[dpdk-dev] [PATCH v2 1/5] lib_vhost: Fix enqueue/dequeue can't handle chained vring descriptors

[Ouyang, Changchun]

> -----Original Message-----
> From: Xie, Huawei
> Sent: Sunday, May 31, 2015 4:41 PM
> To: Ouyang, Changchun; dev at dpdk.org
> Cc: Cao, Waterman
> Subject: Re: [PATCH v2 1/5] lib_vhost: Fix enqueue/dequeue can't handle
> chained vring descriptors
> 
> On 5/28/2015 11:17 PM, Ouyang, Changchun wrote:
> > Vring enqueue need consider the 2 cases:
> >  1. Vring descriptors chained together, the first one is for virtio header, the
> rest are for real
> >     data, virtio driver in Linux usually use this scheme;  2. Only one
> > descriptor, virtio header and real data share one single descriptor, virtio-
> net pmd use
> >     such scheme;
> >
> > So does vring dequeue, it should not assume vring descriptor is
> > chained or not chained, virtio in different Linux version has
> > different behavior, e.g. fedora 20 use chained vring descriptor, while
> fedora 21 use one single vring descriptor for tx.
> >
> > Changes in v2
> >   - drop the uncompleted packet
> >   - refine code logic
> >
> > Signed-off-by: Changchun Ouyang <changchun.ouyang at intel.com>
> > ---
> >  lib/librte_vhost/vhost_rxtx.c | 65
> > +++++++++++++++++++++++++++++++++----------
> >  1 file changed, 50 insertions(+), 15 deletions(-)
> >
> > diff --git a/lib/librte_vhost/vhost_rxtx.c
> > b/lib/librte_vhost/vhost_rxtx.c index 4809d32..06ae2df 100644
> > --- a/lib/librte_vhost/vhost_rxtx.c
> > +++ b/lib/librte_vhost/vhost_rxtx.c
> > @@ -59,7 +59,7 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t
> queue_id,
> >  	struct virtio_net_hdr_mrg_rxbuf virtio_hdr = {{0, 0, 0, 0, 0, 0}, 0};
> >  	uint64_t buff_addr = 0;
> >  	uint64_t buff_hdr_addr = 0;
> > -	uint32_t head[MAX_PKT_BURST], packet_len = 0;
> > +	uint32_t head[MAX_PKT_BURST];
> >  	uint32_t head_idx, packet_success = 0;
> >  	uint16_t avail_idx, res_cur_idx;
> >  	uint16_t res_base_idx, res_end_idx;
> > @@ -113,6 +113,10 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t
> queue_id,
> >  	rte_prefetch0(&vq->desc[head[packet_success]]);
> >
> >  	while (res_cur_idx != res_end_idx) {
> > +		uint32_t offset = 0;
> > +		uint32_t data_len, len_to_cpy;
> > +		uint8_t hdr = 0, uncompleted_pkt = 0;
> > +
> >  		/* Get descriptor from available ring */
> >  		desc = &vq->desc[head[packet_success]];
> >
> > @@ -125,7 +129,6 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t
> > queue_id,
> >
> >  		/* Copy virtio_hdr to packet and increment buffer address */
> >  		buff_hdr_addr = buff_addr;
> > -		packet_len = rte_pktmbuf_data_len(buff) + vq->vhost_hlen;
> >
> >  		/*
> >  		 * If the descriptors are chained the header and data are @@
> > -136,28 +139,55 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t
> queue_id,
> >  			desc = &vq->desc[desc->next];
> >  			/* Buffer address translation. */
> >  			buff_addr = gpa_to_vva(dev, desc->addr);
> > -			desc->len = rte_pktmbuf_data_len(buff);
> Do we got confirm from virtio SPEC that it is OK to only update used->len?

Virtio Spec don't require vhost update desc->len.


> >  		} else {
> >  			buff_addr += vq->vhost_hlen;
> > -			desc->len = packet_len;
> > +			hdr = 1;
> >  		}
> >
> > +		data_len = rte_pktmbuf_data_len(buff);
> > +		len_to_cpy = RTE_MIN(data_len,
> > +			hdr ? desc->len - vq->vhost_hlen : desc->len);
> > +		while (len_to_cpy > 0) {
> > +			/* Copy mbuf data to buffer */
> > +			rte_memcpy((void *)(uintptr_t)buff_addr,
> > +				(const void *)(rte_pktmbuf_mtod(buff,
> const char *) + offset),
> > +				len_to_cpy);
> > +			PRINT_PACKET(dev, (uintptr_t)buff_addr,
> > +				len_to_cpy, 0);
> > +
> > +			offset += len_to_cpy;
> > +
> > +			if (offset == data_len)
> > +				break;
> Ok, i see scatter gather case handling is in patch 5.
> > +
> > +			if (desc->flags & VRING_DESC_F_NEXT) {
> > +				desc = &vq->desc[desc->next];
> > +				buff_addr = gpa_to_vva(dev, desc->addr);
> > +				len_to_cpy = RTE_MIN(data_len - offset,
> desc->len);
> > +			} else {
> > +				/* Room in vring buffer is not enough */
> > +				uncompleted_pkt = 1;
> > +				break;
> > +			}
> > +		};
> > +
> >  		/* Update used ring with desc information */
> >  		vq->used->ring[res_cur_idx & (vq->size - 1)].id =
> >
> 	head[packet_success];
> > -		vq->used->ring[res_cur_idx & (vq->size - 1)].len =
> packet_len;
> >
> > -		/* Copy mbuf data to buffer */
> > -		/* FIXME for sg mbuf and the case that desc couldn't hold the
> mbuf data */
> > -		rte_memcpy((void *)(uintptr_t)buff_addr,
> > -			rte_pktmbuf_mtod(buff, const void *),
> > -			rte_pktmbuf_data_len(buff));
> > -		PRINT_PACKET(dev, (uintptr_t)buff_addr,
> > -			rte_pktmbuf_data_len(buff), 0);
> > +		/* Drop the packet if it is uncompleted */
> > +		if (unlikely(uncompleted_pkt == 1))
> > +			vq->used->ring[res_cur_idx & (vq->size - 1)].len = 0;
> Here things become complicated with the previous lockless reserve.

Why it become complicated? Len = 0 means it contain any meaningful data in the buffer.

> What is the consequence when guest sees zero in used->len? At least, do we
> check with virtio-net implementation?

> 
> > +		else
> > +			vq->used->ring[res_cur_idx & (vq->size - 1)].len =
> > +							offset + vq-
> >vhost_hlen;
> Two questions here,
> 1.  add virtio header len?
> 2.  Why not use packet_len rather than offset?
> >
> >  		res_cur_idx++;
> >  		packet_success++;
> >
> > +		if (unlikely(uncompleted_pkt == 1))
> > +			continue;
> > +
> >  		rte_memcpy((void *)(uintptr_t)buff_hdr_addr,
> >  			(const void *)&virtio_hdr, vq->vhost_hlen);
> >
> > @@ -589,7 +619,14 @@ rte_vhost_dequeue_burst(struct virtio_net *dev,
> uint16_t queue_id,
> >  		desc = &vq->desc[head[entry_success]];
> >
> >  		/* Discard first buffer as it is the virtio header */
> > -		desc = &vq->desc[desc->next];
> > +		if (desc->flags & VRING_DESC_F_NEXT) {
> > +			desc = &vq->desc[desc->next];
> > +			vb_offset = 0;
> > +			vb_avail = desc->len;
> > +		} else {
> > +			vb_offset = vq->vhost_hlen;
> > +			vb_avail = desc->len - vb_offset;
> > +		}
> >
> >  		/* Buffer address translation. */
> >  		vb_addr = gpa_to_vva(dev, desc->addr); @@ -608,8 +645,6
> @@
> > rte_vhost_dequeue_burst(struct virtio_net *dev, uint16_t queue_id,
> >  		vq->used->ring[used_idx].id = head[entry_success];
> >  		vq->used->ring[used_idx].len = 0;
> >
> > -		vb_offset = 0;
> > -		vb_avail = desc->len;
> >  		/* Allocate an mbuf and populate the structure. */
> >  		m = rte_pktmbuf_alloc(mbuf_pool);
> >  		if (unlikely(m == NULL)) {