On 2013-08-02 13:46, Stefan Hajnoczi wrote: > On Thu, Aug 01, 2013 at 07:15:54PM +0200, Jan Kiszka wrote: >> I was digging into the involved code and found something fishy: >> >> net/tap.c: >> static void tap_send(void *opaque) >> { >> ... >> size = qemu_send_packet_async(&s->nc, buf, size, >> tap_send_completed); >> if (size == 0) { >> tap_read_poll(s, false); >> } >> >> So, if tap_send is registered for the mainloop polling (ie. can_receive >> returned true before starting to poll) but qemu_send_packet_async >> returns 0 now as qemu_can_send_packet/can_receive happens to report >> false in the meantime, we will disable read polling. If also write >> polling is off, the fd will be completely removed from the iohandler >> list. But even if write polling remains on, I wonder what should bring >> read polling back? > > This behavior seems fine to me. Once the peer (pcnet) is able to > receive again it must flush the queue, this will re-enable > tap_read_poll(). > > Can you explain a bit more why this would be a problem?
The problem is that I don't see at all what will call tap_read_poll(s, 1), neither in theory nor in reality. As long as the real test case is out of reach, I tried to emulate the faulty behaviour by letting tap_can_send always return 1. Result: reception stalls during boot as even qemu_flush_queued_packets cannot get it running again once tap_read_poll(s, 0) was called. Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux