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

[Stefan Hajnoczi]

On Tue, Apr 23, 2013 at 01:32:11PM +0400, Fedorov Sergey wrote:
> 
> On 04/23/2013 10:58 AM, Stefan Hajnoczi wrote:
> >On Mon, Apr 22, 2013 at 07:27:21PM +0400, Fedorov Sergey wrote:
> >>On 04/22/2013 06:57 PM, Stefan Hajnoczi wrote:
> >>>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
> >>>
> >>Yes, this commit fixes a bug but from other side. I think it's
> >>better to just let the qemu_send_packet() do the right things.
> >>
> >>E.g. network hub has 3 ports. Suppose when iterating through port
> >>list in net_hub_port_can_receive() a packet is successfully
> >>delivered to the first port, and then is queued in the source port
> >>queue because the second port cannot receive packets. Later
> >>net_hub_flush() will flush the packet from the source port queue and
> >>it will be delivered in every port. But it had been already
> >>delivered to one of them. So it will be delivered twice to some
> >>ports. Moreover there is less chance to dequeue the packet if
> >>several clients can't receive periodically.
> >Did you mean "second port" instead of "source port" in the beginning?
> >
> >I don't see a scenario where the packet is delivered to the same port
> >twice.  If one port can receive then the packet will be sent and other
> >ports will queue the packet.
> Whether dump network client can always receive? If so, then the
> packet will always be sent to the dump client regardless of whether
> the other clients can receive. Is there actually any
> synchronization?

Yes, the dump net client can always receive.  Other net clients may not.

There is no explicit synchronization between send queues on the hub.

Stefan