xen: add xenstore watcher infrastructure

A Xen PV frontend communicates its state to the PV backend by writing to
the 'state' key in the frontend area in xenstore. It is therefore
necessary for a XenDevice implementation to be notified whenever the
value of this key changes.

This patch adds code to do this as follows:

- an 'fd handler' is registered on the libxenstore handle which will be
  triggered whenever a 'watch' event occurs
- primitives are added to xen-bus-helper to add or remove watch events
- a list of Notifier objects is added to XenBus to provide a mechanism
  to call the appropriate 'watch handler' when its associated event
  occurs

The xen-block implementation is extended with a 'frontend_changed' method,
which calls as-yet stub 'connect' and 'disconnect' functions when the
relevant frontend state transitions occur. A subsequent patch will supply
a full implementation for these functions.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony Perard <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

2 files changed, 72 insertions, 0 deletions

diff --git a/hw/block/trace-events b/hw/block/trace-events
index 4afbd62..89e2583 100644
--- a/hw/block/trace-events
+++ b/hw/block/trace-events
@@ -130,6 +130,8 @@ xen_disk_free(char *name) "%s"
 
 # hw/block/xen-block.c
 xen_block_realize(const char *type, uint32_t disk, uint32_t partition) "%s d%up%u"
+xen_block_connect(const char *type, uint32_t disk, uint32_t partition) "%s d%up%u"
+xen_block_disconnect(const char *type, uint32_t disk, uint32_t partition) "%s d%up%u"
 xen_block_unrealize(const char *type, uint32_t disk, uint32_t partition) "%s d%up%u"
 xen_disk_realize(void) ""
 xen_disk_unrealize(void) ""
diff --git a/hw/block/xen-block.c b/hw/block/xen-block.c
index d27a286..3a963b0 100644
--- a/hw/block/xen-block.c
+++ b/hw/block/xen-block.c
@@ -21,6 +21,24 @@ static char *xen_block_get_name(XenDevice *xendev, Error **errp)
     return g_strdup_printf("%lu", vdev->number);
 }
 
+static void xen_block_disconnect(XenDevice *xendev, Error **errp)
+{
+    XenBlockDevice *blockdev = XEN_BLOCK_DEVICE(xendev);
+    const char *type = object_get_typename(OBJECT(blockdev));
+    XenBlockVdev *vdev = &blockdev->props.vdev;
+
+    trace_xen_block_disconnect(type, vdev->disk, vdev->partition);
+}
+
+static void xen_block_connect(XenDevice *xendev, Error **errp)
+{
+    XenBlockDevice *blockdev = XEN_BLOCK_DEVICE(xendev);
+    const char *type = object_get_typename(OBJECT(blockdev));
+    XenBlockVdev *vdev = &blockdev->props.vdev;
+
+    trace_xen_block_connect(type, vdev->disk, vdev->partition);
+}
+
 static void xen_block_unrealize(XenDevice *xendev, Error **errp)
 {
     XenBlockDevice *blockdev = XEN_BLOCK_DEVICE(xendev);
@@ -35,6 +53,9 @@ static void xen_block_unrealize(XenDevice *xendev, Error **errp)
 
     trace_xen_block_unrealize(type, vdev->disk, vdev->partition);
 
+    /* Disconnect from the frontend in case this has not already happened */
+    xen_block_disconnect(xendev, NULL);
+
     if (blockdev_class->unrealize) {
         blockdev_class->unrealize(blockdev, errp);
     }
@@ -64,6 +85,54 @@ static void xen_block_realize(XenDevice *xendev, Error **errp)
     }
 }
 
+static void xen_block_frontend_changed(XenDevice *xendev,
+                                       enum xenbus_state frontend_state,
+                                       Error **errp)
+{
+    enum xenbus_state backend_state = xen_device_backend_get_state(xendev);
+    Error *local_err = NULL;
+
+    switch (frontend_state) {
+    case XenbusStateInitialised:
+    case XenbusStateConnected:
+        if (backend_state == XenbusStateConnected) {
+            break;
+        }
+
+        xen_block_disconnect(xendev, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            break;
+        }
+
+        xen_block_connect(xendev, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            break;
+        }
+
+        xen_device_backend_set_state(xendev, XenbusStateConnected);
+        break;
+
+    case XenbusStateClosing:
+        xen_device_backend_set_state(xendev, XenbusStateClosing);
+        break;
+
+    case XenbusStateClosed:
+        xen_block_disconnect(xendev, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            break;
+        }
+
+        xen_device_backend_set_state(xendev, XenbusStateClosed);
+        break;
+
+    default:
+        break;
+    }
+}
+
 static char *disk_to_vbd_name(unsigned int disk)
 {
     char *name, *prefix = (disk >= 26) ?
@@ -272,6 +341,7 @@ static void xen_block_class_init(ObjectClass *class, void *data)
 
     xendev_class->get_name = xen_block_get_name;
     xendev_class->realize = xen_block_realize;
+    xendev_class->frontend_changed = xen_block_frontend_changed;
     xendev_class->unrealize = xen_block_unrealize;
 
     dev_class->props = xen_block_props;