lkl: Fix netperf TCP_STREAM server hangs

The netserver hangs after haijacking in TCP_STREAM test.

To reproduce:
$ LKL_HIJACK_NET_TAP=lkl_ptt1 LKL_HIJACK_NET_IP=192.168.20.2 LKL_HIJACK_NET_NETMASK_LEN=24 LKL_HIJACK_DEBUG=1  ./bin/lkl-hijack.sh netserver -D -f
$ echo "set -e && for i in \`seq 1000\`;do echo Test:\$i; netperf -L
192.168.20.1 -H 192.168.20.2 ; done" > /tmp/test.sh
$ sh /tmp/test.sh

The netperf may fail with error "recv_response_timed_n: no response
received. errno 113 counter 0" after hundreds of runs.

This fix make virtio trigger an irq whenever the avail queue is empty.

Signed-off-by: Yuan Liu <liuyuan@google.com>

tools/lkl/lib/virtio.c

@@ -71,6 +71,7 @@ void virtio_req_complete(struct virtio_req *req, uint32_t len)
 	struct virtio_dev *dev = req->dev;
 	uint16_t idx = le16toh(q->used->idx) & (q->num - 1);
 	uint16_t new = le16toh(q->used->idx) + 1;
+	int send_irq = 0;
 
 	q->used->ring[idx].id = htole16(req->idx);
 	q->used->ring[idx].len = htole16(len);
@@ -83,6 +84,13 @@ void virtio_req_complete(struct virtio_req *req, uint32_t len)
 	__sync_synchronize();
 	q->used->idx = htole16(new);
 
+	/* Triggers the irq whenever there is no available buffer.
+	 * q->last_avail_idx is incremented after calling virtio_req_complete(),
+	 * so here we need to add one to it.
+	 */
+	if (q->last_avail_idx + 1 == q->avail->idx)
+		send_irq = 1;
+
 	/* There are two rings: q->avail and q->used for each of the rx and tx
 	 * queues that are used to pass buffers between kernel driver and the
 	 * virtio device implementation.
@@ -105,16 +113,16 @@ void virtio_req_complete(struct virtio_req *req, uint32_t len)
 	 * increased to a larger one. So we need to trigger the irq when
 	 * virtio_get_used_event(q) < q->used->idx.
 	 *
-	 * To avoid unnessary irqs for each packet after 
+	 * To avoid unnessary irqs for each packet after
 	 * virtio_get_used_event(q) < q->used->idx, last_used_idx_signaled is
-	 * stored and irq is only triggered if 
+	 * stored and irq is only triggered if
 	 * last_used_idx_signaled <= virtio_get_used_event(q) < q->used->idx
 	 *
 	 * This is what lkl_vring_need_event() checks and it evens covers the
-	 * case when those numbers round up.
+	 * case when those numbers wrap up.
 	 */
-	if (lkl_vring_need_event(le16toh(virtio_get_used_event(q)), new,
-					q->last_used_idx_signaled)) {
+	if (send_irq || lkl_vring_need_event(le16toh(virtio_get_used_event(q)),
+					     new, q->last_used_idx_signaled)) {
 		q->last_used_idx_signaled = new;
 		virtio_deliver_irq(dev);
 	}