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.