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

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Sep 25, 2017 at 03:46:47PM +0200, Igor Mammedov wrote:
> 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.

I disagree.  It depends on the purpose and semantics we define
for DeviceClass::hotpluggable.  My goal is to have a reliable way
to tell the user if a device really supports device_add, and I
think it wouldn't make sense to report hotpluggable=true on a
device that is never instantiated directly by the user and only
used internally.

-- 
Eduardo