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

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On 09/06/2021 13.49, Gerd Hoffmann wrote:
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.

I guess we'd need a separate QMP command to fix that, which tries to load the modules first when being called? Something similar to what is being done in qemu_display_help() ? That's certainly doable, too, just a little bit more complex... do we want that? Or is the quick-n-easy way via the schema good enough for most use cases? (I'm not that familiar with "virsh domcapabilities" ... is there any real usage for the <graphics> section or is this rather cosmetical?)

 Thomas


PS: My CI runs with the patch just finished, and apparently I missed some #ifdefs in other parts of the code ... so I need to respin this patch anyway, no matter which direction we decide to go...




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