Re: [Qemu-devel] [PATCH] net: Forbid dealing with packets when VM is not running

[Jason Wang <jasowang@redhat.com>]

On 08/19/2014 08:29 PM, Stefan Hajnoczi wrote:
> On Mon, Aug 18, 2014 at 04:32:42PM +0800, zhanghailiang wrote:
>> On 2014/8/18 14:55, Jason Wang wrote:
>>> On 08/18/2014 12:46 PM, zhanghailiang wrote:
>>>> diff --git a/net/net.c b/net/net.c
>>>> index 6d930ea..21f0d48 100644
>>>> --- a/net/net.c
>>>> +++ b/net/net.c
>>>> @@ -242,6 +242,29 @@ NetClientState *qemu_new_net_client(NetClientInfo *info,
>>>>      return nc;
>>>>  }
>>>>
>>>> +static void nic_vmstate_change_handler(void *opaque,
>>>> +                                       int running,
>>>> +                                       RunState state)
>>>> +{
>>>> +    NICState *nic = opaque;
>>>> +    NetClientState *nc;
>>>> +    int i, queues;
>>>> +
>>>> +    if (!running) {
>>>> +        return;
>>>> +    }
>>>> +
>>>> +    queues = MAX(1, nic->conf->peers.queues);
>>>> +    for (i = 0; i<  queues; i++) {
>>>> +        nc =&nic->ncs[i];
>>>> +        if (nc->receive_disabled
>>>> +            || (nc->info->can_receive&&  !nc->info->can_receive(nc))) {
>>>> +            continue;
>>>> +        }
>>>> +        qemu_flush_queued_packets(nc);
>>> How about simply purge the receive queue during stop? If ok, there's no
>>> need to introduce extra vmstate change handler.
>>>
>> I don't know whether it is OK to purge the receive packages, it was
>> suggested by Stefan Hajnoczi, and i am waiting for his opinion .:)
>>
>> I think we still need the extra vmstate change handler, Without the
>> change handler, we don't know if the VM will go to stop and the time
>> when to call qemu_purge_queued_packets.
> qemu_flush_queued_packets() sets nc->received_disabled = 0.  This may be
> needed to get packets flowing again if ->receive() previously returned 0.
>
> Purging the queue does not clear nc->received_disabled so it is not
> enough.

Confused.

virtio_net_receive() only returns 0 when it does not have enough rx
buffers. In this case, it just wait for the guest to refill and kick
again. Its rx kick handler will call qemu_flush_queued_packets() to
clear nc->received_disabled. So does usbnet and others.

If nic_received_disabled is 1, it means the no available rx buffer. We
need wait guest to do the processing and refilling. Then why need clear
it after vm was started?