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

[Fam Zheng]

On Thu, 07/02 14:30, Stefan Hajnoczi wrote:
> 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;

I'm not sure if it is a good idea to return 0 from either net_hub_port_receive
or net_hub_receive. Because that way if one out of ten ports is down, other
ones will no longer get more packets from tap.

Fam


>           }
>       }
>       return len;
>   }