Re: [PATCH v2 3/7] vhost: refactor virtio_dev_merge_rx

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Mon, Mar 07, 2016 at 07:52:22AM +0000, Xie, Huawei wrote:
> On 2/18/2016 9:48 PM, Yuanhan Liu wrote:
> > Current virtio_dev_merge_rx() implementation just looks like the
> > old rte_vhost_dequeue_burst(), full of twisted logic, that you
> > can see same code block in quite many different places.
> >
> > However, the logic of virtio_dev_merge_rx() is quite similar to
> > virtio_dev_rx().  The big difference is that the mergeable one
> > could allocate more than one available entries to hold the data.
> > Fetching all available entries to vec_buf at once makes the
> [...]
> > -	}
> > +static inline uint32_t __attribute__((always_inline))
> > +copy_mbuf_to_desc_mergeable(struct virtio_net *dev, struct vhost_virtqueue *vq,
> > +			    uint16_t res_start_idx, uint16_t res_end_idx,
> > +			    struct rte_mbuf *m)
> > +{
> > +	struct virtio_net_hdr_mrg_rxbuf virtio_hdr = {{0, 0, 0, 0, 0, 0}, 0};
> > +	uint32_t vec_idx = 0;
> > +	uint16_t cur_idx = res_start_idx;
> > +	uint64_t desc_addr;
> > +	uint32_t mbuf_offset, mbuf_avail;
> > +	uint32_t desc_offset, desc_avail;
> > +	uint32_t cpy_len;
> > +	uint16_t desc_idx, used_idx;
> > +	uint32_t nr_used = 0;
> >  
> > -	cpy_len = RTE_MIN(vb_avail, seg_avail);
> > +	if (m == NULL)
> > +		return 0;
> 
> Is this inherited from old code?

Yes.

> Let us remove the unnecessary check.
> Caller ensures it is not NULL.
...
> > +	desc_avail  = vq->buf_vec[vec_idx].buf_len - vq->vhost_hlen;
> > +	desc_offset = vq->vhost_hlen;
> 
> As we know we are in merge-able path, use sizeof(virtio_net_hdr) to save
> one load for the header len.

Please, it's a refactor patch series. You have mentioned quite many
trivial issues here and there, which I don't care too much and I don't
think they would matter somehow. In addition, they are actually from
the old code.

> 
> > +
> > +	mbuf_avail  = rte_pktmbuf_data_len(m);
> > +	mbuf_offset = 0;
> > +	while (1) {
> > +		/* done with current desc buf, get the next one */
> > +
> [...]
> > +		if (reserve_avail_buf_mergeable(vq, pkt_len, &start, &end) < 0)
> > +			break;
> >  
> > +		nr_used = copy_mbuf_to_desc_mergeable(dev, vq, start, end,
> > +						      pkts[pkt_idx]);
> 
> In which case couldn't we get nr_used from start and end?

When pkts[pkt_idx] is NULL, though you suggest to remove it, the check
is here.

	--yliu