Paolo Bonzini <[email protected]> writes:
> This introduces load-acquire and store-release operations in QEMU.
> For now, just use them as an implementation detail of atomic_mb_read
> and atomic_mb_set.
>
> Since docs/atomics.txt documents that atomic_mb_read only synchronizes
> with an atomic_mb_set of the same variable, we can use the new implementation
> everywhere instead of seq-cst loads and stores.
>
> Signed-off-by: Paolo Bonzini <[email protected]>
> ---
<snip>
> +/* This is more efficient than a store plus a fence. */
> +#if defined(__i386__) || defined(__x86_64__) || defined(__s390x__)
> +#define atomic_mb_set(ptr, i) ((void)atomic_xchg(ptr, i))
> +#endif
Is this working around a compiler issue? Shouldn't it already be using
the best instructions for the constraint?
> +
> +#ifndef atomic_mb_read
> +#define atomic_mb_read(ptr) \
> + atomic_load_acquire(ptr)
> +#endif
> +
> +#ifndef atomic_mb_set
> +#define atomic_mb_set(ptr, i) do { \
> + atomic_store_release(ptr, i); \
> + smp_mb(); \
> +} while(0)
> +#endif
> +
> #endif /* QEMU_ATOMIC_H */
--
Alex Bennée
