On 2012-05-31 11:18, Stefan Hajnoczi 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.
>
> Jan: maybe slirp can do this re-entrant qemu_net_queue_send()?
Yep, when working with a VLAN queue (*): transmission of the NIC will
directly be processed by slirp in the sender's context and can make it
send a reply, also in this context.
Jan
(*) Makes me wonder if this was considered for the hub patches.
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