xen/common/event_channel.c 
 #define bucket_from_port(d,p) \
     ((d)->evtchn[(p)/EVTCHNS_PER_BUCKET])
 #define port_is_valid(d,p)    \
     (((p) >= 0) && ((p) < MAX_EVTCHNS(d)) && \
      (bucket_from_port(d,p) != NULL))
 #define evtchn_from_port(d,p) \
     (&(bucket_from_port(d,p))[(p)&(EVTCHNS_PER_BUCKET-1)])

 xen/include/xen/sched.h 
 #define EVTCHNS_PER_BUCKET 128                    //每个桶里可以容纳128个event-chanel(port)
 #define NR_EVTCHN_BUCKETS  (NR_EVENT_CHANNELS / EVTCHNS_PER_BUCKET) //最大的桶数为(最大envent数/桶的容量)32

 xen/include/public/xen.h 
 /*
  * Event channel endpoints per domain:
  *  1024 if a long is 32 bits; 4096 if a long is 64 bits.
  */
 #define NR_EVENT_CHANNELS (sizeof(unsigned long) * sizeof(unsigned long) * 64)