;
>  	} else if (info->l4_proto == IPPROTO_TCP) {
>  		tcp_hdr = (struct rte_tcp_hdr *)((char *)l3_hdr + info->l3_len);
> -		tcp_hdr->cksum = 0;
>  		if (tso_segsz)
>  			ol_flags |= PKT_TX_TCP_SEG;
> -		else if (tx_offloads & DEV_TX_OFFLOAD_TCP_CKSUM)
> +		else if (tx_offloads & DEV_TX_OFFLOAD_TCP_CKSUM) {
>  			ol_flags |= PKT_TX_TCP_CKSUM;
> -		else {
> +		} else if (tcp_hdr->cksum != 0) {
> +			tcp_hdr->cksum = 0;
>  			tcp_hdr->cksum =
>  				get_udptcp_checksum(l3_hdr, tcp_hdr,
>  					info->ethertype);
> @@ -529,13 +530,13 @@ process_inner_cksums(void *l3_hdr, const struct testpmd_offload_info *info,
>  	} else if (info->l4_proto == IPPROTO_SCTP) {
>  		sctp_hdr = (struct rte_sctp_hdr *)
>  			((char *)l3_hdr + info->l3_len);
> -		sctp_hdr->cksum = 0;
>  		/* sctp payload must be a multiple of 4 to be
>  		 * offloaded */
>  		if ((tx_offloads & DEV_TX_OFFLOAD_SCTP_CKSUM) &&
>  			((ipv4_hdr->total_length & 0x3) == 0)) {
>  			ol_flags |= PKT_TX_SCTP_CKSUM;
> -		} else {
> +		} else if (sctp_hdr->cksum != 0) {
> +			sctp_hdr->cksum = 0;
>  			/* XXX implement CRC32c, example available in
>  			 * RFC3309 */
>  		}
> -- 
> 2.32.0
>