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From: Peter Maydell <peter.maydell@linaro.org>
Date: Wed, 13 May 2015 22:04:41 +0100
Message-ID: <CAFEAcA8f=xy3Tzxq1kWr4m0eWrAxQvEMYZGL=pJyyvoR5WdQRA@mail.gmail.com>
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Subject: Re: [Qemu-devel] [PATCH 3/3] tcg: Add MO_ALIGN, MO_UNALN
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To: Richard Henderson <rth@twiddle.net>
Cc: Yongbok Kim <yongbok.kim@imgtec.com>, Leon Alrae <leon.alrae@imgtec.com>, QEMU Developers <qemu-devel@nongnu.org>

On 13 May 2015 at 20:10, Richard Henderson <rth@twiddle.net> wrote:
> These modifiers control, on a per-memory-op basis, whether
> unaligned memory accesses are allowed.  The default setting
> reflects the target's definition of ALIGNED_ONLY.
>
> Signed-off-by: Richard Henderson <rth@twiddle.net>

> diff --git a/tcg/tcg.h b/tcg/tcg.h
> index f9fb380..ff5bd8a 100644
> --- a/tcg/tcg.h
> +++ b/tcg/tcg.h
> @@ -241,6 +241,15 @@ typedef enum TCGMemOp {
>      MO_TE    = MO_LE,
>  #endif
>
> +    MO_AMASK = 16,  /* Target reverse "align-ness".  */
> +#ifdef ALIGNED_ONLY
> +    MO_ALIGN = 0,
> +    MO_UNALN = MO_AMASK,
> +#else
> +    MO_ALIGN = MO_AMASK,
> +    MO_UNALN = 0,
> +#endif

I feel like the semantics could use a little clarification here.
Something like:

     MO_ALIGN = ...  /* Accesses must be aligned (or CPU
do_unaligned_access hook is called) */
     MO_UNALN = ...  /* Unaligned accesses are permitted */

or put something similar in a block comment before the ifdef:
    /* MO_UNALN accesses are never checked for alignment; MO_ALIGN
     * accesses will result in a call to the CPU's do_unaligned_access
     * hook if the guest address is not aligned. The default depends
     * on whether the target CPU defines ALIGNED_ONLY.
     */

Otherwise
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

Are unaligned accesses always slow-path, by the way? Presumably
they must be, or this code wouldn't work...

thanks
-- PMM