Re: [Qemu-devel] net/tap.c: Possibly a way to stall tap input

[Jan Kiszka <jan.kiszka@siemens.com>]

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

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