On Thu, May 31, 2012 at 11:18 AM, Stefan Hajnoczi <stefa...@gmail.com>wrote:
> On Wed, May 30, 2012 at 8:59 AM, Luigi Rizzo <ri...@iet.unipi.it> 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.
>
i was under the impression that qemu_net_queue_deliver() may also return 0
if the other side
(frontend network device) has no room for the packet. This would cause the
queue to
contain data on entry in the next call, even without reentrancy. Consider
this:
t0. qemu_net_queue_send(pkt-A)
qemu_net_queue_deliver() returns 0, pkt-A is queued and we return
t1. qemu_net_queue_send(pkt-B)
qemu_net_queue_deliver() returns successfully, pkt-B is sent to the
frontend
then we call qemu_net_queue_flush() and this sends pkt-B to the
frontend,
in the wrong order
cheers
luigi
--
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Prof. Luigi RIZZO, ri...@iet.unipi.it . Dip. di Ing. dell'Informazione
http://www.iet.unipi.it/~luigi/ . Universita` di Pisa
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Mobile +39-338-6809875 . 56122 PISA (Italy)
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