Re: [dpdk-dev] [PATCH] nfp: handle packets with length 0 as usual ones

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

On 8/11/2017 11:05 AM, Alejandro Lucero wrote:
> A DPDK app could, whatever the reason, send packets with size 0.
> The PMD is not sending those packets, which does make sense,
> but the problem is the mbuf is not released either. That leads
> to mbufs not being available, because the app trusts the
> PMD will do it.
> 
> Although this is a problem related to app wrong behaviour, we
> should harden the PMD in this regard. Not sending a packet with
> size 0 could be problematic, needing special handling inside the
> PMD xmit function. It could be a burst of those packets, which can
> be easily handled, but it could also be a single packet in a burst,
> what is harder to handle.
> 
> It would be simpler to just send that kind of packets, which will
> likely be dropped by the hw at some point. The main problem is how
> the fw/hw handles the DMA, because a dma read to a hypothetical 0x0
> address could trigger an IOMMU error. It turns out, it is safe to
> send a descriptor with packet size 0 to the hardware: the DMA never
> happens, from the PCIe point of view.
> 
> Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com>
> ---
>  drivers/net/nfp/nfp_net.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/nfp/nfp_net.c b/drivers/net/nfp/nfp_net.c
> index 92b03c4..679a91b 100644
> --- a/drivers/net/nfp/nfp_net.c
> +++ b/drivers/net/nfp/nfp_net.c
> @@ -2094,7 +2094,7 @@ uint32_t nfp_net_txq_full(struct nfp_net_txq *txq)
>  		 */
>  		pkt_size = pkt->pkt_len;
>  
> -		while (pkt_size) {
> +		while (pkt) {
>  			/* Copying TSO, VLAN and cksum info */
>  			*txds = txd;
>  
> @@ -2126,17 +2126,24 @@ uint32_t nfp_net_txq_full(struct nfp_net_txq *txq)
>  				txq->wr_p = 0;
>  
>  			pkt_size -= dma_size;
> -			if (!pkt_size) {
> +			if (!pkt_size)
>  				/* End of packet */
>  				txds->offset_eop |= PCIE_DESC_TX_EOP;
> -			} else {
> +			else
>  				txds->offset_eop &= PCIE_DESC_TX_OFFSET_MASK;
> -				pkt = pkt->next;
> -			}
> +
> +			pkt = pkt->next;
>  			/* Referencing next free TX descriptor */
>  			txds = &txq->txds[txq->wr_p];
>  			lmbuf = &txq->txbufs[txq->wr_p].mbuf;
>  			issued_descs++;
> +
> +			/* Double-checking if we have to use chained mbuf.
> +			 * It seems there are some apps which could wrongly
> +			 * have zeroed mbufs chained leading to send null
> +			 * descriptors to the hw. */
> +			if (!pkt_size)
> +				break;

For the case chained mbufs with all are zero size [1], won't this cause
next mbufs not freed because rte_pktmbuf_free_seg(*lmbuf) used?

[1]
As you mentioned in the commit log, this not correct thing to do, but
since patch is trying to harden PMD for this wrong application behavior..

>  		}
>  		i++;
>  	}
>