with CONFIG_XEN and CONFIG_FTRACE set we get this:

arch/x86/xen/trace.c:22: error: ‘__HYPERVISOR_console_io’ undeclared here (not in a function)
arch/x86/xen/trace.c:22: error: array index in initializer not of integer type
arch/x86/xen/trace.c:22: error: (near initialization for ‘xen_hypercall_names’)
arch/x86/xen/trace.c:23: error: ‘__HYPERVISOR_physdev_op_compat’ undeclared here (not in a function)

Issue was that the definitions of __HYPERVISOR were not pulled
if CONFIG_XEN_PRIVILEGED_GUEST was not set.
Reported-by: NRandy Dunlap <rdunlap@xenotime.net>
Acked-by: NRandy Dunlap <rdunlap@xenotime.net>
Acked-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>