Re: [Qemu-devel] [PATCH v2] net: Flush queued packets when guest resumes

[Jason Wang <jasowang@redhat.com>]

On 07/10/2015 05:24 PM, Fam Zheng wrote:
> On Wed, 07/08 17:40, Jason Wang wrote:
>>
>> On 07/07/2015 05:03 PM, Fam Zheng wrote:
>>> On Tue, 07/07 15:44, Jason Wang wrote:
>>>> On 07/07/2015 09:21 AM, Fam Zheng wrote:
>>>>> Since commit 6e99c63 "net/socket: Drop net_socket_can_send" and friends,
>>>>> net queues need to be explicitly flushed after qemu_can_send_packet()
>>>>> returns false, because the netdev side will disable the polling of fd.
>>>>>
>>>>> This fixes the case of "cont" after "stop" (or migration).
>>>>>
>>>>> Signed-off-by: Fam Zheng <famz@redhat.com>
>>>>>
>>>>> ---
>>>>>
>>>>> v2: Unify with VM stop handler. (Stefan)
>>>>> ---
>>>>>  net/net.c | 19 ++++++++++++-------
>>>>>  1 file changed, 12 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/net/net.c b/net/net.c
>>>>> index 6ff7fec..28a5597 100644
>>>>> --- a/net/net.c
>>>>> +++ b/net/net.c
>>>>> @@ -1257,14 +1257,19 @@ void qmp_set_link(const char *name, bool up, Error **errp)
>>>>>  static void net_vm_change_state_handler(void *opaque, int running,
>>>>>                                          RunState state)
>>>>>  {
>>>>> -    /* Complete all queued packets, to guarantee we don't modify
>>>>> -     * state later when VM is not running.
>>>>> -     */
>>>>> -    if (!running) {
>>>>> -        NetClientState *nc;
>>>>> -        NetClientState *tmp;
>>>>> +    NetClientState *nc;
>>>>> +    NetClientState *tmp;
>>>>>  
>>>>> -        QTAILQ_FOREACH_SAFE(nc, &net_clients, next, tmp) {
>>>>> +    QTAILQ_FOREACH_SAFE(nc, &net_clients, next, tmp) {
>>>>> +        if (running) {
>>>>> +            /* Flush queued packets and wake up backends. */
>>>>> +            if (nc->peer && qemu_can_send_packet(nc)) {
>>>>> +                qemu_flush_queued_packets(nc->peer);
>>>>> +            }
>>>>> +        } else {
>>>>> +            /* Complete all queued packets, to guarantee we don't modify
>>>>> +             * state later when VM is not running.
>>>>> +             */
>>>>>              qemu_flush_or_purge_queued_packets(nc, true);
>>>>>          }
>>>> Looks like qemu_can_send_packet() checks both nc->peer and runstate. So
>>>> probably, we can simplify this to:
>>>>
>>>> if (qemu_can_send_packet(nc))
>>>>     qemu_flush_queued_packets(nc->peer);
>>>> else
>>>>     qemu_flush_or_purge_queued_packets(nc, true);
>>>>
>>>>>      }
>>> qemu_can_send_packet returns 1 if !nc->peer, so this doesn't work.
>>>
>>> Fam
>> Yes, I was wrong.
>>
>> Btw, instead of depending on vm handler (which seems racy with other
>> state change handler). Can we do this in places like vm_start() and
>> vm_stop(). Like we drain and flush block queue during vm stop.
>>
> Because that's a bit hacky.
>
> It won't be racy if we replace vdev->vm_running with runstate_is_running() in
> virtio-net, and/or don't check it in virtio_net_can_receive(). Is that OK?

Ok but not necessary. I believe this patch will be used with the patch
that drops virtio_net_can_receive(). So at any order two vmstate
handlers were called, the sent_cb will be called and poll will be
enabled again. I'm ok with the patch. So

Reviewed-by: Jason Wang <jasowang@redhat.com>

Thanks
>
> Fam