From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [PATCH v3 1/4] vmxnet3: restore tx data ring support
Date: Tue, 5 Jan 2016 16:48:49 -0800
Message-ID: <20160105164849.526442a8@xeon-e3>
References: <1452039178-15635-1-git-send-email-yongwang@vmware.com>
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To: Yong Wang <yongwang@vmware.com>
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On Tue,  5 Jan 2016 16:12:55 -0800
Yong Wang <yongwang@vmware.com> wrote:

> @@ -365,6 +366,14 @@ vmxnet3_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts,
>  			break;
>  		}
>  
> +		if (rte_pktmbuf_pkt_len(txm) <= VMXNET3_HDR_COPY_SIZE) {
> +			struct Vmxnet3_TxDataDesc *tdd;
> +
> +			tdd = txq->data_ring.base + txq->cmd_ring.next2fill;
> +			copy_size = rte_pktmbuf_pkt_len(txm);
> +			rte_memcpy(tdd->data, rte_pktmbuf_mtod(txm, char *), copy_size);
> +		}

Good idea to use a local region which optmizes the copy in the host,
but this implementation needs to be more general.

As written it is broken for multi-segment packets. A multi-segment
packet will have a pktlen >= datalen as in:
  m -> mb_segs=3, pktlen=1200, datalen=200
    -> datalen=900
    -> datalen=100

There are two ways to fix this. You could test for nb_segs == 1
or better yet. Optimize each segment it might be that the first
segment (or tail segment) would fit in the available data area.