hw/xen: Register framebuffer backend via xen_backend_init()

Align the framebuffer backend with the other legacy ones,
register it via xen_backend_init() when '-vga xenfb' is
used. It is safe because MODULE_INIT_XEN_BACKEND is called
in xen_bus_realize(), long after CLI processing initialized
the vga_interface_type variable.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Paul Durrant <paul@xen.org>
Message-Id: <20240510104908.76908-8-philmd@linaro.org>

hw/display/xenfb.c

@@ -29,6 +29,7 @@
 
 #include "ui/input.h"
 #include "ui/console.h"
+#include "sysemu/sysemu.h"
 #include "hw/xen/xen-legacy-backend.h"
 
 #include "hw/xen/interface/io/fbif.h"
@@ -996,8 +997,12 @@ static const GraphicHwOps xenfb_ops = {
     .ui_info     = xenfb_ui_info,
 };
 
-static void xen_vkbd_register_backend(void)
+static void xen_ui_register_backend(void)
 {
     xen_be_register("vkbd", &xen_kbdmouse_ops);
+
+    if (vga_interface_type == VGA_XENFB) {
+        xen_be_register("vfb", &xen_framebuffer_ops);
+    }
 }
-xen_backend_init(xen_vkbd_register_backend);
+xen_backend_init(xen_ui_register_backend);

hw/xenpv/xen_machine_pv.c

@@ -50,8 +50,6 @@ static void xen_init_pv(MachineState *machine)
         break;
     }
 
-    xen_be_register("vfb", &xen_framebuffer_ops);
-
     /* configure framebuffer */
     if (vga_interface_type == VGA_XENFB) {
         xen_config_dev_vfb(0, "vnc");

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

@@ -66,9 +66,6 @@ static inline void xen_be_unmap_grant_ref(struct XenLegacyDevice *xendev,
     return xen_be_unmap_grant_refs(xendev, ptr, &ref, 1);
 }
 
-/* backend drivers not included in all machines */
-extern const struct XenDevOps xen_framebuffer_ops;  /* xenfb.c */
-
 /* configuration (aka xenbus setup) */
 void xen_config_cleanup(void);
 int xen_config_dev_vfb(int vdev, const char *type);