Re: [RFC v6 4/5] virtio_ring: add event idx support in packed ring

[Jason Wang]

On 2018年06月05日 15:40, Tiwei Bie wrote:
  static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
  {
        struct vring_virtqueue *vq = to_vvq(_vq);
+       u16 bufs, used_idx, wrap_counter;
  
  	START_USE(vq);
  
  	/* We optimistically turn back on interrupts, then check if there was
         * more to do. */
+       /* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
+        * either clear the flags bit or point the event index at the next
+        * entry. Always update the event index to keep code simple. */
+

Maybe for packed ring, it's time to treat event index separately to 
avoid a virtio_wmb() for event idx is off.

+       /* TODO: tune this threshold */
+       if (vq->next_avail_idx < vq->last_used_idx)
+               bufs = (vq->vring_packed.num + vq->next_avail_idx -
+                               vq->last_used_idx) * 3 / 4;
+       else
+               bufs = (vq->next_avail_idx - vq->last_used_idx) * 3 / 4;

vq->next_avail-idx could be equal to vq->last_usd_idx when the ring is 
full. Though virito-net is the only user now and it can guarantee this 
won't happen. But consider this is a core API, we should make sure it 
can work for any cases.

It looks to me that bufs is just vq->vring_packed.num - vq->num_free?

+
+       wrap_counter = vq->used_wrap_counter;
+
+       used_idx = vq->last_used_idx + bufs;
+       if (used_idx >= vq->vring_packed.num) {
+               used_idx -= vq->vring_packed.num;
+               wrap_counter ^= 1;
+       }
+
+       vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
+                       used_idx | (wrap_counter << 15));
  
  	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
-               vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
+               /* We need to update event offset and event wrap
+                * counter first before updating event flags. */
+               virtio_wmb(vq->weak_barriers);
+               vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
+                                                    VRING_EVENT_F_ENABLE;
                vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
                                                        vq->event_flags_shadow);
-               /* We need to enable interrupts first before re-checking
-                * for more used buffers. */
-               virtio_mb(vq->weak_barriers);
        }
  
+	/* We need to update event suppression structure first
+        * before re-checking for more used buffers. */
+       virtio_mb(vq->weak_barriers);
+
        if (more_used_packed(vq)) {
                END_USE(vq);
                return false;

I think what we need to to make sure the descriptor used_idx is used? 
Otherwise we may stop and restart qdisc too frequently?

Thanks

--