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

[Fedorov Sergey <s.fedorov@samsung.com>]

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 <s.fedorov@samsung.com>
>> ---
>>   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().

-- 
Best regards,
Sergey Fedorov, Junior Software Engineer,
Samsung R&D Institute Rus.
E-mail: s.fedorov@samsung.com