hw/xen: do not repeatedly try to create a failing backend device

If xen_backend_device_create() fails to instantiate a device, the XenBus
code will just keep trying over and over again each time the bus is
re-enumerated, as long as the backend appears online and in
XenbusStateInitialising.

The only thing which prevents the XenBus code from recreating duplicates
of devices which already exist, is the fact that xen_device_realize()
sets the backend state to XenbusStateInitWait. If the attempt to create
the device doesn't get *that* far, that's when it will keep getting
retried.

My first thought was to handle errors by setting the backend state to
XenbusStateClosed, but that doesn't work for XenConsole which wants to
*ignore* any device of type != "ioemu" completely.

So, make xen_backend_device_create() *keep* the XenBackendInstance for a
failed device, and provide a new xen_backend_exists() function to allow
xen_bus_type_enumerate() to check whether one already exists before
creating a new one.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

hw/xen/xen-backend.c

@@ -101,6 +101,24 @@ static XenBackendInstance *xen_backend_list_find(XenDevice *xendev)
     return NULL;
 }
 
+bool xen_backend_exists(const char *type, const char *name)
+{
+    const XenBackendImpl *impl = xen_backend_table_lookup(type);
+    XenBackendInstance *backend;
+
+    if (!impl) {
+        return false;
+    }
+
+    QLIST_FOREACH(backend, &backend_list, entry) {
+        if (backend->impl == impl && !strcmp(backend->name, name)) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 static void xen_backend_list_remove(XenBackendInstance *backend)
 {
     QLIST_REMOVE(backend, entry);
@@ -122,11 +140,6 @@ void xen_backend_device_create(XenBus *xenbus, const char *type,
     backend->name = g_strdup(name);
 
     impl->create(backend, opts, errp);
-    if (*errp) {
-        g_free(backend->name);
-        g_free(backend);
-        return;
-    }
 
     backend->impl = impl;
     xen_backend_list_add(backend);
@@ -165,7 +178,9 @@ bool xen_backend_try_device_destroy(XenDevice *xendev, Error **errp)
     }
 
     impl = backend->impl;
-    impl->destroy(backend, errp);
+    if (backend->xendev) {
+        impl->destroy(backend, errp);
+    }
 
     xen_backend_list_remove(backend);
     g_free(backend->name);

hw/xen/xen-bus.c

@@ -209,7 +209,8 @@ static void xen_bus_type_enumerate(XenBus *xenbus, const char *type)
                           NULL, "%u", &online) != 1)
             online = 0;
 
-        if (online && state == XenbusStateInitialising) {
+        if (online && state == XenbusStateInitialising &&
+            !xen_backend_exists(type, backend[i])) {
             Error *local_err = NULL;
 
             xen_bus_backend_create(xenbus, type, backend[i], backend_path,

include/hw/xen/xen-backend.h

@@ -33,6 +33,7 @@ XenDevice *xen_backend_get_device(XenBackendInstance *backend);
 void xen_backend_register(const XenBackendInfo *info);
 const char **xen_backend_get_types(unsigned int *nr);
 
+bool xen_backend_exists(const char *type, const char *name);
 void xen_backend_device_create(XenBus *xenbus, const char *type,
                                const char *name, QDict *opts, Error **errp);
 bool xen_backend_try_device_destroy(XenDevice *xendev, Error **errp);