vhost-net: flush outstanding DMAs on memory change

When memory map changes, we need to flush outstanding
DMAs as they might in theory reference old memory addresses.
To do this simply stop initiating new DMAs
and wait for ubufs ref count to drop to 0.
Afterwards reset the count back to 1.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/net.c

@@ -83,6 +83,8 @@ struct vhost_net {
 	/* Number of times zerocopy TX recently failed.
 	 * Protected by tx vq lock. */
 	unsigned tx_zcopy_err;
+	/* Flush in progress. Protected by tx vq lock. */
+	bool tx_flush;
 };
 
 static void vhost_net_tx_packet(struct vhost_net *net)
@@ -101,7 +103,11 @@ static void vhost_net_tx_err(struct vhost_net *net)
 
 static bool vhost_net_tx_select_zcopy(struct vhost_net *net)
 {
-	return net->tx_packets / 64 >= net->tx_zcopy_err;
+	/* TX flush waits for outstanding DMAs to be done.
+	 * Don't start new DMAs.
+	 */
+	return !net->tx_flush &&
+		net->tx_packets / 64 >= net->tx_zcopy_err;
 }
 
 static bool vhost_sock_zcopy(struct socket *sock)
@@ -679,6 +685,17 @@ static void vhost_net_flush(struct vhost_net *n)
 {
 	vhost_net_flush_vq(n, VHOST_NET_VQ_TX);
 	vhost_net_flush_vq(n, VHOST_NET_VQ_RX);
+	if (n->dev.vqs[VHOST_NET_VQ_TX].ubufs) {
+		mutex_lock(&n->dev.vqs[VHOST_NET_VQ_TX].mutex);
+		n->tx_flush = true;
+		mutex_unlock(&n->dev.vqs[VHOST_NET_VQ_TX].mutex);
+		/* Wait for all lower device DMAs done. */
+		vhost_ubuf_put_and_wait(n->dev.vqs[VHOST_NET_VQ_TX].ubufs);
+		mutex_lock(&n->dev.vqs[VHOST_NET_VQ_TX].mutex);
+		n->tx_flush = false;
+		kref_init(&n->dev.vqs[VHOST_NET_VQ_TX].ubufs->kref);
+		mutex_unlock(&n->dev.vqs[VHOST_NET_VQ_TX].mutex);
+	}
 }
 
 static int vhost_net_release(struct inode *inode, struct file *f)
@@ -686,18 +703,10 @@ static int vhost_net_release(struct inode *inode, struct file *f)
 	struct vhost_net *n = f->private_data;
 	struct socket *tx_sock;
 	struct socket *rx_sock;
-	int i;
 
 	vhost_net_stop(n, &tx_sock, &rx_sock);
 	vhost_net_flush(n);
 	vhost_dev_stop(&n->dev);
-	for (i = 0; i < n->dev.nvqs; ++i) {
-		/* Wait for all lower device DMAs done. */
-		if (n->dev.vqs[i].ubufs)
-			vhost_ubuf_put_and_wait(n->dev.vqs[i].ubufs);
-
-		vhost_zerocopy_signal_used(n, &n->dev.vqs[i]);
-	}
 	vhost_dev_cleanup(&n->dev, false);
 	if (tx_sock)
 		fput(tx_sock->file);
@@ -826,6 +835,7 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 
 		n->tx_packets = 0;
 		n->tx_zcopy_err = 0;
+		n->tx_flush = false;
 	}
 
 	mutex_unlock(&vq->mutex);