Re: qom device lifecycle interaction with hotplug/hotunplug ?

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 28 Nov 2019 13:33:58 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Thu, Nov 28, 2019 at 04:00:06PM +0000, Peter Maydell wrote:
> > Hi; this is a question which came up in Damien's reset series
> > which I don't know the answer to:
> > 
> > What is the interaction of the QOM device lifecycle (instance_init/realize/
> > unrealize/instance_finalize) with hotplug and hot-unplug ? I couldn't
> > find any documentation of this but maybe I was looking in the wrong
> > place...
> > 
> > Looking at device_set_realized() it seems like we treat "realize"
> > as meaning "and also do the hot-plug if this is a device we're
> > trying to hotplug". On the other hand hot-unplug is I think the
> > other way around: when we get a hot-unplug event we assume that
> > it should also imply an "unrealize" (but just unrealizing doesn't
> > auto-hot-unplug) ?  
> 
> Your description seems accurate, and I agree it is confusing.
> 
> It would be more consistent if realized=true didn't plug the
> device automatically, and qdev_device_add() asked the hotplug
> handler to plug the device instead.
agreed, it's confusing. But that would not allow to
  o = object_new()
  set props
  o.realize()
reuse the same plug handlers.

we potentially can convert it to device_add input arguments
and then call qdev_device_add() instead, which would then
handle plug handlers, not sure it's doable though.
Other than that I don't have any ideas how to make it less confusing.

> > Once a device is hot-unplugged (and thus unrealized) is it valid
> > for it to be re-hot-plugged, or is the assumption that it's then
> > destroyed and a fresh device is created if the user wants to plug
> > something in again later ? Put another way, is it valid for a qdev
> > device to see state transitions realize -> unrealize -> realize ?  
> 
> My interpretation is that this is valid in theory, but likely to
> crash a large portion of our devices if we tried it.
>