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

[Zhenyu Wang]

On 2018.02.22 09:59:17 -0700, Alex Williamson wrote:
> On Thu, 22 Feb 2018 15:46:17 +0000
> Peter Maydell <address@hidden> wrote:
> 
> > On 22 February 2018 at 15:37, Gerd Hoffmann <address@hidden> 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.
>

I'm not sure, how this is supposed to work?

And there's also case that vgpu's display is not required but still need
device hotplug feature e.g to use render/media acceleration, as supported
by current vfio mdev device on gvt-g.

> 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
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