Re: [RFC v4 4/5] virtio_ring: add event idx support in packed ring

On 2018年05月16日 20:58, Tiwei Bie wrote:

On Wed, May 16, 2018 at 08:17:21PM +0800, Jason Wang wrote:

quoted

On 2018年05月16日 16:37, Tiwei Bie wrote:

[...]

quoted

quoted

@@ -1160,15 +1186,27 @@ static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
   static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
   {
   	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 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_USED_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. */
+
+	wrap_counter = vq->wrap_counter;
+	if (vq->last_used_idx > vq->next_avail_idx)

Should this be ">=" consider rx refill may try to completely fill the ring?

It seems that there are two cases that last_used_idx
equals to next_avail_idx. The first one is that the
ring is empty. And the second one is that the ring
is full. Although in the first case, most probably,
the driver won't enable the interrupt.

Maybe I really should track the used_wrap_counter
instead of calculating it each time I need it.. I'll
give it a try..

Right, good to know and this will match spec sample code.

Thanks