Re: [Qemu-devel] [Qemu-arm] [PATCH 4/9] tcg: Introduce atomic helpers for integer min/max

[Peter Maydell <peter.maydell@linaro.org>]

On 27 April 2018 at 01:26, Richard Henderson
<richard.henderson@linaro.org> wrote:
> Given that this atomic operation will be used by both risc-v
> and aarch64, let's not duplicate code across the two targets.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  accel/tcg/atomic_template.h | 71 +++++++++++++++++++++++++++++++++++++++++++++
>  accel/tcg/tcg-runtime.h     |  8 +++++
>  tcg/tcg-op.h                | 34 ++++++++++++++++++++++
>  tcg/tcg.h                   |  8 +++++
>  tcg/tcg-op.c                |  8 +++++
>  5 files changed, 129 insertions(+)

> @@ -233,6 +270,39 @@ ABI_TYPE ATOMIC_NAME(add_fetch)(CPUArchState *env, target_ulong addr,
>          ldo = ldn;
>      }
>  }
> +
> +/* These helpers are, as a whole, full barriers.  Within the helper,
> + * the leading barrier is explicit and the trailing barrier is within
> + * cmpxchg primitive.
> + */
> +#define GEN_ATOMIC_HELPER_FN(X, FN, XDATA_TYPE, RET)                \
> +ABI_TYPE ATOMIC_NAME(X)(CPUArchState *env, target_ulong addr,       \
> +                        ABI_TYPE xval EXTRA_ARGS)                   \
> +{                                                                   \
> +    ATOMIC_MMU_DECLS;                                               \
> +    XDATA_TYPE *haddr = ATOMIC_MMU_LOOKUP;                          \
> +    XDATA_TYPE ldo, ldn, old, new, val = xval;                      \
> +    smp_mb();                                                       \
> +    ldn = atomic_read__nocheck(haddr);                              \

I see you're using the __nocheck function here. How does this
work for the 32-bit host case where you don't necessarily have
a 64-bit atomic primitive?

> +    do {                                                            \
> +        ldo = ldn; old = BSWAP(ldo); new = FN(old, val);            \
> +        ldn = atomic_cmpxchg__nocheck(haddr, ldo, BSWAP(new));      \
> +    } while (ldo != ldn);                                           \
> +    ATOMIC_MMU_CLEANUP;                                             \
> +    return RET;                                                     \
> +}

I was going to suggest that you could also now use this to
iimplement the currently-hand-coded fetch_add and add_fetch
for the reverse-host-endian case, but those don't have a leading
smp_mb() and this does. Do you know why those are different?

thanks
-- PMM