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

[Jason Wang <jasowang@redhat.com>]

On Fri, Mar 4, 2022 at 12:37 PM Akihiko Odaki <akihiko.odaki@gmail.com> wrote:
>
> On 2022/03/04 10:37, Jason Wang wrote:
> > On Thu, Mar 3, 2022 at 11:43 PM Vladislav Yaroshchuk
> > <vladislav.yaroshchuk@jetbrains.com> wrote:
> >>
> >>
> >>
> >> On Tue, Mar 1, 2022 at 11:21 AM Akihiko Odaki <akihiko.odaki@gmail.com> wrote:
> >>>
> >>> 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.
> >>>
> >>
> >> Yes, it appends, but (net/queue.c):
> >> *  qemu_net_queue_send tries to deliver the packet
> >>      immediately. If the packet cannot be delivered, the
> >>      qemu_net_queue_append is called and 0 is returned
> >>      to say the caller "the receiver is not ready, hold on";
> >> *  qemu_net_queue_append does a probe before adding
> >>      the packet to the queue:
> >>      if (queue->nq_count >= queue->nq_maxlen && !sent_cb) {
> >>          return; /* drop if queue full and no callback */
> >>      }
> >>
> >> The queue is not infinite, so we have three cases:
> >> 1. The queue is not full -> append the packet, no
> >>      problems here
> >> 2. The queue is full, no callback -> we cannot notify
> >>      a caller when we're ready, so just drop the packet
> >>      if we can't append it.
> >> 3. The queue is full, callback present -> we can notify
> >>      a caller when we are ready, so "let's queue this packet,
> >>      but expect no more (!) packets is sent until I call
> >>      sent_cb when the queue is ready"
> >>
> >> Therefore if we provide a callback and keep sending
> >> packets if 0 is returned, this may cause unlimited(!)
> >> queue growth. To prevent this, we should stop sending
> >> packets and wait for notification callback to continue.
> >
> > Right.
> >
> >>
> >> I don't see any contradiction with that comment.
> >>
> >>> 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?
> >>>
> >>
> >> It may be wrong, but I think it's not allowed to send
> >> packets after qemu_send_packet_async returns 0.
> >>
> >> Jason Wang, can you confirm please?
> >
> > With a cb, we can't do this. All users with cb will disable the source
> > polling and depend on the cb to re-read the polling.
> > (tap/l2tpv3/socket).
> >
> > Without a cb, we can. As analyzed above, qemu_net_queue_append() can
> > limit the number of packets queued in this case.
>
> vmnet can read multiple packets at once. What about such a case? Isn't
> calling qemu_send_packet_async for already read packet and stopping
> reading more fine?

It should be fine, I remember I've asked whether or not the source
could be disabled but I get the answer that it can't.

Thanks

>
> Regards,
> Akihiko Odaki
>
> >
> > Thanks
> >
> >>
> >> Best Regards,
> >>
> >> Vladislav Yaroshchuk
> >>
> >>>
> >>> Regards,
> >>> Akihiko Odaki
> >>
> >>
> >>
> >
>