xen-netback: support dynamic unbind/bind

By re-attaching RX, TX, and CTL rings during connect() rather than
assuming they are freshly allocated (i.e. assuming the counters are zero),
and avoiding forcing state to Closed in netback_remove() it is possible
for vif instances to be unbound and re-bound from and to (respectively) a
running guest.

Dynamic unbind/bind is a highly useful feature for a backend module as it
allows it to be unloaded and re-loaded (i.e. updated) without requiring
domUs to be halted.

This has been tested by running iperf as a server in the test VM and
then running a client against it in a continuous loop, whilst also
running:

while true;
  do echo vif-$DOMID-$VIF >unbind;
  echo down;
  rmmod xen-netback;
  echo unloaded;
  modprobe xen-netback;
  cd $(pwd);
  brctl addif xenbr0 vif$DOMID.$VIF;
  ip link set vif$DOMID.$VIF up;
  echo up;
  sleep 5;
  done

in dom0 from /sys/bus/xen-backend/drivers/vif to continuously unbind,
unload, re-load, re-bind and re-plumb the backend.

Clearly a performance drop was seen but no TCP connection resets were
observed during this test and moreover a parallel SSH connection into the
guest remained perfectly usable throughout.
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/interface.c

@@ -585,6 +585,7 @@ int xenvif_connect_ctrl(struct xenvif *vif, grant_ref_t ring_ref,
 	struct net_device *dev = vif->dev;
 	void *addr;
 	struct xen_netif_ctrl_sring *shared;
+	RING_IDX rsp_prod, req_prod;
 	int err;
 
 	err = xenbus_map_ring_valloc(xenvif_to_xenbus_device(vif),
@@ -593,7 +594,14 @@ int xenvif_connect_ctrl(struct xenvif *vif, grant_ref_t ring_ref,
 		goto err;
 
 	shared = (struct xen_netif_ctrl_sring *)addr;
-	BACK_RING_INIT(&vif->ctrl, shared, XEN_PAGE_SIZE);
+	rsp_prod = READ_ONCE(shared->rsp_prod);
+	req_prod = READ_ONCE(shared->req_prod);
+
+	BACK_RING_ATTACH(&vif->ctrl, shared, rsp_prod, XEN_PAGE_SIZE);
+
+	err = -EIO;
+	if (req_prod - rsp_prod > RING_SIZE(&vif->ctrl))
+		goto err_unmap;
 
 	err = bind_interdomain_evtchn_to_irq(vif->domid, evtchn);
 	if (err < 0)

drivers/net/xen-netback/netback.c

@@ -1453,7 +1453,7 @@ int xenvif_map_frontend_data_rings(struct xenvif_queue *queue,
 	void *addr;
 	struct xen_netif_tx_sring *txs;
 	struct xen_netif_rx_sring *rxs;
-
+	RING_IDX rsp_prod, req_prod;
 	int err = -ENOMEM;
 
 	err = xenbus_map_ring_valloc(xenvif_to_xenbus_device(queue->vif),
@@ -1462,7 +1462,14 @@ int xenvif_map_frontend_data_rings(struct xenvif_queue *queue,
 		goto err;
 
 	txs = (struct xen_netif_tx_sring *)addr;
-	BACK_RING_INIT(&queue->tx, txs, XEN_PAGE_SIZE);
+	rsp_prod = READ_ONCE(txs->rsp_prod);
+	req_prod = READ_ONCE(txs->req_prod);
+
+	BACK_RING_ATTACH(&queue->tx, txs, rsp_prod, XEN_PAGE_SIZE);
+
+	err = -EIO;
+	if (req_prod - rsp_prod > RING_SIZE(&queue->tx))
+		goto err;
 
 	err = xenbus_map_ring_valloc(xenvif_to_xenbus_device(queue->vif),
 				     &rx_ring_ref, 1, &addr);
@@ -1470,7 +1477,14 @@ int xenvif_map_frontend_data_rings(struct xenvif_queue *queue,
 		goto err;
 
 	rxs = (struct xen_netif_rx_sring *)addr;
-	BACK_RING_INIT(&queue->rx, rxs, XEN_PAGE_SIZE);
+	rsp_prod = READ_ONCE(rxs->rsp_prod);
+	req_prod = READ_ONCE(rxs->req_prod);
+
+	BACK_RING_ATTACH(&queue->rx, rxs, rsp_prod, XEN_PAGE_SIZE);
+
+	err = -EIO;
+	if (req_prod - rsp_prod > RING_SIZE(&queue->rx))
+		goto err;
 
 	return 0;
 

drivers/net/xen-netback/xenbus.c

@@ -954,12 +954,10 @@ static int netback_remove(struct xenbus_device *dev)
 {
 	struct backend_info *be = dev_get_drvdata(&dev->dev);
 
-	set_backend_state(be, XenbusStateClosed);
-
 	unregister_hotplug_status_watch(be);
 	if (be->vif) {
 		kobject_uevent(&dev->dev.kobj, KOBJ_OFFLINE);
-		xen_unregister_watchers(be->vif);
+		backend_disconnect(be);
 		xenvif_free(be->vif);
 		be->vif = NULL;
 	}
@@ -1131,6 +1129,7 @@ static struct xenbus_driver netback_driver = {
 	.remove = netback_remove,
 	.uevent = netback_uevent,
 	.otherend_changed = frontend_changed,
+	.allow_rebind = true,
 };
 
 int xenvif_xenbus_init(void)