diff --git a/include/xen/interface/hvm/params.h b/include/xen/interface/hvm/params.h
index a6c79911e729..70ad20834e13 100644
--- a/include/xen/interface/hvm/params.h
+++ b/include/xen/interface/hvm/params.h
@@ -27,16 +27,31 @@
  * Parameter space for HVMOP_{set,get}_param.
  */
 
+#define HVM_PARAM_CALLBACK_IRQ 0
 /*
  * How should CPU0 event-channel notifications be delivered?
- * val[63:56] == 0: val[55:0] is a delivery GSI (Global System Interrupt).
- * val[63:56] == 1: val[55:0] is a delivery PCI INTx line, as follows:
- *                  Domain = val[47:32], Bus  = val[31:16],
- *                  DevFn  = val[15: 8], IntX = val[ 1: 0]
- * val[63:56] == 2: val[7:0] is a vector number.
+ *
  * If val == 0 then CPU0 event-channel notifications are not delivered.
+ * If val != 0, val[63:56] encodes the type, as follows:
+ */
+
+#define HVM_PARAM_CALLBACK_TYPE_GSI      0
+/*
+ * val[55:0] is a delivery GSI.  GSI 0 cannot be used, as it aliases val == 0,
+ * and disables all notifications.
+ */
+
+#define HVM_PARAM_CALLBACK_TYPE_PCI_INTX 1
+/*
+ * val[55:0] is a delivery PCI INTx line:
+ * Domain = val[47:32], Bus = val[31:16] DevFn = val[15:8], IntX = val[1:0]
+ */
+
+#define HVM_PARAM_CALLBACK_TYPE_VECTOR   2
+/*
+ * val[7:0] is a vector number.  Check for XENFEAT_hvm_callback_vector to know
+ * if this delivery method is available.
  */
-#define HVM_PARAM_CALLBACK_IRQ 0
 
 #define HVM_PARAM_STORE_PFN    1
 #define HVM_PARAM_STORE_EVTCHN 2