From e3b56cdd4351f0e227d4d847eeadff4c82aef1b9 Mon Sep 17 00:00:00 2001 From: Jason Wang Date: Tue, 7 Feb 2017 15:49:50 +0800 Subject: [PATCH] vhost: try avoiding avail index access when getting descriptor If last avail idx is not equal to cached avail idx, we're sure there's still available buffers in the virtqueue so there's no need to re-read avail idx. So let's skip this to avoid unnecessary userspace memory access and memory barrier. Pktgen test show about 3% improvement on rx pps. Signed-off-by: Jason Wang Signed-off-by: Michael S. Tsirkin --- drivers/vhost/vhost.c | 39 +++++++++++++++++++++++---------------- 1 file changed, 23 insertions(+), 16 deletions(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 8f99fe08de02..1f7e4e4e6f8e 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -1930,25 +1930,32 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq, /* Check it isn't doing very strange things with descriptor numbers. */ last_avail_idx = vq->last_avail_idx; - if (unlikely(vhost_get_user(vq, avail_idx, &vq->avail->idx))) { - vq_err(vq, "Failed to access avail idx at %p\n", - &vq->avail->idx); - return -EFAULT; - } - vq->avail_idx = vhost16_to_cpu(vq, avail_idx); - if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) { - vq_err(vq, "Guest moved used index from %u to %u", - last_avail_idx, vq->avail_idx); - return -EFAULT; - } + if (vq->avail_idx == vq->last_avail_idx) { + if (unlikely(vhost_get_user(vq, avail_idx, &vq->avail->idx))) { + vq_err(vq, "Failed to access avail idx at %p\n", + &vq->avail->idx); + return -EFAULT; + } + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); - /* If there's nothing new since last we looked, return invalid. */ - if (vq->avail_idx == last_avail_idx) - return vq->num; + if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) { + vq_err(vq, "Guest moved used index from %u to %u", + last_avail_idx, vq->avail_idx); + return -EFAULT; + } + + /* If there's nothing new since last we looked, return + * invalid. + */ + if (vq->avail_idx == last_avail_idx) + return vq->num; - /* Only get avail ring entries after they have been exposed by guest. */ - smp_rmb(); + /* Only get avail ring entries after they have been + * exposed by guest. + */ + smp_rmb(); + } /* Grab the next descriptor number they're advertising, and increment * the index we've seen. */ -- 2.20.1