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

[Jason Wang <jasowang@redhat.com>]

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.