Re: [Qemu-devel] [PATCH] net/hub: remove can_receive handler

[Stefan Hajnoczi]

On Mon, Apr 22, 2013 at 04:26:16PM +0400, Fedorov Sergey wrote:
> On 04/22/2013 03:47 PM, Stefan Hajnoczi wrote:
> >On Thu, Apr 18, 2013 at 03:31:55PM +0400, Sergey Fedorov wrote:
> >>Network hub should always receive incoming packets. Then forward them to
> >>the appropriate port queue and let the qemu_send_packet() do the right
> >>things. If the destination queue cannot receive the packet it will be
> >>appended to the queue. When the receiver call
> >>qemu_flush_queued_packets() later the queue will be really flushed and
> >>no packets will be stalled in the sender network queue.
> >>
> >>Signed-off-by: Sergey Fedorov <address@hidden>
> >>---
> >>  net/hub.c |   20 --------------------
> >>  1 file changed, 20 deletions(-)
> >What is the point of this change?  There is no semantic difference for
> >well-behaved net clients.
> >
> >Does it fix a bug, if so, please include details?
> >
> >Stefan
> >
> >
> 
> Yes, this fixes a bug. There were packet stalls when using user-mode
> networking with USB network device. slirp_output() calls
> qemu_send_packet() which eventually calls qemu_net_queue_send().
> qemu_net_queue_send() calls qemu_can_send_packet(), which calls
> can_receive() callback of network hub. Then
> net_hub_port_can_receive() also calls qemu_can_send_packet() for
> each port except packet source port.
> 
> Sometimes USB network device is not able to receive packet and
> qemu_can_send_packet() returns false. In my case there is no more
> ports and net_hub_port_can_receive() returns false. So
> qemu_net_queue_send() call qemu_net_queue_append() instead of
> qemu_net_queue_deliver(). qemu_net_queue_append() appends the packet
> to the receiving port of the network hub which is not flushed when
> USB netork device calls qemu_flush_queued_packets(). It is flushed
> only when slirp resend the packet by timeout.
> 
> Actually there is no need in net_hub_port_can_receive() as the
> network hub can always receive packets and pass it to its port
> network clients with qemu_send_packet(). And if the destination port
> network client cannot receive the packet it will be queued in the
> *destination* port network client queue. Queued packets from that
> queue will be delivered as soon as the network client call
> qemu_flush_queued_packets().

Please confirm the bug is still present in qemu.git/master.  It should
have been fixed by the following commit:

  commit 199ee608f0d08510b5c6c37f31a7fbff211d63c4
  Author: Luigi Rizzo <address@hidden>
  Date:   Tue Feb 5 17:53:31 2013 +0100

      net: fix qemu_flush_queued_packets() in presence of a hub

Stefan