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

[Michael S. Tsirkin]

On Mon, Sep 20, 2010 at 11:56:56AM -0500, Anthony Liguori wrote:
> On 09/20/2010 11:47 AM, Michael S. Tsirkin wrote:
> >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.
> 
> At the management layer instead of doing it magically in the backend.

The amount of pain this inflicts on management would be considerable.
Hotplug commands were designed to be asynchronous
(starts the process, does not wait for it to complete), maybe that
was a mistake but we can not change semantics at will now.

Add new commands, okay, but existing ones should work and get fixed
if there's a bug.

> IOW, if device_del returns and the device isn't actually deleted,
> that's a bug and addressing it like this just means we'll trip over
> it somewhere else.
> 
> We'll have the same problem with drive_del.

Let's fix it there as well then.

> Regards,
> 
> Anthony Liguori
> 
> >>>Signed-off-by: Michael S. Tsirkin<address@hidden>
> >>>---
> >>>  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 {