Re: [dpdk-dev] [PATCH] net/af_packet: try to reinsert the stripped vlan tag

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 8/20/2021 1:46 PM, Tudor Cornea wrote:
> The af_packet pmd driver binds to a raw socket and allows
> sending and receiving of packets through the kernel.
> 
> Since commit bcc6d47903 [1], the kernel strips the vlan tags early in
> __netif_receive_skb_core(), so we receive untagged packets while
> running with the af_packet pmd.
> 
> Luckily for us, the skb vlan-related fields are still populated from the
> stripped vlan tags, so we end up having all the information
> that we need in the mbuf.
> 
> We would like to have the the vlan tag inside the mbuf.
> Let's take a shot at it by trying to reinsert the stripped vlan tag.
> 

PMD already sets 'mbuf->vlan_tci' and 'PKT_RX_VLAN | PKT_RX_VLAN_STRIPPED'
flags, so application can be aware of the vlan tag and can consume it.

Inserting the vlan tag back to packet is costly, what is the motivation to do so?

> As a side note, something similar was done for the netvsc pmd.
> 
> [1] https://github.com/torvalds/linux/commit/bcc6d47903612c3861201cc3a866fb604f26b8b2
> 
> Signed-off-by: Tudor Cornea <tudor.cornea@gmail.com>
> ---
>  drivers/net/af_packet/rte_eth_af_packet.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/net/af_packet/rte_eth_af_packet.c b/drivers/net/af_packet/rte_eth_af_packet.c
> index b73b211..d116583 100644
> --- a/drivers/net/af_packet/rte_eth_af_packet.c
> +++ b/drivers/net/af_packet/rte_eth_af_packet.c
> @@ -148,6 +148,10 @@ eth_af_packet_rx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
>  		if (ppd->tp_status & TP_STATUS_VLAN_VALID) {
>  			mbuf->vlan_tci = ppd->tp_vlan_tci;
>  			mbuf->ol_flags |= (PKT_RX_VLAN | PKT_RX_VLAN_STRIPPED);
> +
> +			/* the kernel always strips the vlan tag, try to reinsert it */
> +			if (rte_vlan_insert(&mbuf))
> +				PMD_LOG(ERR, "Failed to reinsert vlan tag");
>  		}
>  
>  		/* release incoming frame and advance ring buffer */
>