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

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] [PATCH 2/2] hw/vfio/display: add ramfb support
Date: Tue, 11 Sep 2018 06:38:43 +0200
User-agent: NeoMutt/20180716


> >      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.


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