From e928fd0bb0118387f04811970b11504b199b5ed1 Mon Sep 17 00:00:00 2001 From: Huawei Xie Date: Wed, 27 Apr 2016 04:53:58 -0400 Subject: [PATCH] virtio: optimize avail ring update Avail ring is updated by the frontend and consumed by the backend. There are frequent core to core cache transfers for the avail ring. This optmization avoids avail ring entry index update if the entry already holds the same value. As DPDK virtio PMD implements FIFO free descriptor list (also for performance reason of CACHE), in which descriptors are allocated from the head and freed to the tail, with this patch in most cases avail ring will remain the same, then it would be valid in both caches of frontend and backend. Suggested-by: Michael S. Tsirkin Signed-off-by: Huawei Xie Acked-by: Yuanhan Liu --- drivers/net/virtio/virtqueue.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h index 4e9239e0b7..8c46a83d56 100644 --- a/drivers/net/virtio/virtqueue.h +++ b/drivers/net/virtio/virtqueue.h @@ -302,7 +302,8 @@ vq_update_avail_ring(struct virtqueue *vq, uint16_t desc_idx) * descriptor. */ avail_idx = (uint16_t)(vq->vq_avail_idx & (vq->vq_nentries - 1)); - vq->vq_ring.avail->ring[avail_idx] = desc_idx; + if (unlikely(vq->vq_ring.avail->ring[avail_idx] != desc_idx)) + vq->vq_ring.avail->ring[avail_idx] = desc_idx; vq->vq_avail_idx++; } -- 2.20.1