Re: [Qemu-devel] [PATCHv4] net: delay freeing peer host device

[Anthony Liguori <anthony@codemonkey.ws>]

On 10/06/2010 11:04 AM, Michael S. Tsirkin wrote:
> With -netdev, virtio devices present offload
> features to guest, depending on the backend used.
> Thus, removing host netdev peer while guest is
> active leads to guest-visible inconsistency and/or crashes.
>
> As a solution, while guest (NIC) peer device exists,
> we prevent the host peer from being deleted.
> This patch does this by adding peer_deleted flag in nic state:
> if host device is going away while guest device
> is around, set this flag and keep a shell of
> the host device around for as long as guest device exists.
>
> The link is put down so all packets will get discarded.
>
> At the moment, management can detect that device deletion
> is delayed by doing info net. As a next step, we shall add
> commands that control hotplug/unplug without
> removing the device, and an event to report that
> guest has responded to the hotplug event.
>
> Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
>    

I don't think this is a very good idea.  Making netdev_del conditionally 
succeed is going to break management tools and is very non-intuitive.

Regards,

Anthony Liguori

> ---
>
>   Changes from v3:
>     make sure ->cleanup is called for NICs
>   Changes from v2:
>     fix crash on repeated netdev_del
>
>   net.c |   49 ++++++++++++++++++++++++++++++++++++++++++-------
>   net.h |    1 +
>   2 files changed, 43 insertions(+), 7 deletions(-)
>
> diff --git a/net.c b/net.c
> index 3d0fde7..ed74c7f 100644
> --- a/net.c
> +++ b/net.c
> @@ -281,29 +281,64 @@ NICState *qemu_new_nic(NetClientInfo *info,
>       return nic;
>   }
>
> -void qemu_del_vlan_client(VLANClientState *vc)
> +static void qemu_cleanup_vlan_client(VLANClientState *vc)
>   {
>       if (vc->vlan) {
>           QTAILQ_REMOVE(&vc->vlan->clients, vc, next);
>       } else {
> -        if (vc->send_queue) {
> -            qemu_del_net_queue(vc->send_queue);
> -        }
>           QTAILQ_REMOVE(&non_vlan_clients, vc, next);
> -        if (vc->peer) {
> -            vc->peer->peer = NULL;
> -        }
>       }
>
>       if (vc->info->cleanup) {
>           vc->info->cleanup(vc);
>       }
> +}
>
> +static void qemu_free_vlan_client(VLANClientState *vc)
> +{
> +    if (!vc->vlan) {
> +        if (vc->send_queue) {
> +            qemu_del_net_queue(vc->send_queue);
> +        }
> +        if (vc->peer) {
> +            vc->peer->peer = NULL;
> +        }
> +    }
>       qemu_free(vc->name);
>       qemu_free(vc->model);
>       qemu_free(vc);
>   }
>
> +void qemu_del_vlan_client(VLANClientState *vc)
> +{
> +    /* If there is a peer NIC, delete and cleanup client, but do not free. */
> +    if (!vc->vlan&&  vc->peer&&  vc->peer->info->type == NET_CLIENT_TYPE_NIC) {
> +        NICState *nic = DO_UPCAST(NICState, nc, vc->peer);
> +        if (nic->peer_deleted) {
> +            return;
> +        }
> +        nic->peer_deleted = true;
> +        /* Let NIC know peer is gone. */
> +        vc->peer->link_down = true;
> +        if (vc->peer->info->link_status_changed) {
> +            vc->peer->info->link_status_changed(vc->peer);
> +        }
> +        qemu_cleanup_vlan_client(vc);
> +        return;
> +    }
> +
> +    /* If this is a peer NIC and peer has already been deleted, free it now. */
> +    if (!vc->vlan&&  vc->peer&&  vc->info->type == NET_CLIENT_TYPE_NIC) {
> +        NICState *nic = DO_UPCAST(NICState, nc, vc);
> +        if (nic->peer_deleted) {
> +            qemu_free_vlan_client(vc->peer);
> +        }
> +    }
> +
> +    qemu_cleanup_vlan_client(vc);
> +    qemu_free_vlan_client(vc);
> +}
> +
>   VLANClientState *
>   qemu_find_vlan_client_by_name(Monitor *mon, int vlan_id,
>                                 const char *client_str)
> diff --git a/net.h b/net.h
> index 518cf9c..44c31a9 100644
> --- a/net.h
> +++ b/net.h
> @@ -72,6 +72,7 @@ typedef struct NICState {
>       VLANClientState nc;
>       NICConf *conf;
>       void *opaque;
> +    bool peer_deleted;
>   } NICState;
>
>   struct VLANState {
>