Re: [Qemu-devel] [PATCH v5 05/12] stubs: add stubs for io_uring interface

[Stefan Hajnoczi]

On Mon, Jun 17, 2019 at 03:33:01PM +0300, Maxim Levitsky wrote:
> On Mon, 2019-06-10 at 19:18 +0530, Aarushi Mehta wrote:
> > diff --git a/stubs/io_uring.c b/stubs/io_uring.c
> > new file mode 100644
> > index 0000000000..622d1e4648
> > --- /dev/null
> > +++ b/stubs/io_uring.c
> > @@ -0,0 +1,32 @@
> > +/*
> > + * Linux io_uring support.
> > + *
> > + * Copyright (C) 2009 IBM, Corp.
> > + * Copyright (C) 2009 Red Hat, Inc.
> > + *
> > + * This work is licensed under the terms of the GNU GPL, version 2 or 
> > later.
> > + * See the COPYING file in the top-level directory.
> > + */
> > +#include "qemu/osdep.h"
> > +#include "block/aio.h"
> > +#include "block/raw-aio.h"
> > +
> > +void luring_detach_aio_context(LuringState *s, AioContext *old_context)
> > +{
> > +    abort();
> > +}
> > +
> > +void luring_attach_aio_context(LuringState *s, AioContext *new_context)
> > +{
> > +    abort();
> > +}
> > +
> > +LuringState *luring_init(Error **errp)
> > +{
> > +    abort();
> > +}
> > +
> > +void luring_cleanup(LuringState *s)
> > +{
> > +    abort();
> > +}
> 
> I do wonder if there is any value in these stubs (and linux aio stubs as 
> well) since any attempt to use this
> will abort the test, even basic aio initialization.
> 
> I am not yet familiar with qemu unit tests, so I won't yet put ack on this 
> patch.

From docs/devel/build-system.txt:

  The utility code that is used by all binaries is built into a
  static archive called libqemuutil.a, which is then linked to all the
  binaries. In order to provide hooks that are only needed by some of the
  binaries, code in libqemuutil.a may depend on other functions that are
  not fully implemented by all QEMU binaries.  Dummy stubs for all these
  functions are also provided by this library, and will only be linked
  into the binary if the real implementation is not present.  In a way,
  the stubs can be thought of as a portable implementation of the weak
  symbols concept.

Take util/async.c as an example.  It calls laio_init(), which lives in
block/.  But our binary using libqemuutil.a (which contains async.o)
never actually calls into it.  We need to a stub just to make the linker
happy but don't worry, it will never be called by the program.

Stefan

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