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From: | Fedorov Sergey |
Subject: | Re: [Qemu-devel] [PATCH] net/hub: remove can_receive handler |
Date: | Tue, 23 Apr 2013 15:58:02 +0400 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130329 Thunderbird/17.0.5 |
On 04/23/2013 03:48 PM, Stefan Hajnoczi wrote:
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? StefanYes, 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 StefanYes, 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
So why don't we apply this patch? -- Best regards, Sergey Fedorov, Junior Software Engineer, Samsung R&D Institute Rus. E-mail: address@hidden
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