Re: qom device lifecycle interaction with hotplug/hotunplug ?

[Eduardo Habkost]

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.

> 
> 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.

-- 
Eduardo