Re: [Qemu-devel] [PATCH v3 0/5] virtio-9p: hotplug and migration support

[Greg Kurz]

On Tue, 20 Oct 2015 15:53:08 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> On Tue, Oct 20, 2015 at 11:16:40AM +0200, Greg Kurz wrote:
> > We already have a blocker to prevent migration of an active virtio-9p 
> > device.
> > But in fact, there is no migration support at all for 9p, even if the device
> > is considered to be quiescent (when the VirtFS share is not mounted): 
> > migration
> > succeeds but the device is lost in the restarted guest.
> > Hotunplug of a virtio-9p device is not supported either (no unrealize 
> > handler)
> > and leads to a QEMU crash on the source node, if one unplugs and migrates.
> > 
> > This series tries to fix that and brings hotplug and migration support of
> > *quiescent* virtio-9p devices.
> > 
> > v2->v3:
> >  - renamed QDEV handler @unpluggable to @unplug_is_blocked (patches 2/5
> >    and 3/5)
> > 
> > v1->v2:
> >  - introduced unplug blocker (patches 2/5 and 3/5)
> >  - moved fixes to separate patches (see individual changelogs)
> 
> I have some doubts about how hotunplug is handled, but migration
> looks ok.
> Is there a dependency, or can I just pick savevm things meanwhile?
> 

Well... you can drop the unplug blocker patches (2/5 and 3/5) but the
savevm patch (5/5) modifies a function introduced by the unrealize
patch (4/5):

@@ -146,6 +157,7 @@ static void virtio_9p_device_unrealize(DeviceState *dev, 
Error **errp)
     v9fs_release_worker_threads();
     g_free(s->ctx.fs_root);
     g_free(s->tag);
+    unregister_savevm(dev, "virtio-9p", s);
     virtio_cleanup(vdev);
 }

and not having an unrealize handler calling unregister_savevm() means
unplug will succeed and leave a dangling pointer in the vm_change_state
list... This causes a systematic crash of the source QEMU when attempting
a migration.

> > ---
> > 
> > Greg Kurz (5):
> >       virtio-9p-coth: fix init function
> >       qdev: add the @unplug_is_blocked handler
> >       virtio-9p: block hot-unplug when device is active
> >       virtio-9p: add unrealize handler
> >       virtio-9p: add savem handlers
> > 
> > 
> >  hw/9pfs/virtio-9p-coth.c   |   22 ++++++++++++++++++----
> >  hw/9pfs/virtio-9p-coth.h   |    2 ++
> >  hw/9pfs/virtio-9p-device.c |   24 ++++++++++++++++++++++++
> >  hw/9pfs/virtio-9p.c        |   14 ++++++++++++++
> >  hw/9pfs/virtio-9p.h        |    2 ++
> >  hw/core/qdev.c             |    4 ++++
> >  hw/s390x/virtio-ccw.c      |    8 ++++++++
> >  hw/virtio/virtio-pci.c     |    8 ++++++++
> >  include/hw/qdev-core.h     |    4 ++++
> >  9 files changed, 84 insertions(+), 4 deletions(-)
>