Re: [Qemu-devel] [RfC PATCH 0/3] sdl2: add opengl rendering support

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Dec 11, 2014 at 04:29:00PM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > > Second for the long term there will be 3d support in a number of UIs:
> > > I expect sdl2, gtk, egl (using render nodes, for headless) and spice.
> > > Having a global switch for them all looks easier.
> > 
> > In libvirt we model the ability to turn on 3d support for guests against
> > the video device, rather than the backend device, since it is a guest
> > visible feature. So should we do this against the virtual virtio-vga
> > device and have the backends just tie their behaviour to that ?
> 
> There will be a switch for the video device anyway, probably something
> along the lines of '-device virtio-vga,opengl={on,off,auto}'.
> 
> 'off' behavior is obvious ;)
> 'on' must fail in case the ui has no 3d support, but the video doesn't
> know that at initialization time.
> 'auto' should enable 3d in case ui supports it, but again we have the
> problem here is that the device doesn't know at init time.
> 
> With a global switch we can tell the video device 'we want run
> with/without 3d support' and have it initialize accordingly.  Likewise
> ui initialization can check the flag and figure whenever it should take
> the 3d or non-3d code paths.  Finally after ui init is done we can look
> whenever we actually have 3d support when it was requested and error out
> in case we havn't.

Consider mutli-head for a minute. There could be a scenario where we
want to allow 3d usage in one head, but not in the other head. This
might in turn mean we would want to enable opengl in one display
backend but not the other, depending on which head was connected.
eg so only one tenant gets exclusive usage of the 3d features.
A single -global wouldn't allow that to be done.


Regards,
Daniel
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