vhost: introduce last available index for dequeue

So far, we retrieve both the used ring and avail ring idx by the var
last_used_idx; it won't be a problem because the used ring is updated
immediately after those avail entries are consumed.

But that's not true when dequeue zero copy is enabled, that used ring is
updated only when the mbuf is consumed. Thus, we need use another var to
note the last avail ring idx we have consumed.

Therefore, last_avail_idx is introduced.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Qian Xu <qian.q.xu@intel.com>

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 2d5298745f2c239c9c114ef52027ca209b9c8a9a..8565fa1c193a661d96103bd7515190d9e52006b2 100644 (file)
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -70,7 +70,7 @@ struct vhost_virtqueue {
        struct vring_used       *used;
        uint32_t                size;
 
-       /* Last index used on the available ring */
+       uint16_t                last_avail_idx;
        volatile uint16_t       last_used_idx;
 #define VIRTIO_INVALID_EVENTFD         (-1)
 #define VIRTIO_UNINITIALIZED_EVENTFD   (-2)

diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
index e651912c5260c8cdde10dd68c268102e0b8bbed7..a92377a5a3e093c9c837ebf42abebd2fef52bb23 100644 (file)
--- a/lib/librte_vhost/vhost_user.c
+++ b/lib/librte_vhost/vhost_user.c
@@ -343,7 +343,8 @@ vhost_user_set_vring_addr(struct virtio_net *dev, struct vhost_vring_addr *addr)
                        "last_used_idx (%u) and vq->used->idx (%u) mismatches; "
                        "some packets maybe resent for Tx and dropped for Rx\n",
                        vq->last_used_idx, vq->used->idx);
-               vq->last_used_idx     = vq->used->idx;
+               vq->last_used_idx  = vq->used->idx;
+               vq->last_avail_idx = vq->used->idx;
        }
 
        vq->log_guest_addr = addr->log_guest_addr;
@@ -367,7 +368,8 @@ static int
 vhost_user_set_vring_base(struct virtio_net *dev,
                          struct vhost_vring_state *state)
 {
-       dev->virtqueue[state->index]->last_used_idx = state->num;
+       dev->virtqueue[state->index]->last_used_idx  = state->num;
+       dev->virtqueue[state->index]->last_avail_idx = state->num;
 
        return 0;
 }

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index a59c39bb949d557adcdcf766671c469d3e796328..70301a583840562f75771e016280004c2243d268 100644 (file)
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -851,16 +851,17 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
                }
        }
 
-       avail_idx =  *((volatile uint16_t *)&vq->avail->idx);
-       free_entries = avail_idx - vq->last_used_idx;
+       free_entries = *((volatile uint16_t *)&vq->avail->idx) -
+                       vq->last_avail_idx;
        if (free_entries == 0)
                goto out;
 
        LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
 
-       /* Prefetch available ring to retrieve head indexes. */
-       used_idx = vq->last_used_idx & (vq->size - 1);
-       rte_prefetch0(&vq->avail->ring[used_idx]);
+       /* Prefetch available and used ring */
+       avail_idx = vq->last_avail_idx & (vq->size - 1);
+       used_idx  = vq->last_used_idx  & (vq->size - 1);
+       rte_prefetch0(&vq->avail->ring[avail_idx]);
        rte_prefetch0(&vq->used->ring[used_idx]);
 
        count = RTE_MIN(count, MAX_PKT_BURST);
@@ -870,8 +871,9 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 
        /* Retrieve all of the head indexes first to avoid caching issues. */
        for (i = 0; i < count; i++) {
-               used_idx = (vq->last_used_idx + i) & (vq->size - 1);
-               desc_indexes[i] = vq->avail->ring[used_idx];
+               avail_idx = (vq->last_avail_idx + i) & (vq->size - 1);
+               used_idx  = (vq->last_used_idx  + i) & (vq->size - 1);
+               desc_indexes[i] = vq->avail->ring[avail_idx];
 
                vq->used->ring[used_idx].id  = desc_indexes[i];
                vq->used->ring[used_idx].len = 0;
@@ -921,7 +923,8 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
        rte_smp_wmb();
        rte_smp_rmb();
        vq->used->idx += i;
-       vq->last_used_idx += i;
+       vq->last_avail_idx += i;
+       vq->last_used_idx  += i;
        vhost_log_used_vring(dev, vq, offsetof(struct vring_used, idx),
                        sizeof(vq->used->idx));