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From: Sergey Fedorov <serge.fdrv@gmail.com>
Message-ID: <573CB671.5040204@gmail.com>
Date: Wed, 18 May 2016 21:37:37 +0300
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Subject: Re: [Qemu-devel] [PATCH v5 04/18] include/processor.h: define
 cpu_relax()
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To: "Emilio G. Cota" <cota@braap.org>
Cc: QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@listserver.greensocs.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>

On 18/05/16 21:29, Emilio G. Cota wrote:
> On Wed, May 18, 2016 at 20:47:56 +0300, Sergey Fedorov wrote:
>> Why don't do like this:
>>
>> #if defined(__foo__)
>> #  define ...
>> #elif defined(__bar__)
>> #  define ...
>> #else
>> #  define ...
>> #endif
> Good point. Changed to:
>
> commit ad31d6cff8e309e41bd4bed110f173e473c27c5a
> Author: Emilio G. Cota <cota@braap.org>
> Date:   Wed Apr 6 18:21:08 2016 -0400
>
>     include/processor.h: define cpu_relax()
>     
>     Taken from the linux kernel.
>     
>     Reviewed-by: Richard Henderson <rth@twiddle.net>
>     Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
>     Signed-off-by: Emilio G. Cota <cota@braap.org>
>
> diff --git a/include/qemu/processor.h b/include/qemu/processor.h
> new file mode 100644
> index 0000000..42bcc99
> --- /dev/null
> +++ b/include/qemu/processor.h
> @@ -0,0 +1,30 @@
> +/*
> + * Copyright (C) 2016, Emilio G. Cota <cota@braap.org>
> + *
> + * 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")
> +
> +#elif defined(__ia64__)
> +# define cpu_relax() asm volatile("hint @pause" ::: "memory")
> +
> +#elif defined(__aarch64__)
> +# define cpu_relax() asm volatile("yield" ::: "memory")
> +
> +#elif 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")
> +
> +#else
> +# define cpu_relax() barrier()
> +#endif
> +
> +#endif /* QEMU_PROCESSOR_H */

Looks like you prefer "sparse" code :)

-Sergey