[RFC QEMU PATCH] ui: Make the DisplayType enum entries conditional

[Gerd Hoffmann]

On Wed, Jun 09, 2021 at 12:29:58PM +0100, Daniel P. Berrangé wrote:
> On Wed, Jun 09, 2021 at 01:24:05PM +0200, Gerd Hoffmann wrote:
> > On Wed, Jun 09, 2021 at 12:02:40PM +0200, Thomas Huth wrote:
> > > Libvirt's "domcapabilities" command has a way to state whether
> > > certain graphic frontends are available in QEMU or not. Originally,
> > > libvirt looked at the "--help" output of the QEMU binary to determine
> > > whether SDL was available or not (by looking for the "-sdl" parameter
> > > in the help text), but since libvirt stopped doing this analysis of
> > > the help text, the detection of SDL is currently broken, see:
> > > 
> > >  https://bugzilla.redhat.com/show_bug.cgi?id=1790902
> > > 
> > > QEMU should provide a way via the QMP interface instead. The simplest
> > > way, without introducing additional commands, is to make the DisplayType
> > > enum entries conditional, so that the enum only contains the entries if
> > > the corresponding CONFIG_xxx switches have been set.
> > 
> > Hmm, that'll break for the "dnf remove qemu-ui-sdl" case ...
> 
> Note tht libvirt invalidates its cache of QEMU capabilities when it
> sees the /usr/lib64/qemu directory timestamp change. So it ought to
> pick up changes caused by installing/removing QEMU modules, and apply
> this to future queries for domcapabilities, or when starting future
> QEMU guests.

That'll work fine for modules implementing qom objects / devices,
because the list of available objects changes accordingly and libvirt
can see that.

The #if CONFIG_SDL approach will not work because qemu will continue to
report sdl as supported even when the sdl module is not installed any
more.

take care,
  Gerd