[RFC 1/2] virtio: expose used buffers

[Guo Zhi]

Follow VIRTIO 1.1 spec, we can only writing out a single used ring for a
batch of descriptors, and only notify guest when the batch of
descriptors have all been used.

We do that batch for tx, because the driver doesn't need to know the
length of tx buffer, while for rx, we don't apply the batch strategy.

Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
---
 hw/net/virtio-net.c | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index dd0d056f..c8e83921 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -2542,8 +2542,10 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
     VirtIONet *n = q->n;
     VirtIODevice *vdev = VIRTIO_DEVICE(n);
     VirtQueueElement *elem;
+    VirtQueueElement *elems[VIRTQUEUE_MAX_SIZE];
     int32_t num_packets = 0;
     int queue_index = vq2q(virtio_get_queue_index(q->tx_vq));
+    size_t j;
     if (!(vdev->status & VIRTIO_CONFIG_S_DRIVER_OK)) {
         return num_packets;
     }
@@ -2621,14 +2623,35 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
         }
 
 drop:
-        virtqueue_push(q->tx_vq, elem, 0);
-        virtio_notify(vdev, q->tx_vq);
-        g_free(elem);
+        if (!virtio_vdev_has_feature(vdev, VIRTIO_F_IN_ORDER)) {
+            virtqueue_push(q->tx_vq, elem, 0);
+            virtio_notify(vdev, q->tx_vq);
+            g_free(elem);
+        } else {
+            elems[num_packets] = elem;
+        }
 
         if (++num_packets >= n->tx_burst) {
             break;
         }
     }
+
+    if (virtio_vdev_has_feature(vdev, VIRTIO_F_IN_ORDER) && num_packets) {
+        /**
+         * If in order feature negotiated, devices can notify the use of a 
batch
+         * of buffers to the driver by only writing out a single used ring 
entry
+         * with the id corresponding to the head entry of the descriptor chain
+         * describing the last buffer in the batch.
+         */
+        virtqueue_fill(q->tx_vq, elems[num_packets - 1], 0, 0);
+        for (j = 0; j < num_packets; j++) {
+            g_free(elems[j]);
+        }
+
+        virtqueue_flush(q->tx_vq, num_packets);
+        virtio_notify(vdev, q->tx_vq);
+    }
+
     return num_packets;
 }
 
-- 
2.17.1