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

[Zhi Yong Wu]

On Thu, May 31, 2012 at 9:48 PM, Jan Kiszka <address@hidden> wrote:
> On 2012-05-31 15:45, Zhi Yong Wu wrote:
>> On Thu, May 31, 2012 at 5:18 PM, Stefan Hajnoczi <address@hidden> wrote:
>>> 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
>> If qemu_net_queue_send() are re-entranced, what issue or badness will
>> it introduce?
>> I haven't found what issue will take place when queue_flush() is moved
>> before queue_deliver().
>>
>
> Delayed packets that are hold back during delivering==1. Having a flush
Sorry, i don't get what it mean? You know, english is not my first
language.:) You mean that delayed packets may be reenqueued?
> before and after may be fine - if we really need it before.
>
> Jan
>
> --
> Siemens AG, Corporate Technology, CT T DE IT 1
> Corporate Competence Center Embedded Linux



-- 
Regards,

Zhi Yong Wu