[libvirt] [RFC PATCH 0/7] Enable vfio-pci 'property' for mediated devices

[Gerd Hoffmann]

On Wed, Jun 06, 2018 at 02:49:24PM +0200, Erik Skultety wrote:
> On Tue, Jun 05, 2018 at 11:52:03AM +0200, Gerd Hoffmann wrote:
> >   Hi,
> >
> > > > TL;DR:
> > > > - we have a new attribute value for vfio-pci mediated devices called 'display'
> > > >     -> devices can now format this new 'display=on/off' property to the cmdline
> >
> > Good.
> >
> > > > - if user enables the vfio display (display=on) but doesn't enable OpenGL for
> > > >     Spice, we automatically assume the usage of '-display egl-headless'
> >
> > Hmm, I think it would be better to have egl-headless explicitly
> > configured in the domain xml instead of doing it automagically depending
> > on configuration.
> 
> I was thinking about that earlier, then we had a private conversation where I
> asked why there's no documentation on egl-headless in QEMU to which you replied
> that the future of egl-headless being uncertain, especially once there's remote
> opengl renderer support (as opposed to local nowadays). So I took a bit of a
> safe path here and only made the changes to the command line, thus leaving us
> with a way of easily ditching that if QEMU decides to deprecate egl-headless
> completely. So, if QEMU can guarantee supporting egl-headless (of course you
> can discourage usage of it...) then we surely can expose it through the XML,

Well, I guess if libvirt uses it it pretty much has to stay anyway ...

It isn't much of a burden code-wise, most of the underlying egl/opengl
code is needed for -spice gl=on too.  It is just that once spice got
remote support for gl=on mode it is alot less useful because native
spice support clearly is the better route performance-wise.

> egl-headless was introduced in 2.10 IIRC, but I don't see any way for libvirt
> to use any kind of introspection here, there's no capability for egl-headless,

Yea, right, isn't there.

> I see it defined as a value for DisplayType in the qapi scheme (ui.json), but
> I don't see any query command.

Also the enum is alwayws present, no matter whenever qemu was compiled
with opengl support or not.  So even in case you could query it it
wouldn't buy you anything.

How is spice/vnc support probed for btw?

cheers,
  Gerd