Re: [RFC v3 0/4] tls: add macros for coroutine-safe TLS variables

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Dec 07, 2021 at 01:55:34PM +0000, Peter Maydell wrote:
> On Tue, 7 Dec 2021 at 13:53, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > On Mon, Dec 06, 2021 at 02:34:45PM +0000, Peter Maydell wrote:
> > > On Mon, 6 Dec 2021 at 14:33, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > >
> > > > v3:
> > > > - Added __attribute__((weak)) to get_ptr_*() [Florian]
> > >
> > > Do we really need it *only* on get_ptr_*() ? If we need to
> > > noinline the other two we probably also should use the same
> > > attribute weak to force no optimizations at all.
> >
> > The weak attribute can't be used on static functions, so I think we need
> > a different approach:
> >
> > In file included from ../util/async.c:35:
> > /builds/stefanha/qemu/include/qemu/coroutine-tls.h:201:11: error: weak declaration of 'get_ptr_my_aiocontext' must be public
> >      type *get_ptr_##var(void)                                                \
> >            ^~~~~~~~
> > ../util/async.c:673:1: note: in expansion of macro 'QEMU_DEFINE_STATIC_CO_TLS'
> >  QEMU_DEFINE_STATIC_CO_TLS(AioContext *, my_aiocontext)
> >  ^~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Adding asm volatile("") seems to work though:
> > https://godbolt.org/z/3hn8Gh41d
> 
> You can see in the clang disassembly there that this isn't
> sufficient. The compiler puts in both calls, but it ignores
> the return results and always returns "true" from the function.

Specifying a input operand forces the compiler to evaluate the return
value of get_ptr_i():

  static __attribute__((noinline)) long get_ptr_i()
  {
    long l = (long)&i;
    asm volatile("" : "+rm" (l));
    return l;
  }

https://godbolt.org/z/e6ddGdPMv

Stefan

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