vhost: introduce vring call API

Users of librte_vhost currently implement the vring call operation
themselves.  Each caller performs the operation slightly differently.

This patch introduces a new librte_vhost API called
rte_vhost_vring_call() that performs the operation so that vhost-user
applications don't have to duplicate it.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>

examples/vhost/virtio_net.c

@@ -178,13 +178,8 @@ vs_enqueue_pkts(struct vhost_dev *dev, uint16_t queue_id,
 	*(volatile uint16_t *)&vr->used->idx += count;
 	queue->last_used_idx += count;
 
-	/* flush used->idx update before we read avail->flags. */
-	rte_mb();
+	rte_vhost_vring_call(dev->vid, queue_id);
 
-	/* Kick the guest if necessary. */
-	if (!(vr->avail->flags & VRING_AVAIL_F_NO_INTERRUPT)
-			&& (vr->callfd >= 0))
-		eventfd_write(vr->callfd, (eventfd_t)1);
 	return count;
 }
 
@@ -367,9 +362,7 @@ vs_dequeue_pkts(struct vhost_dev *dev, uint16_t queue_id,
 
 	vr->used->idx += i;
 
-	if (!(vr->avail->flags & VRING_AVAIL_F_NO_INTERRUPT)
-			&& (vr->callfd >= 0))
-		eventfd_write(vr->callfd, (eventfd_t)1);
+	rte_vhost_vring_call(dev->vid, queue_id);
 
 	return i;
 }

examples/vhost_scsi/vhost_scsi.c

@@ -81,7 +81,7 @@ descriptor_is_wr(struct vring_desc *cur_desc)
 }
 
 static void
-submit_completion(struct vhost_scsi_task *task)
+submit_completion(struct vhost_scsi_task *task, uint32_t q_idx)
 {
 	struct rte_vhost_vring *vq;
 	struct vring_used *used;
@@ -102,7 +102,7 @@ submit_completion(struct vhost_scsi_task *task)
 	/* Send an interrupt back to the guest VM so that it knows
 	 * a completion is ready to be processed.
 	 */
-	eventfd_write(vq->callfd, (eventfd_t)1);
+	rte_vhost_vring_call(task->bdev->vid, q_idx);
 }
 
 static void
@@ -234,7 +234,7 @@ process_requestq(struct vhost_scsi_ctrlr *ctrlr, uint32_t q_idx)
 			task->resp->status = 0;
 			task->resp->resid = 0;
 		}
-		submit_completion(task);
+		submit_completion(task, q_idx);
 		rte_free(task);
 	}
 }

lib/librte_vhost/rte_vhost.h

@@ -57,7 +57,9 @@ struct rte_vhost_vring {
 	struct vring_used	*used;
 	uint64_t		log_guest_addr;
 
+	/** Deprecated, use rte_vhost_vring_call() instead. */
 	int			callfd;
+
 	int			kickfd;
 	uint16_t		size;
 };
@@ -407,6 +409,19 @@ int rte_vhost_get_mem_table(int vid, struct rte_vhost_memory **mem);
 int rte_vhost_get_vhost_vring(int vid, uint16_t vring_idx,
 			      struct rte_vhost_vring *vring);
 
+/**
+ * Notify the guest that used descriptors have been added to the vring.  This
+ * function acts as a memory barrier.
+ *
+ * @param vid
+ *  vhost device ID
+ * @param vring_idx
+ *  vring index
+ * @return
+ *  0 on success, -1 on failure
+ */
+int rte_vhost_vring_call(int vid, uint16_t vring_idx);
+
 /**
  * Get vhost RX queue avail count.
  *

lib/librte_vhost/rte_vhost_version.map

@@ -52,3 +52,10 @@ DPDK_17.08 {
 	rte_vhost_rx_queue_count;
 
 } DPDK_17.05;
+
+DPDK_18.02 {
+	global:
+
+	rte_vhost_vring_call;
+
+} DPDK_17.08;

lib/librte_vhost/vhost.c

@@ -490,6 +490,27 @@ rte_vhost_get_vhost_vring(int vid, uint16_t vring_idx,
 	return 0;
 }
 
+int
+rte_vhost_vring_call(int vid, uint16_t vring_idx)
+{
+	struct virtio_net *dev;
+	struct vhost_virtqueue *vq;
+
+	dev = get_device(vid);
+	if (!dev)
+		return -1;
+
+	if (vring_idx >= VHOST_MAX_VRING)
+		return -1;
+
+	vq = dev->virtqueue[vring_idx];
+	if (!vq)
+		return -1;
+
+	vhost_vring_call(vq);
+	return 0;
+}
+
 uint16_t
 rte_vhost_avail_entries(int vid, uint16_t queue_id)
 {