Re: [Qemu-devel] Re: [PATCH] vhost-net: Do not set features for backend when peer was deleted

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

On Wed, Oct 27, 2010 at 03:26:04PM +0200, Michael S. Tsirkin wrote:
> On Wed, Oct 27, 2010 at 02:59:16PM +0800, Jason Wang wrote:
> > Michael S. Tsirkin writes:
> >  > On Wed, Oct 27, 2010 at 12:43:51AM +0800, Jason Wang wrote:
> >  > > When hot-unplug a virtio nic with vhost-net backend, guest may
> >  > > continue to program the nic even if its peer have been deleted. We can
> >  > > not set features at this time as vhost_net_ack_features() may still
> >  > > try to use the tap related vhost_net structure which have been freed
> >  > > in tap_cleanup(). And setting offload features for a deleted backend
> >  > > is also meaningless in this situation
> >  > > 
> >  > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> >  > 
> >  > Hmm. Actually, this is not enough.
> >  > We really must stop and cleanup vhost net.
> >  > 
> > 
> > But this patch just prevent the guest from programming a nic with its
> > vhost-net backend deleted. The vhost net was still stopped and cleaned
> > when the peer have been removed.
> 
> No, I think it was cleanup but not stopped first. E.g. try to migrate at
> this point and see it crash. I suspect we also leak fds for notifiers
> and what not.

Hmm. I missed the fact that we bring the link down, which
will stop vhost_net. So no, it's not broken.

But I'd rather we didn't look at peer_deleted and leave
this as an implementation detail in net.c: I think the
bug is in returning an invalid pointer from get_vhost_net.
Posted a patch to fix that.

> >  > Two issues are 
> >  > 1. vhost_net_stop needs virtio device pointer
> >  > 2. virtio net has a vhost_started flag
> >  > 
> >  > Two ways to fix it that I see:
> >  > 1. add a callback in nic and invoke when peer_deleted is set
> >  > 2. add vhost_started and virtio device pointers in vhost
> >  > 
> >  > Path 1 seems easier ...
> >  > 
> >  > MST
> >  > 
> >  > > ---
> >  > >  hw/virtio-net.c |    3 +++
> >  > >  1 files changed, 3 insertions(+), 0 deletions(-)
> >  > > 
> >  > > diff --git a/hw/virtio-net.c b/hw/virtio-net.c
> >  > > index 7e1688c..68c8e48 100644
> >  > > --- a/hw/virtio-net.c
> >  > > +++ b/hw/virtio-net.c
> >  > > @@ -245,6 +245,9 @@ static void virtio_net_set_features(VirtIODevice *vdev, uint32_t features)
> >  > >  {
> >  > >      VirtIONet *n = to_virtio_net(vdev);
> >  > >  
> >  > > +    if (n->nic->peer_deleted)
> >  > > +        return;
> >  > > +
> >  > >      n->mergeable_rx_bufs = !!(features & (1 << VIRTIO_NET_F_MRG_RXBUF));
> >  > >  
> >  > >      if (n->has_vnet_hdr) {
> >  >