xen: register legacy backends via xen_backend_init

It is okay to register legacy backends in the middle of xen_bus_init().
All that the registration does is record the existence of the backend
in xenstore.

This makes it possible to remove them from the build without introducing
undefined symbols in xen_be_init().  It also removes the need for the
backend_register callback, whose only purpose is to avoid registering
nonfunctional backends.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240509170044.190795-8-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/9pfs/xen-9p-backend.c

@@ -513,7 +513,7 @@ static void xen_9pfs_alloc(struct XenLegacyDevice *xendev)
     xenstore_write_be_int(xendev, "max-ring-page-order", MAX_RING_ORDER);
 }
 
-struct XenDevOps xen_9pfs_ops = {
+static struct XenDevOps xen_9pfs_ops = {
     .size       = sizeof(Xen9pfsDev),
     .flags      = DEVOPS_FLAG_NEED_GNTDEV,
     .alloc      = xen_9pfs_alloc,
@@ -522,3 +522,9 @@ struct XenDevOps xen_9pfs_ops = {
     .disconnect = xen_9pfs_disconnect,
     .free       = xen_9pfs_free,
 };
+
+static void xen_9pfs_register_backend(void)
+{
+    xen_be_register("9pfs", &xen_9pfs_ops);
+}
+xen_backend_init(xen_9pfs_register_backend);

hw/display/xenfb.c

@@ -972,7 +972,7 @@ static void fb_event(struct XenLegacyDevice *xendev)
 
 /* -------------------------------------------------------------------- */
 
-struct XenDevOps xen_kbdmouse_ops = {
+static struct XenDevOps xen_kbdmouse_ops = {
     .size       = sizeof(struct XenInput),
     .init       = input_init,
     .initialise = input_initialise,
@@ -995,3 +995,9 @@ static const GraphicHwOps xenfb_ops = {
     .gfx_update  = xenfb_update,
     .ui_info     = xenfb_ui_info,
 };
+
+static void xen_vkbd_register_backend(void)
+{
+    xen_be_register("vkbd", &xen_kbdmouse_ops);
+}
+xen_backend_init(xen_vkbd_register_backend);

hw/usb/xen-usb.c

@@ -1083,7 +1083,7 @@ static void usbback_event(struct XenLegacyDevice *xendev)
     qemu_bh_schedule(usbif->bh);
 }
 
-struct XenDevOps xen_usb_ops = {
+static struct XenDevOps xen_usb_ops = {
     .size            = sizeof(struct usbback_info),
     .flags           = DEVOPS_FLAG_NEED_GNTDEV,
     .init            = usbback_init,
@@ -1095,15 +1095,9 @@ struct XenDevOps xen_usb_ops = {
     .event           = usbback_event,
 };
 
-#else /* USBIF_SHORT_NOT_OK */
-
-static int usbback_not_supported(void)
+static void xen_usb_register_backend(void)
 {
-    return -EINVAL;
+    xen_be_register("qusb", &xen_usb_ops);
 }
-
-struct XenDevOps xen_usb_ops = {
-    .backend_register = usbback_not_supported,
-};
-
+xen_backend_init(xen_usb_register_backend);
 #endif

hw/xen/xen-legacy-backend.c

@@ -622,27 +622,11 @@ void xen_be_init(void)
     qbus_set_bus_hotplug_handler(xen_sysbus);
 
     xen_set_dynamic_sysbus();
-
-    xen_be_register("vkbd", &xen_kbdmouse_ops);
-#ifdef CONFIG_VIRTFS
-    xen_be_register("9pfs", &xen_9pfs_ops);
-#endif
-#ifdef CONFIG_USB_LIBUSB
-    xen_be_register("qusb", &xen_usb_ops);
-#endif
 }
 
 int xen_be_register(const char *type, struct XenDevOps *ops)
 {
     char path[50];
-    int rc;
-
-    if (ops->backend_register) {
-        rc = ops->backend_register();
-        if (rc) {
-            return rc;
-        }
-    }
 
     snprintf(path, sizeof(path), "device-model/%u/backends/%s", xen_domid,
              type);

include/hw/xen/xen-legacy-backend.h

@@ -66,18 +66,8 @@ static inline void xen_be_unmap_grant_ref(struct XenLegacyDevice *xendev,
     return xen_be_unmap_grant_refs(xendev, ptr, &ref, 1);
 }
 
-/* actual backend drivers */
-extern struct XenDevOps xen_console_ops;      /* xen_console.c     */
-extern struct XenDevOps xen_kbdmouse_ops;     /* xen_framebuffer.c */
-extern struct XenDevOps xen_framebuffer_ops;  /* xen_framebuffer.c */
-extern struct XenDevOps xen_blkdev_ops;       /* xen_disk.c        */
-#ifdef CONFIG_VIRTFS
-extern struct XenDevOps xen_9pfs_ops;       /* xen-9p-backend.c        */
-#endif
-extern struct XenDevOps xen_netdev_ops;       /* xen_nic.c         */
-#ifdef CONFIG_USB_LIBUSB
-extern struct XenDevOps xen_usb_ops;          /* xen-usb.c         */
-#endif
+/* backend drivers not included in all machines */
+extern struct XenDevOps xen_framebuffer_ops;  /* xenfb.c */
 
 /* configuration (aka xenbus setup) */
 void xen_config_cleanup(void);

include/hw/xen/xen_pvdev.h

@@ -29,7 +29,6 @@ struct XenDevOps {
                                  const char *node);
     void      (*frontend_changed)(struct XenLegacyDevice *xendev,
                                   const char *node);
-    int       (*backend_register)(void);
 };
 
 struct XenLegacyDevice {