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>

3 files changed, 249 insertions, 2 deletions

diff --git a/hw/xen/trace-events b/hw/xen/trace-events
index 75dc226..22055b5 100644
--- a/hw/xen/trace-events
+++ b/hw/xen/trace-events
@@ -16,13 +16,19 @@ xen_domid_restrict(int err) "err: %u"
 # include/hw/xen/xen-bus.c
 xen_bus_realize(void) ""
 xen_bus_unrealize(void) ""
+xen_bus_add_watch(const char *node, const char *key, char *token) "node: %s key: %s token: %s"
+xen_bus_remove_watch(const char *node, const char *key, char *token) "node: %s key: %s token: %s"
+xen_bus_watch(const char *token) "token: %s"
 xen_device_realize(const char *type, char *name) "type: %s name: %s"
 xen_device_unrealize(const char *type, char *name) "type: %s name: %s"
 xen_device_backend_state(const char *type, char *name, const char *state) "type: %s name: %s -> %s"
 xen_device_frontend_state(const char *type, char *name, const char *state) "type: %s name: %s -> %s"
+xen_device_frontend_changed(const char *type, char *name) "type: %s name: %s"
 
 # include/hw/xen/xen-bus-helper.c
 xs_node_create(const char *node) "%s"
 xs_node_destroy(const char *node) "%s"
 xs_node_vprintf(char *path, char *value) "%s %s"
 xs_node_vscanf(char *path, char *value) "%s %s"
+xs_node_watch(char *path) "%s"
+xs_node_unwatch(char *path) "%s"
diff --git a/hw/xen/xen-bus-helper.c b/hw/xen/xen-bus-helper.c
index 15b3ad8..5f7a4b2 100644
--- a/hw/xen/xen-bus-helper.c
+++ b/hw/xen/xen-bus-helper.c
@@ -148,3 +148,37 @@ int xs_node_scanf(struct xs_handle *xsh,  xs_transaction_t tid,
 
     return rc;
 }
+
+void xs_node_watch(struct xs_handle *xsh, const char *node, const char *key,
+                   char *token, Error **errp)
+{
+    char *path;
+
+    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
+        g_strdup(key);
+
+    trace_xs_node_watch(path);
+
+    if (!xs_watch(xsh, path, token)) {
+        error_setg_errno(errp, errno, "failed to watch node '%s'", path);
+    }
+
+    g_free(path);
+}
+
+void xs_node_unwatch(struct xs_handle *xsh, const char *node,
+                     const char *key, const char *token, Error **errp)
+{
+    char *path;
+
+    path = (strlen(node) != 0) ? g_strdup_printf("%s/%s", node, key) :
+        g_strdup(key);
+
+    trace_xs_node_unwatch(path);
+
+    if (!xs_unwatch(xsh, path, token)) {
+        error_setg_errno(errp, errno, "failed to unwatch node '%s'", path);
+    }
+
+    g_free(path);
+}
diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index 843fda2..5e19592 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -6,6 +6,8 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/main-loop.h"
+#include "qemu/uuid.h"
 #include "hw/hw.h"
 #include "hw/sysbus.h"
 #include "hw/xen/xen.h"
@@ -59,6 +61,87 @@ static char *xen_bus_get_dev_path(DeviceState *dev)
     return xen_device_get_backend_path(XEN_DEVICE(dev));
 }
 
+struct XenWatch {
+    char *node, *key;
+    char *token;
+    XenWatchHandler handler;
+    void *opaque;
+    Notifier notifier;
+};
+
+static void watch_notify(Notifier *n, void *data)
+{
+    XenWatch *watch = container_of(n, XenWatch, notifier);
+    const char *token = data;
+
+    if (!strcmp(watch->token, token)) {
+        watch->handler(watch->opaque);
+    }
+}
+
+static XenWatch *new_watch(const char *node, const char *key,
+                           XenWatchHandler handler, void *opaque)
+{
+    XenWatch *watch = g_new0(XenWatch, 1);
+    QemuUUID uuid;
+
+    qemu_uuid_generate(&uuid);
+
+    watch->token = qemu_uuid_unparse_strdup(&uuid);
+    watch->node = g_strdup(node);
+    watch->key = g_strdup(key);
+    watch->handler = handler;
+    watch->opaque = opaque;
+    watch->notifier.notify = watch_notify;
+
+    return watch;
+}
+
+static void free_watch(XenWatch *watch)
+{
+    g_free(watch->token);
+    g_free(watch->key);
+    g_free(watch->node);
+
+    g_free(watch);
+}
+
+static XenWatch *xen_bus_add_watch(XenBus *xenbus, const char *node,
+                                   const char *key, XenWatchHandler handler,
+                                   void *opaque, Error **errp)
+{
+    XenWatch *watch = new_watch(node, key, handler, opaque);
+    Error *local_err = NULL;
+
+    trace_xen_bus_add_watch(watch->node, watch->key, watch->token);
+
+    notifier_list_add(&xenbus->watch_notifiers, &watch->notifier);
+
+    xs_node_watch(xenbus->xsh, node, key, watch->token, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+
+        notifier_remove(&watch->notifier);
+        free_watch(watch);
+
+        return NULL;
+    }
+
+    return watch;
+}
+
+static void xen_bus_remove_watch(XenBus *xenbus, XenWatch *watch,
+                                 Error **errp)
+{
+    trace_xen_bus_remove_watch(watch->node, watch->key, watch->token);
+
+    xs_node_unwatch(xenbus->xsh, watch->node, watch->key, watch->token,
+                    errp);
+
+    notifier_remove(&watch->notifier);
+    free_watch(watch);
+}
+
 static void xen_bus_unrealize(BusState *bus, Error **errp)
 {
     XenBus *xenbus = XEN_BUS(bus);
@@ -69,9 +152,33 @@ static void xen_bus_unrealize(BusState *bus, Error **errp)
         return;
     }
 
+    qemu_set_fd_handler(xs_fileno(xenbus->xsh), NULL, NULL, NULL);
+
     xs_close(xenbus->xsh);
 }
 
+static void xen_bus_watch(void *opaque)
+{
+    XenBus *xenbus = opaque;
+    char **v;
+    const char *token;
+
+    g_assert(xenbus->xsh);
+
+    v = xs_check_watch(xenbus->xsh);
+    if (!v) {
+        return;
+    }
+
+    token = v[XS_WATCH_TOKEN];
+
+    trace_xen_bus_watch(token);
+
+    notifier_list_notify(&xenbus->watch_notifiers, (void *)token);
+
+    free(v);
+}
+
 static void xen_bus_realize(BusState *bus, Error **errp)
 {
     XenBus *xenbus = XEN_BUS(bus);
@@ -92,6 +199,9 @@ static void xen_bus_realize(BusState *bus, Error **errp)
         xenbus->backend_id = 0; /* Assume lack of node means dom0 */
     }
 
+    notifier_list_init(&xenbus->watch_notifiers);
+    qemu_set_fd_handler(xs_fileno(xenbus->xsh), xen_bus_watch, NULL,
+                        xenbus);
     return;
 
 fail:
@@ -139,8 +249,25 @@ static void xen_device_backend_printf(XenDevice *xendev, const char *key,
     }
 }
 
-static void xen_device_backend_set_state(XenDevice *xendev,
-                                         enum xenbus_state state)
+static int xen_device_backend_scanf(XenDevice *xendev, const char *key,
+                                    const char *fmt, ...)
+{
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
+    va_list ap;
+    int rc;
+
+    g_assert(xenbus->xsh);
+
+    va_start(ap, fmt);
+    rc = xs_node_vscanf(xenbus->xsh, XBT_NULL, xendev->backend_path, key,
+                        NULL, fmt, ap);
+    va_end(ap);
+
+    return rc;
+}
+
+void xen_device_backend_set_state(XenDevice *xendev,
+                                  enum xenbus_state state)
 {
     const char *type = object_get_typename(OBJECT(xendev));
 
@@ -155,6 +282,11 @@ static void xen_device_backend_set_state(XenDevice *xendev,
     xen_device_backend_printf(xendev, "state", "%u", state);
 }
 
+enum xenbus_state xen_device_backend_get_state(XenDevice *xendev)
+{
+    return xendev->backend_state;
+}
+
 static void xen_device_backend_create(XenDevice *xendev, Error **errp)
 {
     XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
@@ -218,6 +350,23 @@ static void xen_device_frontend_printf(XenDevice *xendev, const char *key,
     }
 }
 
+static int xen_device_frontend_scanf(XenDevice *xendev, const char *key,
+                                     const char *fmt, ...)
+{
+    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
+    va_list ap;
+    int rc;
+
+    g_assert(xenbus->xsh);
+
+    va_start(ap, fmt);
+    rc = xs_node_vscanf(xenbus->xsh, XBT_NULL, xendev->frontend_path, key,
+                        NULL, fmt, ap);
+    va_end(ap);
+
+    return rc;
+}
+
 static void xen_device_frontend_set_state(XenDevice *xendev,
                                           enum xenbus_state state)
 {
@@ -234,6 +383,50 @@ static void xen_device_frontend_set_state(XenDevice *xendev,
     xen_device_frontend_printf(xendev, "state", "%u", state);
 }
 
+static void xen_device_frontend_changed(void *opaque)
+{
+    XenDevice *xendev = opaque;
+    XenDeviceClass *xendev_class = XEN_DEVICE_GET_CLASS(xendev);
+    const char *type = object_get_typename(OBJECT(xendev));
+    enum xenbus_state state;
+
+    trace_xen_device_frontend_changed(type, xendev->name);
+
+    if (xen_device_frontend_scanf(xendev, "state", "%u", &state) != 1) {
+        state = XenbusStateUnknown;
+    }
+
+    xen_device_frontend_set_state(xendev, state);
+
+    if (xendev_class->frontend_changed) {
+        Error *local_err = NULL;
+
+        xendev_class->frontend_changed(xendev, state, &local_err);
+
+        if (local_err) {
+            error_reportf_err(local_err, "frontend change error: ");
+        }
+    }
+
+    /*
+     * If a backend is still 'online' then its state should be cycled
+     * back round to InitWait in order for a new frontend instance to
+     * connect. This may happen when, for example, a frontend driver is
+     * re-installed or updated.
+     */
+    if (xendev->backend_state == XenbusStateClosed) {
+        unsigned int online;
+
+        if (xen_device_backend_scanf(xendev, "online", "%u", &online) != 1) {
+            online = 0;
+        }
+
+        if (online) {
+            xen_device_backend_set_state(xendev, XenbusStateInitWait);
+        }
+    }
+}
+
 static void xen_device_frontend_create(XenDevice *xendev, Error **errp)
 {
     XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
@@ -254,6 +447,15 @@ static void xen_device_frontend_create(XenDevice *xendev, Error **errp)
     if (local_err) {
         error_propagate_prepend(errp, local_err,
                                 "failed to create frontend: ");
+        return;
+    }
+
+    xendev->frontend_state_watch =
+        xen_bus_add_watch(xenbus, xendev->frontend_path, "state",
+                          xen_device_frontend_changed, xendev, &local_err);
+    if (local_err) {
+        error_propagate_prepend(errp, local_err,
+                                "failed to watch frontend state: ");
     }
 }
 
@@ -262,6 +464,11 @@ static void xen_device_frontend_destroy(XenDevice *xendev)
     XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
     Error *local_err = NULL;
 
+    if (xendev->frontend_state_watch) {
+        xen_bus_remove_watch(xenbus, xendev->frontend_state_watch, NULL);
+        xendev->frontend_state_watch = NULL;
+    }
+
     if (!xendev->frontend_path) {
         return;
     }