diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
index 3277f0e..944eaeb 100644
--- a/drivers/tty/hvc/hvc_xen.c
+++ b/drivers/tty/hvc/hvc_xen.c
@@ -214,14 +214,19 @@
 	/* already configured */
 	if (info->intf != NULL)
 		return 0;
-
+	/*
+	 * If the toolstack (or the hypervisor) hasn't set these values, the
+	 * default value is 0. Even though mfn = 0 and evtchn = 0 are
+	 * theoretically correct values, in practice they never are and they
+	 * mean that a legacy toolstack hasn't initialized the pv console correctly.
+	 */
 	r = hvm_get_parameter(HVM_PARAM_CONSOLE_EVTCHN, &v);
-	if (r < 0)
+	if (r < 0 || v == 0)
 		goto err;
 	info->evtchn = v;
 	v = 0;
 	r = hvm_get_parameter(HVM_PARAM_CONSOLE_PFN, &v);
-	if (r < 0)
+	if (r < 0 || v == 0)
 		goto err;
 	mfn = v;
 	info->intf = ioremap(mfn << PAGE_SHIFT, PAGE_SIZE);