[PATCH v2] xen-bus: reduce scope of backend watch

[Paul Durrant <paul@xxxxxxx>]

From: Paul Durrant <pdurrant@xxxxxxxxxx>

Currently a single watch on /local/domain/X/backend is registered by each
QEMU process running in service domain X (where X is usually 0). The purpose
of this watch is to ensure that QEMU is notified when the Xen toolstack
creates a new device backend area.
Such a backend area is specific to a single frontend area created for a
specific guest domain and, since each QEMU process is also created to service
a specfic guest domain, it is unnecessary and inefficient to notify all QEMU
processes.
Only the QEMU process associated with the same guest domain need
receive the notification. This patch re-factors the watch registration code
such that notifications are targetted appropriately.

Reported-by: Jerome Leseinne <jerome.leseinne@xxxxxxxxx>
Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
---
Cc: Edwin Torok <edvin.torok@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>

v2:
 - Re-work casting
 - Check for a NULL watch before trying to remove it
---
 hw/xen/xen-backend.c         | 11 ++++++++++
 hw/xen/xen-bus.c             | 40 ++++++++++++++++++++++++++++--------
 include/hw/xen/xen-backend.h |  1 +
 include/hw/xen/xen-bus.h     |  3 ++-
 4 files changed, 46 insertions(+), 9 deletions(-)

diff --git a/hw/xen/xen-backend.c b/hw/xen/xen-backend.c
index 10199fb58d..5b0fb76eae 100644
--- a/hw/xen/xen-backend.c
+++ b/hw/xen/xen-backend.c
@@ -41,6 +41,12 @@ static void xen_backend_table_add(XenBackendImpl *impl)
     g_hash_table_insert(xen_backend_table_get(), (void *)impl->type, impl);
 }
 
+static const char **xen_backend_table_keys(unsigned int *count)
+{
+    return (const char **)g_hash_table_get_keys_as_array(
+        xen_backend_table_get(), count);
+}
+
 static const XenBackendImpl *xen_backend_table_lookup(const char *type)
 {
     return g_hash_table_lookup(xen_backend_table_get(), type);
@@ -70,6 +76,11 @@ void xen_backend_register(const XenBackendInfo *info)
     xen_backend_table_add(impl);
 }
 
+const char **xen_backend_get_types(unsigned int *count)
+{
+    return xen_backend_table_keys(count);
+}
+
 static QLIST_HEAD(, XenBackendInstance) backend_list;
 
 static void xen_backend_list_add(XenBackendInstance *backend)
diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index 9ce1c9540b..8c588920d9 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -430,7 +430,15 @@ static void xen_bus_unrealize(BusState *bus)
     trace_xen_bus_unrealize();
 
     if (xenbus->backend_watch) {
-        xen_bus_remove_watch(xenbus, xenbus->backend_watch, NULL);
+        unsigned int i;
+
+        for (i = 0; i < xenbus->backend_types; i++) {
+            if (xenbus->backend_watch[i]) {
+                xen_bus_remove_watch(xenbus, xenbus->backend_watch[i], NULL);
+            }
+        }
+
+        g_free(xenbus->backend_watch);
         xenbus->backend_watch = NULL;
     }
 
@@ -446,8 +454,11 @@ static void xen_bus_unrealize(BusState *bus)
 
 static void xen_bus_realize(BusState *bus, Error **errp)
 {
+    char *key = g_strdup_printf("%u", xen_domid);
     XenBus *xenbus = XEN_BUS(bus);
     unsigned int domid;
+    const char **type;
+    unsigned int i;
     Error *local_err = NULL;
 
     trace_xen_bus_realize();
@@ -469,19 +480,32 @@ static void xen_bus_realize(BusState *bus, Error **errp)
 
     module_call_init(MODULE_INIT_XEN_BACKEND);
 
-    xenbus->backend_watch =
-        xen_bus_add_watch(xenbus, "", /* domain root node */
-                          "backend", xen_bus_backend_changed, &local_err);
-    if (local_err) {
-        /* This need not be treated as a hard error so don't propagate */
-        error_reportf_err(local_err,
-                          "failed to set up enumeration watch: ");
+    type = xen_backend_get_types(&xenbus->backend_types);
+    xenbus->backend_watch = g_new(XenWatch *, xenbus->backend_types);
+
+    for (i = 0; i < xenbus->backend_types; i++) {
+        char *node = g_strdup_printf("backend/%s", type[i]);
+
+        xenbus->backend_watch[i] =
+            xen_bus_add_watch(xenbus, node, key, xen_bus_backend_changed,
+                              &local_err);
+        if (local_err) {
+            /* This need not be treated as a hard error so don't propagate */
+            error_reportf_err(local_err,
+                              "failed to set up '%s' enumeration watch: ",
+                              type[i]);
+        }
+
+        g_free(node);
     }
 
+    g_free(type);
+    g_free(key);
     return;
 
 fail:
     xen_bus_unrealize(bus);
+    g_free(key);
 }
 
 static void xen_bus_unplug_request(HotplugHandler *hotplug,
diff --git a/include/hw/xen/xen-backend.h b/include/hw/xen/xen-backend.h
index 010d712638..aac2fd454d 100644
--- a/include/hw/xen/xen-backend.h
+++ b/include/hw/xen/xen-backend.h
@@ -31,6 +31,7 @@ void xen_backend_set_device(XenBackendInstance *backend,
 XenDevice *xen_backend_get_device(XenBackendInstance *backend);
 
 void xen_backend_register(const XenBackendInfo *info);
+const char **xen_backend_get_types(unsigned int *nr);
 
 void xen_backend_device_create(XenBus *xenbus, const char *type,
                                const char *name, QDict *opts, Error **errp);
diff --git a/include/hw/xen/xen-bus.h b/include/hw/xen/xen-bus.h
index 3df696136f..6bdbf3ff82 100644
--- a/include/hw/xen/xen-bus.h
+++ b/include/hw/xen/xen-bus.h
@@ -66,7 +66,8 @@ struct XenBus {
     domid_t backend_id;
     struct xs_handle *xsh;
     XenWatchList *watch_list;
-    XenWatch *backend_watch;
+    unsigned int backend_types;
+    XenWatch **backend_watch;
     QLIST_HEAD(, XenDevice) inactive_devices;
 };
 
-- 
2.20.1