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

[Eduardo Habkost <ehabkost@redhat.com>]

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().

-- 
Eduardo