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Message-ID: <20251021003018.GC3520958@port70.net>
Date: Tue, 21 Oct 2025 02:30:18 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Pincheng Wang <pincheng.plct@...c.iscas.ac.cn>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH 1/1] riscv: add Zacas extension support for atomic
CAS
* Pincheng Wang <pincheng.plct@...c.iscas.ac.cn> [2025-09-19 00:47:20 +0800]:
> Add compile-time detection for RISC-V Zacas extension and use
> amocas.w.aqrl/amocas.d.aqrl instructions when available.
>
> When __riscv_zacas is defined, a_cas() and a_cas_p() use single amocas
> instructions instead of lr/sc loops. Falls back to existing lr/sc
> implementation when Zacas is not available.
is this a supported extension? are there users?
(implemented on existing cpus with released toolchain versions)
what cflags enable the extension? (how to test)
i can't review if the instructions have the right semantics,
but the code looks ok, with some comments below.
>
> Signed-off-by: Pincheng Wang <pincheng.plct@...c.iscas.ac.cn>
> ---
> arch/riscv32/atomic_arch.h | 17 +++++++++++++++++
> arch/riscv64/atomic_arch.h | 30 ++++++++++++++++++++++++++++++
> 2 files changed, 47 insertions(+)
>
> diff --git a/arch/riscv32/atomic_arch.h b/arch/riscv32/atomic_arch.h
> index 4d418f63..64ef05b7 100644
> --- a/arch/riscv32/atomic_arch.h
> +++ b/arch/riscv32/atomic_arch.h
> }
> +#ifdef __riscv_zacas
newline before #ifdef
> +#else /* Fallback to lr/sc when Zacas is not available */
...
> +#endif /* __riscv_zacas */
> \ No newline at end of file
newline after endif
i think ifdef comments are not needed in such a simple file.
> +++ b/arch/riscv64/atomic_arch.h
> @@ -4,6 +4,34 @@ static inline void a_barrier()
> __asm__ __volatile__ ("fence rw,rw" : : : "memory");
> }
>
> +#ifdef __riscv_zacas
> +
> +#define a_cas a_cas
> +static inline int a_cas(volatile int *p, int t, int s)
> +{
> + int old = t;
> + __asm__ __volatile__ (
> + "amocas.w.aqrl %0, %2, %1"
> + : "+r"(old), "+A"(*(volatile int *)p)
> + : "r"(s)
> + : "memory");
existing cas does not use +A constraint (check git log
why and ensure this is ok).
the ptr cast should not be needed. (same for rv32)
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