[net-next 1/2] xen-netback: add module parameter to disable ctrl-ring

[ChiaHao Hsu <andyhsu@xxxxxxxxxx>]

In order to support live migration of guests between kernels
that do and do not support 'feature-ctrl-ring', we add a
module parameter that allows the feature to be disabled
at run time, instead of using hardcode value.
The default value is enable.

Signed-off-by: ChiaHao Hsu <andyhsu@xxxxxxxxxx>
---
 drivers/net/xen-netback/common.h  |  2 ++
 drivers/net/xen-netback/netback.c |  6 ++++++
 drivers/net/xen-netback/xenbus.c  | 23 ++++++++++++++---------
 3 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
index 4a16d6e33c09..bfb7a3054917 100644
--- a/drivers/net/xen-netback/common.h
+++ b/drivers/net/xen-netback/common.h
@@ -276,6 +276,7 @@ struct backend_info {
        u8 have_hotplug_status_watch:1;
 
        const char *hotplug_script;
+       bool ctrl_ring_enabled;
 };
 
 struct xenvif {
@@ -413,6 +414,7 @@ static inline pending_ring_idx_t nr_pending_reqs(struct 
xenvif_queue *queue)
 
 irqreturn_t xenvif_interrupt(int irq, void *dev_id);
 
+extern bool control_ring;
 extern bool separate_tx_rx_irq;
 extern bool provides_xdp_headroom;
 
diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 39a01c2a3058..a119ae673862 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -48,6 +48,12 @@
 
 #include <asm/xen/hypercall.h>
 
+/* Provide an option to disable control ring which is used to pass
+ * large quantities of data from frontend to backend.
+ */
+bool control_ring = true;
+module_param(control_ring, bool, 0644);
+
 /* Provide an option to disable split event channels at load time as
  * event channels are limited resource. Split event channels are
  * enabled by default.
diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
index a5439c130130..9801b8d10239 100644
--- a/drivers/net/xen-netback/xenbus.c
+++ b/drivers/net/xen-netback/xenbus.c
@@ -755,10 +755,12 @@ static void connect(struct backend_info *be)
        xen_register_watchers(dev, be->vif);
        read_xenbus_vif_flags(be);
 
-       err = connect_ctrl_ring(be);
-       if (err) {
-               xenbus_dev_fatal(dev, err, "connecting control ring");
-               return;
+       if (be->ctrl_ring_enabled) {
+               err = connect_ctrl_ring(be);
+               if (err) {
+                       xenbus_dev_fatal(dev, err, "connecting control ring");
+                       return;
+               }
        }
 
        /* Use the number of queues requested by the frontend */
@@ -1123,11 +1125,14 @@ static int netback_probe(struct xenbus_device *dev,
        if (err)
                pr_debug("Error writing multi-queue-max-queues\n");
 
-       err = xenbus_printf(XBT_NIL, dev->nodename,
-                           "feature-ctrl-ring",
-                           "%u", true);
-       if (err)
-               pr_debug("Error writing feature-ctrl-ring\n");
+       be->ctrl_ring_enabled = READ_ONCE(control_ring);
+       if (be->ctrl_ring_enabled) {
+               err = xenbus_printf(XBT_NIL, dev->nodename,
+                                   "feature-ctrl-ring",
+                                   "%u", true);
+               if (err)
+                       pr_debug("Error writing feature-ctrl-ring\n");
+       }
 
        backend_switch_state(be, XenbusStateInitWait);
 
-- 
2.23.3