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Subject[PATCH net-next] xen-netback: support dynamic unbind/bind
Date
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>
---
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Paul Durrant <paul@xen.org>
Cc: "David S. Miller" <davem@davemloft.net>
---
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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