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

From: Luigi Rizzo
Subject: [Qemu-devel] frame reordering in qemu_net_queue_send() ?
Date: Wed, 30 May 2012 09:59:13 +0200
User-agent: Mutt/

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 ?


    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, 

        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;

        return ret;

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