xen: add variable hypercall caller

Allow non-constant hypercall to be called, for privcmd.

[ Impact: make arbitrary hypercalls; needed for privcmd ]
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/include/asm/xen/hypercall.h

@@ -200,6 +200,23 @@ extern struct { char _entry[32]; } hypercall_page[];
 	(type)__res;							\
 })
 
+static inline long
+privcmd_call(unsigned call,
+	     unsigned long a1, unsigned long a2,
+	     unsigned long a3, unsigned long a4,
+	     unsigned long a5)
+{
+	__HYPERCALL_DECLS;
+	__HYPERCALL_5ARG(a1, a2, a3, a4, a5);
+
+	asm volatile("call *%[call]"
+		     : __HYPERCALL_5PARAM
+		     : [call] "a" (&hypercall_page[call])
+		     : __HYPERCALL_CLOBBER5);
+
+	return (long)__res;
+}
+
 static inline int
 HYPERVISOR_set_trap_table(struct trap_info *table)
 {