I found the poll in virEventPollRunOnce (vireventpoll.c),  a POLLHUP event occurred for a pipe descriptor,  thus the poll always returned because of it.( the pipe broken). this may be the best I could track temporarily£¬I don't know what to do netx , to mask the hangup event or to do something to handle the hangup event , what to do ?   Is it a bug or something ?