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Re: [Qemu-devel] frame reordering in qemu_net_queue_send() ?

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] frame reordering in qemu_net_queue_send() ?
Date: Thu, 31 May 2012 10:18:51 +0100

On Wed, May 30, 2012 at 8:59 AM, Luigi Rizzo <address@hidden> wrote:
> Hi,
> while investigating rx performance for emulated network devices
> (i am looking at the userspace version, relying on net=tap
> or similar approaches) i noticed the code
> in net/queue.c :: qemu_net_queue_send()
> which look strange to me (same goes for the iov version).
> The whole function is below, just for reference.
> My impression is that the call to qemu_net_queue_flush()
> is misplaced, and it should be before qemu_net_queue_deliver()
> otherwise the current packet is pushed out before anything
> was already in the queue.
> Does it make sense ?
> cheers
> luigi
>    ssize_t qemu_net_queue_send(NetQueue *queue,
>                                VLANClientState *sender,
>                                unsigned flags,
>                                const uint8_t *data,
>                                size_t size,
>                                NetPacketSent *sent_cb)
>    {
>        ssize_t ret;
>        if (queue->delivering) {
>            return qemu_net_queue_append(queue, sender, flags, data, size, 
> NULL);
>        }
>        ret = qemu_net_queue_deliver(queue, sender, flags, data, size);
>        if (ret == 0) {
>            qemu_net_queue_append(queue, sender, flags, data, size, sent_cb);
>            return 0;
>        }
>        qemu_net_queue_flush(queue);

This of the case where delivering a packet causes additional
(re-entrant) qemu_net_queue_send() calls to this queue.  We'll be in
->delivering state and queue those packets.  After we've finished
delivering the initial packet we flush the queue.

This is a weird case but this is how I read the intention of the code.

Jan: maybe slirp can do this re-entrant qemu_net_queue_send()?


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