Re: [Qemu-devel] [PATCH] net: delay peer host device delete

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

On Mon, Sep 20, 2010 at 11:41:45AM -0500, Anthony Liguori wrote:
> On 09/20/2010 11:30 AM, Michael S. Tsirkin wrote:
> >With -netdev, virtio devices present offload
> >features to guest, depending on the backend used.
> >Thus, removing host ntedev peer while guest is
> >active leads to guest-visible inconsistency and/or crashes.
> >See e.g. https://bugzilla.redhat.com/show_bug.cgi?id=623735
> >
> >As a solution, while guest (NIC) peer device exists,
> >we must 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 host device around
> >for as long as guest device exists.
> 
> Having an unclear life cycle really worries me.
> 
> Wouldn't the more correct solution be to avoid removing the netdev
> device until after the peer has successfully been removed?
> 
> Regards,
> 
> Anthony Liguori

This is exactly what the patch does.


> >Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> >---
> >  net.c |   21 ++++++++++++++++++++-
> >  net.h |    1 +
> >  2 files changed, 21 insertions(+), 1 deletions(-)
> >
> >diff --git a/net.c b/net.c
> >index 3d0fde7..10855d1 100644
> >--- a/net.c
> >+++ b/net.c
> >@@ -286,12 +286,31 @@ void qemu_del_vlan_client(VLANClientState *vc)
> >      if (vc->vlan) {
> >          QTAILQ_REMOVE(&vc->vlan->clients, vc, next);
> >      } else {
> >+        /* Even if client will not be deleted yet, remove it from list so it
> >+         * does not appear in monitor.  */
> >+        QTAILQ_REMOVE(&non_vlan_clients, vc, next);
> >+        /* Detect that guest-visible (NIC) peer is active, and delay deletion.
> >+         * */
> >+        if (vc->peer&&  vc->peer->info->type == NET_CLIENT_TYPE_NIC) {
> >+            NICState *nic = DO_UPCAST(NICState, nc, vc->peer);
> >+            assert(!nic->peer_deleted);
> >+            nic->peer_deleted = true;
> >+            return;
> >+        }
> >          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 this is a guest-visible (NIC) device,
> >+             * and peer has already been removed from monitor,
> >+             * delete it here. */
> >+            if (vc->info->type == NET_CLIENT_TYPE_NIC) {
> >+                NICState *nic = DO_UPCAST(NICState, nc, vc);
> >+                if (nic->peer_deleted) {
> >+                    qemu_del_vlan_client(vc->peer);
> >+                }
> >+            }
> >          }
> >      }
> >
> >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 {