[Xen-devel] [PATCH net-next] xen-netback: support dynamic unbind/bind

[Paul Durrant <pdurrant@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
Cc: Wei Liu <wei.liu@xxxxxxxxxx>
Cc: Paul Durrant <paul@xxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
---
 drivers/net/xen-netback/interface.c | 10 +++++++++-
 drivers/net/xen-netback/netback.c   | 20 +++++++++++++++++---
 drivers/net/xen-netback/xenbus.c    |  5 ++---
 3 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/drivers/net/xen-netback/interface.c 
b/drivers/net/xen-netback/interface.c
index f15ba3de6195..0c8a02a1ead7 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/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)
diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 0020b2e8c279..315dfc6ea297 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/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;
 
diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
index 17b4950ec051..286054b60d47 100644
--- a/drivers/net/xen-netback/xenbus.c
+++ b/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)
-- 
2.20.1


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