On 2012-05-31 11:34, Luigi Rizzo wrote:
> 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
This ordering requires a state change in the frontend (from "can't
receive" to "can receive") that is not possible. Such a change could
only be triggered if the guest interacts with the NIC. But we are
holding a lock that prevents this while walking the queue.
Jan
> then we call qemu_net_queue_flush() and this sends pkt-B to the
> frontend,
> in the wrong order
>
> cheers
> luigi
>
--
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
