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

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Sep 25, 2017 at 07:42:13PM +0200, Thomas Huth wrote:
> On 25.09.2017 17:26, Peter Maydell wrote:
> > On 25 September 2017 at 16:19, Thomas Huth <thuth@redhat.com> wrote:
> >> Not sure whether this works for the virtio-xxx-device devices,
> >> though, since they are marked as user_creatable = true currently...
> > 
> > That's deliberate -- for the arm boards with virtio-mmio
> > the board creates a bunch of virtio-mmio transports and the
> > virtio-foo-device can be user created to plug into those.
> 
> Yes, I know ... I'm just wondering whether the virtio-xxx-device devices
> should be non-user_creatable on the non-ARM targets, since they
> apparently can't be used with "-device" there...?

I wouldn't like to make DeviceClass fields depend on the target.
Being user-creatable doesn't mean they will work on all machines,
anyway.  If user/management need more specific info, they need
something like the query-device-slots command I've proposed some
time ago.

> 
> > If overall trying to do the 'right thing' is tricky here
> > then perhaps we can start by flipping the default to
> > not-hotpluggable and marking some devices hotpluggable
> > that in theory shouldn't be with a comment about why.
> 
> Yes, if Eduardo's idea to move the test to qmp_device_add() does not
> work out (I'll try that next), your suggestion is certainly the best
> thing we can do right now.

I think it would work, but we would lose the feature Peter
mentions below:

> 
> > Incidentally I think there's still some advantage in the
> > "created as part of some other device" devices having
> > to be explicitly marked hotpluggable, because those
> > devices do still need some specific code in them to
> > handle it (ie code to release the resources that are
> > created in the device's realize method).
> 
> Ok ... by the way, does anybody know more devices like
> virtio-xxx-device, i.e. devices that are indirectly plugged when adding
> other devices?

"make check" found a few candidates:
https://travis-ci.org/ehabkost/qemu/jobs/278743999

  Initialization of device dpcd failed: Device 'dpcd' does not support hotplugging
[...]
  Initialization of device nand failed: Device 'nand' does not support hotplugging

Finding the full list of devices that can be instantiated
internally at hotplug-time sounds tricky.

-- 
Eduardo