Re: [dpdk-dev] [PATCH 2/5] net/tap: do not touch Tx offload flags

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

On Thu, Apr 01, 2021 at 11:52:40AM +0200, David Marchand wrote:
> Tx offload flags are of the application responsibility.
> Leave the mbuf alone and check for TSO where needed.
> 
> Signed-off-by: David Marchand <david.marchand@redhat.com>

Maybe the problem being solved should be better described in the commit
log. Is it a problem (other than cosmetic) to touch a mbuf in the Tx
function of a driver, where we could expect that the mbuf is owned by
the driver?

The only problem I can think about is in case we transmit a direct mbuf
whose refcnt is increased, but I wonder how much this is really
supported: for instance, several drivers add vlans using
rte_vlan_insert() in their Tx path.


> ---
>  drivers/net/tap/rte_eth_tap.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/tap/rte_eth_tap.c b/drivers/net/tap/rte_eth_tap.c
> index c36d4bf76e..285fe395c5 100644
> --- a/drivers/net/tap/rte_eth_tap.c
> +++ b/drivers/net/tap/rte_eth_tap.c
> @@ -562,6 +562,7 @@ tap_tx_l3_cksum(char *packet, uint64_t ol_flags, unsigned int l2_len,
>  		uint16_t *l4_phdr_cksum, uint32_t *l4_raw_cksum)
>  {
>  	void *l3_hdr = packet + l2_len;
> +	uint64_t csum_l4;
>  
>  	if (ol_flags & (PKT_TX_IP_CKSUM | PKT_TX_IPV4)) {
>  		struct rte_ipv4_hdr *iph = l3_hdr;
> @@ -571,13 +572,17 @@ tap_tx_l3_cksum(char *packet, uint64_t ol_flags, unsigned int l2_len,
>  		cksum = rte_raw_cksum(iph, l3_len);
>  		iph->hdr_checksum = (cksum == 0xffff) ? cksum : ~cksum;
>  	}
> -	if (ol_flags & PKT_TX_L4_MASK) {
> +
> +	csum_l4 = ol_flags & PKT_TX_L4_MASK;
> +	if (ol_flags & PKT_TX_TCP_SEG)
> +		csum_l4 |= PKT_TX_TCP_CKSUM;
> +	if (csum_l4) {
>  		void *l4_hdr;
>  
>  		l4_hdr = packet + l2_len + l3_len;
> -		if ((ol_flags & PKT_TX_L4_MASK) == PKT_TX_UDP_CKSUM)
> +		if (csum_l4 == PKT_TX_UDP_CKSUM)
>  			*l4_cksum = &((struct rte_udp_hdr *)l4_hdr)->dgram_cksum;
> -		else if ((ol_flags & PKT_TX_L4_MASK) == PKT_TX_TCP_CKSUM)
> +		else if (csum_l4 == PKT_TX_TCP_CKSUM)
>  			*l4_cksum = &((struct rte_tcp_hdr *)l4_hdr)->cksum;
>  		else
>  			return;
> @@ -648,7 +653,8 @@ tap_write_mbufs(struct tx_queue *txq, uint16_t num_mbufs,
>  		if (txq->csum &&
>  		    ((mbuf->ol_flags & (PKT_TX_IP_CKSUM | PKT_TX_IPV4) ||
>  		     (mbuf->ol_flags & PKT_TX_L4_MASK) == PKT_TX_UDP_CKSUM ||
> -		     (mbuf->ol_flags & PKT_TX_L4_MASK) == PKT_TX_TCP_CKSUM))) {
> +		     (mbuf->ol_flags & PKT_TX_L4_MASK) == PKT_TX_TCP_CKSUM) ||
> +		     (mbuf->ol_flags & PKT_TX_TCP_SEG))) {
>  			is_cksum = 1;
>  
>  			/* Support only packets with at least layer 4
> @@ -742,9 +748,6 @@ pmd_tx_burst(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
>  		if (tso) {
>  			struct rte_gso_ctx *gso_ctx = &txq->gso_ctx;
>  
> -			/* TCP segmentation implies TCP checksum offload */
> -			mbuf_in->ol_flags |= PKT_TX_TCP_CKSUM;
> -
>  			/* gso size is calculated without RTE_ETHER_CRC_LEN */
>  			hdrs_len = mbuf_in->l2_len + mbuf_in->l3_len +
>  					mbuf_in->l4_len;
> -- 
> 2.23.0
>