Re: [Qemu-devel] [PATCH v2] qdev: Mark devices as non-hotpluggable by default

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 25 Sep 2017 10:31:53 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Mon, Sep 25, 2017 at 01:53:16PM +0200, Cornelia Huck wrote:
> > On Fri, 22 Sep 2017 11:16:34 +0200
> > Thomas Huth <thuth@redhat.com> wrote:
> >   
> > > Historically we've marked all devices as hotpluggable by default. However,
> > > most devices are not hotpluggable, and you also need a HotplugHandler to
> > > support these devices. So if the user tries to "device_add" or "device_del"
> > > such a non-hotpluggable device during runtime, either nothing really usable
> > > happens, or QEMU even crashes/aborts unexpectedly (see for example commit
> > > 84ebd3e8c7d4fe955b - "Mark diag288 watchdog as non-hotpluggable").
> > > So let's change this dangerous default behaviour and mark the devices as
> > > non-hotpluggable by default. Certain parent devices classes which are known
> > > as hotpluggable (e.g. PCI, USB, etc.) are marked with "hotpluggable = true",
> > > so that devices that are derived from these classes continue to work as
> > > expected.
> > > 
> > > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > > ---
> > >  v2: Add missing devices and dropped "RFC" status. See Eduardo's reply on
> > >  the previous version of this patch for the rationale which devices need
> > >  to be hotpluggable:
> > >  https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg06128.html
> > > 
> > >  hw/char/virtio-serial-bus.c   |  1 +
> > >  hw/core/qdev.c                | 10 ++++------
> > >  hw/mem/nvdimm.c               |  3 +++
> > >  hw/mem/pc-dimm.c              |  1 +
> > >  hw/pci/pci.c                  |  1 +
> > >  hw/ppc/spapr_cpu_core.c       |  1 +
> > >  hw/s390x/ccw-device.c         |  1 +
> > >  hw/scsi/scsi-bus.c            |  1 +
> > >  hw/usb/bus.c                  |  1 +
> > >  hw/usb/dev-smartcard-reader.c |  1 +
> > >  hw/xen/xen_backend.c          |  1 +
> > >  target/i386/cpu.c             |  4 ++--
> > >  target/s390x/cpu.c            |  1 +
> > >  13 files changed, 19 insertions(+), 8 deletions(-)  
> > 
> > Hm, this seems to break hotplug of virtio devices:
> > 
> > (qemu) device_add virtio-net-pci,id=xxx
> > Device 'virtio-net-device' does not support hotplugging
> > 
> > (qemu) device_add virtio-net-ccw,id=yyy
> > Device 'virtio-net-device' does not support hotplugging
> > 
> > We probably need to enable hotplug for virtio devices as well?  
> 
> I've seen a similar case broken by "[PATCH] hw/core/qdev: Do not
> allow hot-plugging without hotplug controller", because we're
> blocking creation of devices created internally by other devices,
> not just the ones created by device_add.  I need to find the
> exact reproducer again.
> 
> It would probably simplify things if we move the check for
> DeviceClass::hotpluggable to qmp_device_add(), instead of
> device_set_realized().
if we would have to do that, than we are probably doing
something wrong and not using property right. Perhaps we
should fix property instead of trying find a place to check
it where it would hurt less.