[dpdk-dev] [RFC] vhost: Add indirect descriptors support to the TX path

[Maxime Coquelin <maxime.coquelin@redhat.com>]

Indirect descriptors are usually supported by virtio-net devices,
allowing to dispatch a large number of large requests.

When the virtio device sends a packet using indirect descriptors,
only one slot is used in the ring, even for large packets.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
I have a two questions regarding the implementation of this feature:

1. Should I add a check to ensure the indirect feature is supported
(i.e. the negociation succeeded) when having an indirect desc?

2. Should I check in copy_desc_to_mbuf() that we don't have a nested
indirect descriptor?

Both these sanity checks are recommended from the virtio spec, but
since it is to be done in the hot path, it may introduce some
performance penalties.

Note that the first check is not done in the Kernel vhost driver, whereas
the second one is.

Cheers,
Maxime
---
 lib/librte_vhost/vhost_rxtx.c | 31 ++++++++++++++++++++++---------
 lib/librte_vhost/virtio-net.c |  3 ++-
 2 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
index 15ca956..74cbc75 100644
--- a/lib/librte_vhost/vhost_rxtx.c
+++ b/lib/librte_vhost/vhost_rxtx.c
@@ -669,8 +669,8 @@ make_rarp_packet(struct rte_mbuf *rarp_mbuf, const struct ether_addr *mac)
 }
 
 static inline int __attribute__((always_inline))
-copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
-		  struct rte_mbuf *m, uint16_t desc_idx,
+copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
+		  uint16_t max_desc, struct rte_mbuf *m, uint16_t desc_idx,
 		  struct rte_mempool *mbuf_pool)
 {
 	struct vring_desc *desc;
@@ -683,7 +683,7 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	/* A counter to avoid desc dead loop chain */
 	uint32_t nr_desc = 1;
 
-	desc = &vq->desc[desc_idx];
+	desc = &descs[desc_idx];
 	if (unlikely(desc->len < dev->vhost_hlen))
 		return -1;
 
@@ -698,7 +698,7 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	 */
 	if (likely((desc->len == dev->vhost_hlen) &&
 		   (desc->flags & VRING_DESC_F_NEXT) != 0)) {
-		desc = &vq->desc[desc->next];
+		desc = &descs[desc->next];
 
 		desc_addr = gpa_to_vva(dev, desc->addr);
 		rte_prefetch0((void *)(uintptr_t)desc_addr);
@@ -731,10 +731,10 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 			if ((desc->flags & VRING_DESC_F_NEXT) == 0)
 				break;
 
-			if (unlikely(desc->next >= vq->size ||
-				     ++nr_desc >= vq->size))
+			if (unlikely(desc->next >= max_desc ||
+				     ++nr_desc >= max_desc))
 				return -1;
-			desc = &vq->desc[desc->next];
+			desc = &descs[desc->next];
 
 			desc_addr = gpa_to_vva(dev, desc->addr);
 			rte_prefetch0((void *)(uintptr_t)desc_addr);
@@ -859,19 +859,32 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 	/* Prefetch descriptor index. */
 	rte_prefetch0(&vq->desc[desc_indexes[0]]);
 	for (i = 0; i < count; i++) {
+		struct vring_desc *desc;
+		uint16_t size, idx;
 		int err;
 
 		if (likely(i + 1 < count))
 			rte_prefetch0(&vq->desc[desc_indexes[i + 1]]);
 
+		if (vq->desc[desc_indexes[i]].flags & VRING_DESC_F_INDIRECT) {
+			desc = (struct vring_desc *)gpa_to_vva(dev,
+					vq->desc[desc_indexes[i]].addr);
+			rte_prefetch0(desc);
+			size = vq->desc[desc_indexes[i]].len / sizeof(*desc);
+			idx = 0;
+		} else {
+			desc = vq->desc;
+			size = vq->size;
+			idx = desc_indexes[i];
+		}
+
 		pkts[i] = rte_pktmbuf_alloc(mbuf_pool);
 		if (unlikely(pkts[i] == NULL)) {
 			RTE_LOG(ERR, VHOST_DATA,
 				"Failed to allocate memory for mbuf.\n");
 			break;
 		}
-		err = copy_desc_to_mbuf(dev, vq, pkts[i], desc_indexes[i],
-					mbuf_pool);
+		err = copy_desc_to_mbuf(dev, desc, size, pkts[i], idx, mbuf_pool);
 		if (unlikely(err)) {
 			rte_pktmbuf_free(pkts[i]);
 			break;
diff --git a/lib/librte_vhost/virtio-net.c b/lib/librte_vhost/virtio-net.c
index 1785695..0304812 100644
--- a/lib/librte_vhost/virtio-net.c
+++ b/lib/librte_vhost/virtio-net.c
@@ -76,7 +76,8 @@ struct virtio_net_device_ops const *notify_ops;
 				(1ULL << VIRTIO_NET_F_CSUM)    | \
 				(1ULL << VIRTIO_NET_F_GUEST_CSUM) | \
 				(1ULL << VIRTIO_NET_F_GUEST_TSO4) | \
-				(1ULL << VIRTIO_NET_F_GUEST_TSO6))
+				(1ULL << VIRTIO_NET_F_GUEST_TSO6) | \
+				(1ULL << VIRTIO_RING_F_INDIRECT_DESC))
 
 static uint64_t VHOST_FEATURES = VHOST_SUPPORTED_FEATURES;

-- 
2.7.4