xen/hvm: If we fail to fetch an HVM parameter print out which flag it is.

Makes it easier to troubleshoot in the field.
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[v1: Use macro per Ian's suggestion]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

include/xen/hvm.h

@@ -5,6 +5,36 @@
 #include <xen/interface/hvm/params.h>
 #include <asm/xen/hypercall.h>
 
+static const char *param_name(int op)
+{
+#define PARAM(x) [HVM_PARAM_##x] = #x
+	static const char *const names[] = {
+		PARAM(CALLBACK_IRQ),
+		PARAM(STORE_PFN),
+		PARAM(STORE_EVTCHN),
+		PARAM(PAE_ENABLED),
+		PARAM(IOREQ_PFN),
+		PARAM(BUFIOREQ_PFN),
+		PARAM(TIMER_MODE),
+		PARAM(HPET_ENABLED),
+		PARAM(IDENT_PT),
+		PARAM(DM_DOMAIN),
+		PARAM(ACPI_S_STATE),
+		PARAM(VM86_TSS),
+		PARAM(VPT_ALIGN),
+		PARAM(CONSOLE_PFN),
+		PARAM(CONSOLE_EVTCHN),
+	};
+#undef PARAM
+
+	if (op >= ARRAY_SIZE(names))
+		return "unknown";
+
+	if (!names[op])
+		return "reserved";
+
+	return names[op];
+}
 static inline int hvm_get_parameter(int idx, uint64_t *value)
 {
 	struct xen_hvm_param xhv;
@@ -14,8 +44,8 @@ static inline int hvm_get_parameter(int idx, uint64_t *value)
 	xhv.index = idx;
 	r = HYPERVISOR_hvm_op(HVMOP_get_param, &xhv);
 	if (r < 0) {
-		printk(KERN_ERR "Cannot get hvm parameter %d: %d!\n",
-			idx, r);
+		printk(KERN_ERR "Cannot get hvm parameter %s (%d): %d!\n",
+			param_name(idx), idx, r);
 		return r;
 	}
 	*value = xhv.value;