Re: [PATCH v5 02/24] include: define constant for early constructor priority

On Fri, Jan 09, 2026 at 12:39:59PM +0100, Paolo Bonzini wrote:
> On 1/8/26 18:03, Daniel P. Berrangé wrote:
> > Functions marked with __attribute__((__constructor__)) will be
> > invoked in linker order. In theory this is well defined, but
> > in practice, it is hard to determine what this order will be
> > with the layers of indirection through meson, ninja and the
> > static libraries QEMU builds.
> > 
> > Notably, the order currently appears different between Linux
> > and Windows (as tested with Wine on Linux). This can cause
> > problems when certain QEMU constructors have a dependancy on
> > other QEMU constructors.
> 
> What dependency are you seeing, or introducing?
> 
> In theory QEMU constructors should not have any dependency, as we only use
> them to initialize static data structures (such as the various *_init
> macros).
> 
> Not an objection, but I'd like to understand this better.

See the commit message in patch 2.   Essentially the RCU thread
gets spawned from a constructor, and if anything it does results
in an error_report() call, it will trigger use of monitor APIs
before the monitor constructor has had a chance to run.

IMHO the root problem is that it is pretty dubious for us to be
spawning the RCU thread from a constructor. Constructors should
be limited to minimal initialization of state. Spawning threads
is outside the boundary of what I'd consider reasonable for
a constructor todo. I didn't want try to tackle refactoring the
RCU thread creation though, hence this gross workaround.

> 
> Paolo
> 
> > To address this define a QEMU_CONSTRUCTOR_EARLY constant which
> > provides a priority value that will run before other default
> > constructors. This is to be used for QEMU constructors that
> > are themselves self-contained, but may be relied upon by other
> > constructors.
> > 
> > Reviewed-by: Richard Henderson <[email protected]>
> > Reviewed-by: Dr. David Alan Gilbert <[email protected]>
> > Signed-off-by: Daniel P. Berrangé <[email protected]>
> > ---
> >   include/qemu/compiler.h | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
> > index 1c2b673c05..4c49f52eb0 100644
> > --- a/include/qemu/compiler.h
> > +++ b/include/qemu/compiler.h
> > @@ -194,6 +194,14 @@
> >   # define QEMU_USED
> >   #endif
> > +/*
> > + * A priority for __attribute__((constructor(...))) that
> > + * will run earlier than the default constructors. Must
> > + * only be used for functions that have no dependency
> > + * on global initialization of other QEMU subsystems.
> > + */
> > +#define QEMU_CONSTRUCTOR_EARLY 101
> > +
> >   /*
> >    * Disable -ftrivial-auto-var-init on a local variable.
> >    *
> 

With regards,
Daniel
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