Re: [PATCH v15 3/8] net/vmnet: implement shared mode (vmnet-shared)

[Akihiko Odaki <akihiko.odaki@gmail.com>]

On 2022/03/01 17:09, Vladislav Yaroshchuk wrote:
>      > Not sure that only one field is enough, cause
>      > we may have two states on bh execution start:
>      > 1. There are packets in vmnet buffer s->packets_buf
>      >      that were rejected by qemu_send_async and waiting
>      >      to be sent. If this happens, we should complete sending
>      >      these waiting packets with qemu_send_async firstly,
>      >      and after that we should call vmnet_read to get
>      >      new ones and send them to QEMU;
>      > 2. There are no packets in s->packets_buf to be sent to
>      >      qemu, we only need to get new packets from vmnet
>      >      with vmnet_read and send them to QEMU
> 
>     In case 1, you should just keep calling qemu_send_packet_async.
>     Actually
>     qemu_send_packet_async adds the packet to its internal queue and calls
>     the callback when it is consumed.
> 
> 
> I'm not sure we can keep calling qemu_send_packet_async,
> because as docs from net/queue.c says:
> 
> /* [...]
>   * If a sent callback is provided to send(), the caller must handle a
>   * zero return from the delivery handler by not sending any more packets
>   * until we have invoked the callback. Only in that case will we queue
>   * the packet.
>   *
>   * If a sent callback isn't provided, we just drop the packet to avoid
>   * unbounded queueing.
>   */
> 
> So after we did vmnet_read and read N packets
> into temporary s->packets_buf, we begin calling
> qemu_send_packet_async. If it returns 0 - it says
> "no more packets until sent_cb called please".
> At this moment we have N packets in s->packets_buf
> and already queued K < N of them. But, packets K..N
> are not queued and keep waiting for sent_cb to be sent
> with qemu_send_packet_async.
> Thus when sent_cb called, we should finish
> our transfer of packets K..N from s->packets_buf
> to qemu calling qemu_send_packet_async.
> I meant this.

I missed the comment. The description is contradicting with the actual 
code; qemu_net_queue_send_iov appends the packet to the queue whenever 
it cannot send one immediately.

Jason Wang, I saw you are in the MAINTAINERS for net/. Can you tell if 
calling qemu_send_packet_async is allowed after it returns 0?

Regards,
Akihiko Odaki