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


From: Stefan Hajnoczi
Subject: Re: [RFC v3 0/4] tls: add macros for coroutine-safe TLS variables
Date: Tue, 7 Dec 2021 13:53:03 +0000

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

The GCC documentation mentions combining noinline with asm(""):
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noinline-function-attribute

Stefan

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