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

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

On 04/23/2013 04:00 PM, Stefan Hajnoczi wrote:
> On Tue, Apr 23, 2013 at 11:41:42AM +0400, Fedorov Sergey wrote:
>>> Beyond that, we also want to avoid growing net queues indefinitely.  If
>>> the hub does not implement .can_receive() then it relies on growing
>>> queues (keeping packets buffered in memory).
>> No, net_hub_receive() calls qemu_send_packet(). If the destination
>> queue cannot receive the packet qemu_net_queue_append() will take
>> care of queue->nq_maxlen.
> You are right, sorry.  We do discard packets at nq_maxlen.
>
> The problem with ignoring .can_receive() on the hub is that it breaks
> flow control.  For example, net/tap.c is designed to avoid reading more
> packets if its peer cannot receive (see tap_can_send()).
>
> If the hub claims it can always receive we waste cycles reading packets
> from the tap device only to discard them.
>
> Since qemu.git already has a fix which preserves flow control, I am not
> going to merge your patch.
>
> Stefan
>
OK, I see. Thanks for attention.

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