hvc_xen: fix xenboot for DomUs

The xenboot early console has been partially broken for DomU for a long
time: the output would only go to the hypervisor via hypercall
(HYPERVISOR_console_io), while it wouldn't actually go to the DomU
console. The reason is that domU_write_console would return early as no
xencons structs are configured for it.

Add an appropriate xencons struct for xenboot from the xenboot setup
callback.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/tty/hvc/hvc_xen.c

@@ -246,6 +246,18 @@ static int xen_hvm_console_init(void)
 	return -ENODEV;
 }
 
+static int xencons_info_pv_init(struct xencons_info *info, int vtermno)
+{
+	info->evtchn = xen_start_info->console.domU.evtchn;
+	/* GFN == MFN for PV guest */
+	info->intf = gfn_to_virt(xen_start_info->console.domU.mfn);
+	info->vtermno = vtermno;
+
+	list_add_tail(&info->list, &xenconsoles);
+
+	return 0;
+}
+
 static int xen_pv_console_init(void)
 {
 	struct xencons_info *info;
@@ -265,13 +277,8 @@ static int xen_pv_console_init(void)
 		/* already configured */
 		return 0;
 	}
-	info->evtchn = xen_start_info->console.domU.evtchn;
-	/* GFN == MFN for PV guest */
-	info->intf = gfn_to_virt(xen_start_info->console.domU.mfn);
-	info->vtermno = HVC_COOKIE;
-
 	spin_lock(&xencons_lock);
-	list_add_tail(&info->list, &xenconsoles);
+	xencons_info_pv_init(info, HVC_COOKIE);
 	spin_unlock(&xencons_lock);
 
 	return 0;
@@ -599,6 +606,18 @@ static int xen_cons_init(void)
 console_initcall(xen_cons_init);
 
 #ifdef CONFIG_EARLY_PRINTK
+static int __init xenboot_setup_console(struct console *console, char *string)
+{
+	static struct xencons_info xenboot;
+
+	if (xen_initial_domain())
+		return 0;
+	if (!xen_pv_domain())
+		return -ENODEV;
+
+	return xencons_info_pv_init(&xenboot, 0);
+}
+
 static void xenboot_write_console(struct console *console, const char *string,
 				  unsigned len)
 {
@@ -629,6 +648,7 @@ static void xenboot_write_console(struct console *console, const char *string,
 struct console xenboot_console = {
 	.name		= "xenboot",
 	.write		= xenboot_write_console,
+	.setup		= xenboot_setup_console,
 	.flags		= CON_PRINTBUFFER | CON_BOOT | CON_ANYTIME,
 	.index		= -1,
 };