[dpdk-dev] [PATCH 2/3] net/virtio: allocate fake mbuf in Rx queue

[Maxime Coquelin]

While it is worth clarifying whether the fake mbuf
in virtnet_rx struct is really necessary, it is sure
that it heavily impacts cache usage by being part of
the struct. Indeed, it takes uses cachelines, and
requires alignement on a cacheline.

Before this series, it means it took 120 bytes in
virtnet_rx struct:

struct virtnet_rx {
	struct virtqueue *         vq;                   /*     0     8 */

	/* XXX 56 bytes hole, try to pack */

	/* --- cacheline 1 boundary (64 bytes) --- */
	struct rte_mbuf            fake_mbuf __attribute__((__aligned__(64))); /*    64   128 */
	/* --- cacheline 3 boundary (192 bytes) --- */

This patch allocates it using malloc in order to optimize
virtnet_rx cache usage and so virtqueue cache usage.

Signed-off-by: Maxime Coquelin <maxime.coquelin at redhat.com>
---
 drivers/net/virtio/virtio_ethdev.c | 10 ++++++++++
 drivers/net/virtio/virtio_rxtx.c   |  8 +++-----
 drivers/net/virtio/virtio_rxtx.h   |  2 +-
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/virtio_ethdev.c
index 297c01a70d..a1351b36ca 100644
--- a/drivers/net/virtio/virtio_ethdev.c
+++ b/drivers/net/virtio/virtio_ethdev.c
@@ -539,6 +539,7 @@ virtio_init_queue(struct rte_eth_dev *dev, uint16_t queue_idx)
 	}
 
 	if (queue_type == VTNET_RQ) {
+		struct rte_mbuf *fake_mbuf;
 		size_t sz_sw = (RTE_PMD_VIRTIO_RX_MAX_BURST + vq_size) *
 			       sizeof(vq->sw_ring[0]);
 
@@ -550,10 +551,18 @@ virtio_init_queue(struct rte_eth_dev *dev, uint16_t queue_idx)
 			goto fail_q_alloc;
 		}
 
+		fake_mbuf = malloc(sizeof(*fake_mbuf));
+		if (!fake_mbuf) {
+			PMD_INIT_LOG(ERR, "can not allocate fake mbuf");
+			ret = -ENOMEM;
+			goto fail_q_alloc;
+		}
+
 		vq->sw_ring = sw_ring;
 		rxvq = &vq->rxq;
 		rxvq->port_id = dev->data->port_id;
 		rxvq->mz = mz;
+		rxvq->fake_mbuf = fake_mbuf;
 	} else if (queue_type == VTNET_TQ) {
 		txvq = &vq->txq;
 		txvq->port_id = dev->data->port_id;
@@ -636,6 +645,7 @@ virtio_free_queues(struct virtio_hw *hw)
 
 		queue_type = virtio_get_queue_type(hw, i);
 		if (queue_type == VTNET_RQ) {
+			free(vq->rxq.fake_mbuf);
 			rte_free(vq->sw_ring);
 			rte_memzone_free(vq->rxq.mz);
 		} else if (queue_type == VTNET_TQ) {
diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
index 1fcce36cbd..d147d7300a 100644
--- a/drivers/net/virtio/virtio_rxtx.c
+++ b/drivers/net/virtio/virtio_rxtx.c
@@ -703,11 +703,9 @@ virtio_dev_rx_queue_setup_finish(struct rte_eth_dev *dev, uint16_t queue_idx)
 		virtio_rxq_vec_setup(rxvq);
 	}
 
-	memset(&rxvq->fake_mbuf, 0, sizeof(rxvq->fake_mbuf));
-	for (desc_idx = 0; desc_idx < RTE_PMD_VIRTIO_RX_MAX_BURST;
-	     desc_idx++) {
-		vq->sw_ring[vq->vq_nentries + desc_idx] =
-			&rxvq->fake_mbuf;
+	memset(rxvq->fake_mbuf, 0, sizeof(*rxvq->fake_mbuf));
+	for (desc_idx = 0; desc_idx < RTE_PMD_VIRTIO_RX_MAX_BURST; desc_idx++) {
+		vq->sw_ring[vq->vq_nentries + desc_idx] = rxvq->fake_mbuf;
 	}
 
 	if (hw->use_vec_rx && !virtio_with_packed_queue(hw)) {
diff --git a/drivers/net/virtio/virtio_rxtx.h b/drivers/net/virtio/virtio_rxtx.h
index 7f1036be6f..6ce5d67d15 100644
--- a/drivers/net/virtio/virtio_rxtx.h
+++ b/drivers/net/virtio/virtio_rxtx.h
@@ -19,7 +19,7 @@ struct virtnet_stats {
 
 struct virtnet_rx {
 	/* dummy mbuf, for wraparound when processing RX ring. */
-	struct rte_mbuf fake_mbuf;
+	struct rte_mbuf *fake_mbuf;
 	uint64_t mbuf_initializer; /**< value to init mbufs. */
 	struct rte_mempool *mpool; /**< mempool for mbuf allocation */
 
-- 
2.29.2