Hello Oliver,
Please see my 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.
>
You are right. I'll update the patch comment in v3.
> > 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?
>
Correct. Inner checksum is offloaded and outer computed in software.
Consider this example:
Tunneled packet arrived at port A and being forwarded through port B.
The packet arrived at port A with correct inner checksums - L3 and L4.
Port B TX offloads inner L3 only.
process_inner_cksums() sets "ipv4_hdr->hdr_checksum = 0;" unconditionally.
Inner L3 checksum value will be restored by port B TX checksum offload, but
when
process_outer_cksums() runs software calculation on outer L4 it will use 0 and
produce wrong result.
Therefore, the patch zeros inner checksum values only before actual software
calculations.
> > 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: sta...@dpdk.org
> >
> > Signed-off-by: Gregory Etelson <getel...@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
> >
