Re: [Qemu-devel] [PATCH 2/2] hw/vfio/display: add ramfb support

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

On Tue, 11 Sep 2018 06:38:43 +0200
Gerd Hoffmann <kraxel@redhat.com> wrote:

>   Hi,
> 
> > >      type_register_static(&vfio_pci_dev_info);
> > > +    type_register_static(&vfio_pci_ramfb_dev_info);  
> 
> > My concern here is still all of the extra tooling that needs to be
> > added to management layers above QEMU for this device that exists only
> > because we can't hotplug the primary display in QEMU.  What happens when
> > we can hotplug the primary display?  
> 
> Ramfb uses fw_cfg, and fw_cfg files can't be added or removed at
> runtime, the interface simply isn't designed for that.
> 
> > Aren't disabling hotplug of a
> > vfio-pci device and supporting ramfb two separate things?  I think
> > we're leaking current implementation issues out to the device options
> > when really we'd rather have a "ramfb" (or perhaps "console") option on
> > the vfio-pci device and the hotplug capability determined automatically
> > and available through introspection of the device.  
> 
> Well, I don't think libvirt will have too much trouble handling this.
> We have two variants (with and without vga compatibility) of other
> devices: qxl-vga and qxl, virtio-vga and virtio-gpu-pci.  libvirt copes
> just fine and picks the right one (I think depending on video model
> 'primary' property).
> 
> Also libvirt manages hotpluggability per device *class*, not per device
> *instance*.  So a device being hotpluggable or not depending on some
> device property is a problem for libvirt ...
> 
> I'm open to suggestions how to handle this better, as long as the
> libvirt people are on board with the approach.

Ok, so we need a new class to handle making a device non-hotpluggable,
but I'm still not sure whether we should make:

 -device vfio-pci-ramfb

or

 -device vfio-pci-nohotplug,ramfb=on

Where ramfb would be a property only available on the nohotplug class
variant.  The latter seems to provide a lot more flexibility, but which
is more practical for libvirt?  Thanks,

Alex