Re: [PATCH] net/tap: fix L4 checksum

[David Marchand <david.marchand@redhat.com>]

On Tue, Aug 22, 2023 at 10:55 AM Olivier Matz <olivier.matz@6wind.com> wrote:
>
> Hi David,
>
> On Tue, Aug 22, 2023 at 09:32:44AM +0200, David Marchand wrote:
> > The L4 checksum offloading API does not require l4_len to be set.
> > Make the driver discover the L4 headers size by itself.
> >
> > Fixes: 6546e76056e3 ("net/tap: calculate checksums of multi segs packets")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: David Marchand <david.marchand@redhat.com>
> > Tested-by: Ales Musil <amusil@redhat.com>
> > ---
> >  .mailmap                      |  1 +
> >  drivers/net/tap/rte_eth_tap.c | 13 +++++++++++--
> >  2 files changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/.mailmap b/.mailmap
> > index 864d33ee46..b6a21b35cb 100644
> > --- a/.mailmap
> > +++ b/.mailmap
> > @@ -40,6 +40,7 @@ Aleksandr Loktionov <aleksandr.loktionov@intel.com>
> >  Aleksandr Miloshenko <a.miloshenko@f5.com>
> >  Aleksey Baulin <aleksey.baulin@gmail.com>
> >  Aleksey Katargin <gureedo@gmail.com>
> > +Ales Musil <amusil@redhat.com>
> >  Alexander Bechikov <asb.tyum@gmail.com>
> >  Alexander Belyakov <abelyako@gmail.com>
> >  Alexander Chernavin <achernavin@netgate.com>
> > diff --git a/drivers/net/tap/rte_eth_tap.c b/drivers/net/tap/rte_eth_tap.c
> > index bf98f75559..0ab214847a 100644
> > --- a/drivers/net/tap/rte_eth_tap.c
> > +++ b/drivers/net/tap/rte_eth_tap.c
> > @@ -645,13 +645,22 @@ tap_write_mbufs(struct tx_queue *txq, uint16_t num_mbufs,
> >                   ((mbuf->ol_flags & (RTE_MBUF_F_TX_IP_CKSUM | RTE_MBUF_F_TX_IPV4) ||
> >                     (mbuf->ol_flags & RTE_MBUF_F_TX_L4_MASK) == RTE_MBUF_F_TX_UDP_CKSUM ||
> >                     (mbuf->ol_flags & RTE_MBUF_F_TX_L4_MASK) == RTE_MBUF_F_TX_TCP_CKSUM))) {
>
> While looking at the patch, I noticed this line:
>
>   mbuf->ol_flags & (RTE_MBUF_F_TX_IP_CKSUM | RTE_MBUF_F_TX_IPV4)
>
> I think only RTE_MBUF_F_TX_IP_CKSUM should be checked.

And tap_tx_l3_cksum is wrong too:
if (ol_flags & (RTE_MBUF_F_TX_IP_CKSUM | RTE_MBUF_F_TX_IPV4)) {

This is a separate issue, I'll send another patch.

>
> > +                     unsigned int l4_len = 0;
> > +
> >                       is_cksum = 1;
> >
> > +                     if ((mbuf->ol_flags & RTE_MBUF_F_TX_L4_MASK) ==
> > +                                     RTE_MBUF_F_TX_UDP_CKSUM)
> > +                             l4_len = sizeof(struct rte_udp_hdr);
> > +                     else if ((mbuf->ol_flags & RTE_MBUF_F_TX_L4_MASK) ==
> > +                                     RTE_MBUF_F_TX_TCP_CKSUM)
> > +                             l4_len = sizeof(struct rte_tcp_hdr);
> > +
> >                       /* Support only packets with at least layer 4
> >                        * header included in the first segment
> >                        */
> >                       seg_len = rte_pktmbuf_data_len(mbuf);
> > -                     l234_hlen = mbuf->l2_len + mbuf->l3_len + mbuf->l4_len;
> > +                     l234_hlen = mbuf->l2_len + mbuf->l3_len + l4_len;
> >                       if (seg_len < l234_hlen)
> >                               return -1;
> >
> > @@ -661,7 +670,7 @@ tap_write_mbufs(struct tx_queue *txq, uint16_t num_mbufs,
> >                       rte_memcpy(m_copy, rte_pktmbuf_mtod(mbuf, void *),
> >                                       l234_hlen);
> >                       tap_tx_l3_cksum(m_copy, mbuf->ol_flags,
> > -                                    mbuf->l2_len, mbuf->l3_len, mbuf->l4_len,
> > +                                    mbuf->l2_len, mbuf->l3_len, l4_len,
> >                                      &l4_cksum, &l4_phdr_cksum,
> >                                      &l4_raw_cksum);
> >                       iovecs[k].iov_base = m_copy;
> > --
> > 2.41.0
> >
>
> Using rte_ipv4_udptcp_cksum() in this code would probably simplify it, and may
> solve other issues (for instance the 0 checksum for UDP which has a special
> meaning).

I agree such a rework would make the code easier to read, and may
solve other issues.
But I prefer to keep my original fix as is, and do what you propose as
a followup patch.


-- 
David Marchand