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

[Fam Zheng]

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 <address@hidden>
> >>>
> >>> ---
> >>>
> >>> 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?

Fam