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

["Xie, Huawei" <huawei.xie@intel.com>]

On 3/7/2016 4:36 PM, Yuanhan Liu wrote:
> 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.

For normal code, it would be better using vq->vhost_hlen for example for
future compatibility. For DPDK, we don't waste cycles whenever possible,
especially vhost is the centralized bottleneck.
For the check of m == NULL, it should be removed, which not only
occupies unnecessary branch predication resource but also causes
confusion for return nr_used from copy_mbuf_to_desc_mergeable.
It is OK if you don't want to fix this in this patchset.

>>> +
>>> +	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
>