Re: [Qemu-devel] [PATCH v4 4/7] qdev: add hotpluggable to DeviceState

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 22 Feb 2018 15:46:17 +0000
Peter Maydell <peter.maydell@linaro.org> wrote:

> On 22 February 2018 at 15:37, Gerd Hoffmann <kraxel@redhat.com> wrote:
> > On Mon, Feb 19, 2018 at 12:14:55PM +0100, Gerd Hoffmann wrote:  
> >> vfio display support wants disable hotplug for certain devices, because
> >> qemu doesn't support hotplugging display devices and qemu consoles.
> >>
> >> Add a hotpluggable bool to DeviceState, initialize it from
> >> DeviceClass->hotpluggable, update device_get_hotpluggable accordingly.
> >>
> >> Devices can flip the new variable from true to false if needed.  
> >
> > Alex wants an ack for this one.  Who maintains it these days?
> >
> > MAINTAINERS doesn't list qdev.  For QOM which is closest probably
> > Andreas Färber is listed.  Havn't seen him on the list for a while
> > though.
> >
> > Ok, lets try some usual suspects...
> > Markus? Eric? Paolo? Peter?  Any comments on this one?  
> 
> What type of device is only sometimes hotpluggable ?
> The commit message says "display devices" and "consoles",
> but I would expect those to both be types of device which
> have a class which is never hotpluggable, so you can mark
> them non-hotpluggable with the existing class flag rather
> than needing a per-instance flag.

With this series, a vfio-pci device optionally supports a display.  The
vfio-pci device is hotpluggable, but QEMU display support is not.  So
the solution here is to make the vfio-pci device non-hotpluggable only
when it supports and enables a display.

Gerd, is there another solution that the display object is instantiated
separately from the vfio-pci object and the display support in the
vfio-pci device references the display object via an id.  Possibly
vfio-pci could remain hotpluggable while the display class device is
not.  Potentially one display could be switched between multiple
display capable devices, like an input control button on a monitor,
losing signal if none are connected.  Possible?  Clearly I have no idea
how display objects actually work in QEMU.  Thanks,

Alex