Re: [Qemu-devel] TAP network breaks after debugger break-in

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jul 06, 2015 at 02:34:24PM +0300, Max Filippov wrote:
> On Mon, Jul 6, 2015 at 10:57 AM, Fam Zheng <famz@redhat.com> wrote:
> > On Mon, 07/06 10:45, Max Filippov wrote:
> >> On Mon, Jul 6, 2015 at 10:36 AM, Fam Zheng <famz@redhat.com> wrote:
> >> > On Mon, 07/06 10:27, Max Filippov wrote:
> >> >> On Mon, Jul 6, 2015 at 4:55 AM, Fam Zheng <famz@redhat.com> wrote:
> >> >> > On Sat, 07/04 10:47, Max Filippov wrote:
> >> >> >> Hello,
> >> >> >>
> >> >> >> I'm using QEMU with TAP network and after the commit
> >> >> >> 0a2df857a703 "Merge remote-tracking branch
> >> >> >> 'remotes/stefanha/tags/net-pull-request' into staging"
> >> >> >> I've noticed that activation of debugger connected to QEMU's
> >> >> >> gdbstub during network I/O almost always breaks network
> >> >> >> connection: network stops working completely after return
> >> >> >> from the debugger.
> >> >> >>
> >> >> >> Stefan, Fam, any hint on where to start debugging it?
> >> >> >>
> >> >> >
> >> >> > Which NIC are you using?
> >> >>
> >> >> opencores_eth.
> >> >>
> >> >
> >> > Does reverting a90a7425cf592a3afeff3eaf32f543b83050ee5c work?
> >>
> >> Looks like it does, though the revert isn't clean.
> >
> > I can't really tell what happened to opencores_eth because it has proper
> > open_eth_notify_can_receive() calls that should flush the queue as expected
> > since a90a7425cf592a3afeff3eaf32f543b83050ee5c (some other NICs are broken
> > because of missing such flushes).
> >
> > FWIW, what is changed by that patch is that if NIC's .can_receive() callback
> > returns 0, it should later call qemu_flush_queued_packets() explicitly, when
> > conditions becomes ready again (i.e. .can_receive() would return true on next
> > invocation).
> 
> Ok, what I see is the following:
> 
> #0  tap_update_fd_handler (s=0x555555fbe290) at qemu/net/tap.c:69
> #1  0x00005555556ec0d1 in tap_read_poll (s=0x555555fbe290,
> enable=false) at qemu/net/tap.c:80
> #2  0x00005555556ec68d in tap_send (opaque=0x555555fbe290) at qemu/net/tap.c:202
> 
> tap_read_poll is called with 'enable' set to false from tap_send, because size
> returned from qemu_send_packet_async is 0, because of this piece of code
> in the qemu_net_queue_send:
> 
>     if (queue->delivering || !qemu_can_send_packet(sender)) {
>         qemu_net_queue_append(queue, sender, flags, data, size, sent_cb);
>         return 0;
>     }
> 
> Previously this function wasn't called after qemu_can_send_packet returns 0,
> which was changed by the patch a90a7425cf592a.
> 
> tap_read_poll is never called with 'enable' set to true after that.

The GDB stub starts/stops the VM.  Code for the runstate STOPPED ->
START transition was missing in the net subsystem.

Please try this patch:
http://patchwork.ozlabs.org/patch/489941/

Stefan

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