Re: [Qemu-devel] [PATCH for-2.4] net-hub: Drop can_receive

[Stefan Hajnoczi]

On Tue, Jun 30, 2015 at 10:29:20AM +0800, Fam Zheng wrote:
> It returns true as long as there is another attached port. This is not
> strictly necessary because even if there is only one port (the sender),
> net_hub_port_receive could succeed with a NOP. So always deliver the
> packets, instead of queuing them.
> 
> This fixes the possible hanging issue after net layer changed how
> can_read is handled. That is, if net_hub_port_can_receive returned
> false, the peer would disable the queue until it's explicitly flushed
> (for example, a call to qemu_flush_queued_packets() in net_hub_add_port,
> where net_hub_port_can_receive() would become true). This patch avoids
> that complication.
> 
> Signed-off-by: Fam Zheng <address@hidden>
> ---
>  net/hub.c | 20 --------------------
>  1 file changed, 20 deletions(-)

Hmm...I misread the hub code:

net_hub_port_can_receive() returns true if *any* port can receive.

net_hub_receive_iov() always accepts packets.  It never discards or
queues.

So in order to move to the semantics you want, let's drop
net_hub_port_can_receive() *and* change net_hub_receive_iov():

  static ssize_t net_hub_receive(NetHub *hub, NetHubPort *source_port,
                                 const uint8_t *buf, size_t len)
  {
      NetHubPort *port;

      QLIST_FOREACH(port, &hub->ports, next) {
          if (port == source_port) {
              continue;
          }

          /* No need for a callback because net_hub_flush() is called
           * when the peer flushes the queue anyway.
           *
           * Note that packets are duplicated if there are multiple
           * ports and some of them accepted a packet before a later
           * port queued it.  Live with it, the network tolerates
           * duplicates.
           */
          if (qemu_send_packet(&port->nc, buf, len) == 0) {
              return 0;
          }
      }
      return len;
  }

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