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

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

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.

> +			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).