xen64: set up syscall and sysenter entrypoints for 64-bit

We set up entrypoints for syscall and sysenter.  sysenter is only used
for 32-bit compat processes, whereas syscall can be used in by both 32
and 64-bit processes.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/enlighten.c

@@ -1139,6 +1139,10 @@ static const struct pv_cpu_ops xen_cpu_ops __initdata = {
 
 	.iret = xen_iret,
 	.irq_enable_sysexit = xen_sysexit,
+#ifdef CONFIG_X86_64
+	.usergs_sysret32 = xen_sysret32,
+	.usergs_sysret64 = xen_sysret64,
+#endif
 
 	.load_tr_desc = paravirt_nop,
 	.set_ldt = xen_set_ldt,