xen_vcputime_systime_ns
local = xen_vcputime_systime_ns();
static uint64_t xen_vcputime_systime_ns(void);
ci->ci_xen_hardclock_systime_ns = xen_vcputime_systime_ns();
now = xen_vcputime_systime_ns();
next = xen_vcputime_systime_ns() + ns_per_tick / 2;