This patch reorders the code to delay virtio header write to optimize cache
access efficiency for cases where the mrg_rxbuf feature is turned on. It
reduces CPU pipeline stall cycles significantly.
Signed-off-by: Zhihong Wang <zhihong.wang at intel.com>
---
Changes in v3:
1. Remove unnecessary memset which causes frontend stall on SNB & IVB.
2. Rename variables to follow naming convention.
lib/librte_vhost/virtio_net.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)
diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index e9f6353..0086bcb 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -197,6 +197,7 @@ enqueue_packet(struct virtio_net *dev, struct
vhost_virtqueue *vq,
uint32_t mbuf_len;
uint32_t mbuf_avail;
uint32_t cpy_len;
+ uint32_t copy_virtio_hdr;
uint32_t num_buffers = 0;
/* start with the first mbuf of the packet */
@@ -211,12 +212,12 @@ enqueue_packet(struct virtio_net *dev, struct
vhost_virtqueue *vq,
if (unlikely(!desc_addr))
goto error;
- /* handle virtio header */
+ /*
+ * handle virtio header, the actual write operation is delayed
+ * for cache optimization, to reduce CPU pipeline stall cycles.
+ */
virtio_hdr = (struct virtio_net_hdr_mrg_rxbuf *)(uintptr_t)desc_addr;
- virtio_enqueue_offload(mbuf, &(virtio_hdr->hdr));
- if (is_mrg_rxbuf)
- virtio_hdr->num_buffers = 1;
-
+ copy_virtio_hdr = 1;
vhost_log_write(dev, desc->addr, dev->vhost_hlen);
PRINT_PACKET(dev, (uintptr_t)desc_addr, dev->vhost_hlen, 0);
desc_offset = dev->vhost_hlen;
@@ -266,8 +267,15 @@ enqueue_packet(struct virtio_net *dev, struct
vhost_virtqueue *vq,
goto error;
}
- /* copy mbuf data */
+ /* copy virtio header and mbuf data */
cpy_len = RTE_MIN(desc->len - desc_offset, mbuf_avail);
+ if (copy_virtio_hdr) {
+ copy_virtio_hdr = 0;
+ virtio_enqueue_offload(mbuf, &(virtio_hdr->hdr));
+ if (is_mrg_rxbuf)
+ virtio_hdr->num_buffers = num_buffers + 1;
+ }
+
rte_memcpy((void *)(uintptr_t)desc_addr,
rte_pktmbuf_mtod_offset(mbuf, void *,
mbuf_len - mbuf_avail),
--
2.7.4
