Re: [Qemu-devel] [PATCH v4 3/3] hw/vfio/display: add ramfb support

[Gerd Hoffmann <kraxel@redhat.com>]

> > Well, it's simliar to qxl vs. qxl-vga.  It's not qxl,vga={on,off} and
> > libvirt has no problems to deal with that ...
> >
> > Another more technical reason is (again) hotplug.  ramfb needs an fw_cfg
> > entry for configuration, and fw_cfg entries can't be hotplugged.  So
> > hotplugging vfio-pci with ramfb=on isn't going to fly.  So we need a
> > separate device with hotplug turned off.
> 
> Well if that's not supposed to work ever, libvirt's hotplug code could format
> the following FWIW:
> "-device vfio-pci [opts],ramfb=off"
> 
> As such, new device wouldn't be that of an issue for libvirt if vfio-pci and
> vfio-pci-ramfb are back to back compatible with all the device options that are
> available for vfio-pci (I mean in terms of using an mdev). Because in that
> case, what libvirt could do is to look whether we're supposed to turn on the
> display, if so, then we need support for this in capabilities to query and then
> we could prefer this new device over the "legacy" vfio-pci one. However, if we
> expect a case where QEMU would succeed to start with an mdev mapped to this
> new ramfb device but not with vfio-pci, then that's an issue.

vfio-pci and vfio-pci-ramfb are identical, except for the later having a
boot display (with display=on), and vfio-pci-ramfb not being
hotplugable.  So, yes, all pcu/mdev devices should work with both
vfio-pci variants.

> But I'm still curious about the ramfb=off possibility I asked above
> for hotplug nonetheless.

Well, the problem is introspection.  qemu can report via qmp whenever a
specific device supports hotplug.  A device which can both be
hot-pluggable or not hot-pluggable depending on some condition is a
problem here ...

cheers,
  Gerd