Re: [Qemu-devel] [PATCHv3 4/6] virtio-net: stop/start bh when appropriate

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Dec 01, 2010 at 01:45:09PM +0800, Jason Wang wrote:
> Michael S. Tsirkin writes:
>  > Avoid sending out packets, and modifying
>  > device state, when VM is stopped.
>  > Add assert statements to verify this does not happen.
>  > 
>  > Avoid scheduling bh when vhost-net is started.
>  > 
>  > Stop bh when driver disabled bus mastering
>  > (we must not access memory after this).
>  > 
>  > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>  > 
> 
> There's no need to disable it bh we call qemu_aio_flush() after
> vm_state_notify() in do_vm_stop().

Yes, but bh can resubmit itself, so flush is not enough.
Note that vm stop is not the only reason to avoid
running bh: when vhost-net is enabled we should avoid it too.
Makes sense?

> And for timer, looks like every device should
> stop its timer in vm state change handler, not only for virtio-net?

Yes. We'll have to fix these things one at a time, I don't
have a general solution.

>  > ---
>  > 
>  > Respinning just this one patch:
>  > virtio-net: stop/start bh on vm start/stop
>  > 
>  > it turns out under kvm vcpu can be running after vm stop
>  > notifier callbacks are done. And it makes sense: vcpu is
>  > just another device, so we should not
>  > depend on the order of vmstate notifiers.
>  > The state callback is thus a request for device to avoid changing state
>  > of other devices, not a guarantee that other devices
>  > will not send requests to it.
>  > 
>  > diff --git a/hw/virtio-net.c b/hw/virtio-net.c
>  > index 43a2b3d..260433f 100644
>  > --- a/hw/virtio-net.c
>  > +++ b/hw/virtio-net.c
>  > @@ -99,7 +99,13 @@ static void virtio_net_set_config(VirtIODevice *vdev, const uint8_t *config)
>  >      }
>  >  }
>  >  
>  > -static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t status)
>  > +bool virtio_net_started(VirtIONet *n, uint8_t status)
>  > +{
>  > +    return (status & VIRTIO_CONFIG_S_DRIVER_OK) &&
>  > +        (n->status & VIRTIO_NET_S_LINK_UP) && n->vm_running;
>  > +}
>  > +
>  > +static void virtio_net_vhost_status(struct VirtIODevice *vdev, uint8_t status)
>  >  {
>  >      VirtIONet *n = to_virtio_net(vdev);
>  >      if (!n->nic->nc.peer) {
>  > @@ -112,9 +118,7 @@ static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t status)
>  >      if (!tap_get_vhost_net(n->nic->nc.peer)) {
>  >          return;
>  >      }
>  > -    if (!!n->vhost_started == ((status & VIRTIO_CONFIG_S_DRIVER_OK) &&
>  > -                               (n->status & VIRTIO_NET_S_LINK_UP) &&
>  > -                               n->vm_running)) {
>  > +    if (!!n->vhost_started == virtio_net_started(n, status)) {
>  >          return;
>  >      }
>  >      if (!n->vhost_started) {
>  > @@ -131,6 +135,30 @@ static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t status)
>  >      }
>  >  }
>  >  
>  > +static void virtio_net_set_status(struct VirtIODevice *vdev, uint8_t status)
>  > +{
>  > +    virtio_net_vhost_status(vdev, status);
>  > +
>  > +    if (!n->tx_waiting) {
>  > +        return;
>  > +    }
>  > +
>  > +    if (virtio_net_started(n, status) && !n->vhost_started) {
>  > +        if (n->tx_timer) {
>  > +            qemu_mod_timer(n->tx_timer,
>  > +                           qemu_get_clock(vm_clock) + n->tx_timeout);
>  > +        } else {
>  > +            qemu_bh_schedule(n->tx_bh);
>  > +        }
>  > +    } else {
>  > +        if (n->tx_timer) {
>  > +            qemu_del_timer(n->tx_timer);
>  > +        } else {
>  > +            qemu_bh_cancel(n->tx_bh);
>  > +        }
>  > +    }
>  > +}
>  > +
>  >  static void virtio_net_set_link_status(VLANClientState *nc)
>  >  {
>  >      VirtIONet *n = DO_UPCAST(NICState, nc, nc)->opaque;
>  > @@ -675,11 +703,13 @@ static int32_t virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
>  >  {
>  >      VirtQueueElement elem;
>  >      int32_t num_packets = 0;
>  >  
>  >      if (!(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK)) {
>  >          return num_packets;
>  >      }
>  >  
>  > +    assert(n->vm_running);
>  > +
>  >      if (n->async_tx.elem.out_num) {
>  >          virtio_queue_set_notification(n->tx_vq, 0);
>  >          return num_packets;
>  > @@ -738,6 +767,12 @@ static void virtio_net_handle_tx_timer(VirtIODevice *vdev, VirtQueue *vq)
>  >  {
>  >      VirtIONet *n = to_virtio_net(vdev);
>  >  
>  > +    /* This happens when device was stopped but VCPU wasn't. */
>  > +    if (!n->vm_running) {
>  > +        n->tx_waiting = 1;
>  > +        return;
>  > +    }
>  > +
>  >      if (n->tx_waiting) {
>  >          virtio_queue_set_notification(vq, 1);
>  >          qemu_del_timer(n->tx_timer);
>  > @@ -758,14 +793,19 @@ static void virtio_net_handle_tx_bh(VirtIODevice *vdev, VirtQueue *vq)
>  >      if (unlikely(n->tx_waiting)) {
>  >          return;
>  >      }
>  > +    n->tx_waiting = 1;
>  > +    /* This happens when device was stopped but VCPU wasn't. */
>  > +    if (!n->vm_running) {
>  > +        return;
>  > +    }
>  >      virtio_queue_set_notification(vq, 0);
>  >      qemu_bh_schedule(n->tx_bh);
>  > -    n->tx_waiting = 1;
>  >  }
>  >  
>  >  static void virtio_net_tx_timer(void *opaque)
>  >  {
>  >      VirtIONet *n = opaque;
>  > +    assert(n->vm_running);
>  >  
>  >      n->tx_waiting = 0;
>  >  
>  > @@ -782,6 +822,8 @@ static void virtio_net_tx_bh(void *opaque)
>  >      VirtIONet *n = opaque;
>  >      int32_t ret;
>  >  
>  > +    assert(n->vm_running);
>  > +
>  >      n->tx_waiting = 0;
>  >  
>  >      /* Just in case the driver is not ready on more */
>  > @@ -926,15 +968,6 @@ static int virtio_net_load(QEMUFile *f, void *opaque, int version_id)
>  >          }
>  >      }
>  >      n->mac_table.first_multi = i;
>  > -
>  > -    if (n->tx_waiting) {
>  > -        if (n->tx_timer) {
>  > -            qemu_mod_timer(n->tx_timer,
>  > -                           qemu_get_clock(vm_clock) + n->tx_timeout);
>  > -        } else {
>  > -            qemu_bh_schedule(n->tx_bh);
>  > -        }
>  > -    }
>  >      return 0;
>  >  }
>  >  
>  >