Re: [PATCH RFC 00/10] qapi: remove all TARGET_* conditionals from the schema

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

On Sat, May 10, 2025 at 08:08:02AM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > On Fri, May 09, 2025 at 03:43:30PM +0200, Markus Armbruster wrote:
> >> Daniel P. Berrangé <berrange@redhat.com> writes:
> >> > Even if we had a QAPI schema that didn't vary per target, this is
> >> > repeated probing is tricky to avoid when we have completely independant
> >> > binaries. We would need QEMU to have some internal "build id", so that
> >> > we could detect that all binaries came from the same build, to let us
> >> > avoid re-probing each binary.
> >> 
> >> Back when I created QAPI/QMP introspection, I floated the idea to put
> >> something into the QMP greeting to enable safe caching.  Libvirt
> >> developers told me they didn't need that.  I don't remember the details,
> >> but I guess the cache invalidation they already had was deemed good
> >> enough.
> >
> > I don't recall that discussion, but I would think the problem is
> > that we probe much more than just QMP schema. Actually thinking
> > about it, the fact that we probe more than just QMP schema means
> > my idea of probing once and getting the answer for all targets
> > may not be practical. Some of the query-xxx  commands we run will
> > likely need to know the target.
> 
> Yes.
> 
> We could split the cache along validity conditions.  Possibly worth the
> extra complexity if expensive probes then live longer in their cache.
> 
> I'd love to have a list of probes libvirt runs and why.

We don't record the 'why' I'm afraid, so answer the 'why' is pretty
much a case of having to read the code :-(

The list of probes is also somewhat metadata driven, starting
from:

 https://gitlab.com/libvirt/libvirt/-/blob/6ff8d08777ebbcb9a1e11534c3a3341fbf0343e8/src/qemu/qemu_capabilities.c?page=6#L5718

you would have to then work backwards for each virQMEUCapsProbeQMP
method we call.

With regards,
Daniel
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