Re: [Qemu-devel] [PATCH v5 04/18] include/processor.h: define cpu_relax()

[Sergey Fedorov]

On 14/05/16 06:34, Emilio G. Cota wrote:
> Taken from the linux kernel.
>
> Reviewed-by: Richard Henderson <address@hidden>
> Reviewed-by: Alex Bennée <address@hidden>
> Signed-off-by: Emilio G. Cota <address@hidden>
> ---
>  include/qemu/processor.h | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>  create mode 100644 include/qemu/processor.h
>
> diff --git a/include/qemu/processor.h b/include/qemu/processor.h
> new file mode 100644
> index 0000000..4e6a71f
> --- /dev/null
> +++ b/include/qemu/processor.h
> @@ -0,0 +1,34 @@
> +/*
> + * Copyright (C) 2016, Emilio G. Cota <address@hidden>
> + *
> + * License: GNU GPL, version 2.
> + *   See the COPYING file in the top-level directory.
> + */
> +#ifndef QEMU_PROCESSOR_H
> +#define QEMU_PROCESSOR_H
> +
> +#include "qemu/atomic.h"
> +
> +#if defined(__i386__) || defined(__x86_64__)
> +#define cpu_relax() asm volatile("rep; nop" ::: "memory")
> +#endif
> +
> +#ifdef __ia64__
> +#define cpu_relax() asm volatile("hint @pause" ::: "memory")
> +#endif
> +
> +#ifdef __aarch64__
> +#define cpu_relax() asm volatile("yield" ::: "memory")
> +#endif
> +
> +#if defined(__powerpc64__)
> +/* set Hardware Multi-Threading (HMT) priority to low; then back to medium */
> +#define cpu_relax() asm volatile("or 1, 1, 1;"
> +                                 "or 2, 2, 2;" ::: "memory")
> +#endif
> +
> +#ifndef cpu_relax
> +#define cpu_relax() barrier()
> +#endif
> +
> +#endif /* QEMU_PROCESSOR_H */

Why don't do like this:

#if defined(__foo__)
#  define ...
#elif defined(__bar__)
#  define ...
#else
#  define ...
#endif

Kind regards,
Sergey