linux-kernel - Re: [PATCH v2 03/10] riscv: Implement cmpxchg8/16() using Zabha

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Message-ID: <ZoBeTTdMYdXU8gxD@andrea>
Date: Sat, 29 Jun 2024 21:19:41 +0200
From: Andrea Parri <parri.andrea@...il.com>
To: Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Jonathan Corbet <corbet@....net>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Nathan Chancellor <nathan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
	Arnd Bergmann <arnd@...db.de>, Leonardo Bras <leobras@...hat.com>,
	Guo Ren <guoren@...nel.org>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 03/10] riscv: Implement cmpxchg8/16() using Zabha

> I admit that I found this all quite difficult to read; IIUC, this is
> missing an IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) check.  How about adding
> such a check under the zabha: label (replacing/in place of the second
> IS_ENABLED(CONFIG_RISCV_ISA_ZABHA) check) and moving the corresponding
> asm goto statement there, perhaps as follows? (on top of this patch)

[...]

> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index b9a3fdcec919..3c913afec150 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -110,15 +110,12 @@
>  	__label__ no_zacas, zabha, end;					\
>  									\
>  	if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA)) {			\
> -		asm goto(ALTERNATIVE("j %[no_zacas]", "nop", 0,		\
> -				     RISCV_ISA_EXT_ZACAS, 1)		\
> -			 : : : : no_zacas);				\
>  		asm goto(ALTERNATIVE("nop", "j %[zabha]", 0,		\
>  				     RISCV_ISA_EXT_ZABHA, 1)		\
>  			 : : : : zabha);				\
>  	}								\
>  									\
> -no_zacas:;								\
> +no_zacas:								\
>  	u32 *__ptr32b = (u32 *)((ulong)(p) & ~0x3);			\
>  	ulong __s = ((ulong)(p) & (0x4 - sizeof(*p))) * BITS_PER_BYTE;	\
>  	ulong __mask = GENMASK(((sizeof(*p)) * BITS_PER_BYTE) - 1, 0)	\
> @@ -148,16 +145,20 @@ no_zacas:;								\
>  	goto end;							\
>  									\
>  zabha:									\
> -	if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA)) {			\
> -		__asm__ __volatile__ (					\
> -			prepend						\
> -			"	amocas" cas_sfx " %0, %z2, %1\n"	\
> -			append						\
> -			: "+&r" (r), "+A" (*(p))			\
> -			: "rJ" (n)					\
> -			: "memory");					\
> +	if (IS_ENABLED(CONFIG_RISCV_ISA_ZAZAS)) {			\
> +		asm goto(ALTERNATIVE("j %[no_zacas]", "nop", 0,		\
> +				     RISCV_ISA_EXT_ZACAS, 1)		\
> +			 : : : : no_zacas);				\
>  	}								\
> -end:;									\
> +									\
> +	__asm__ __volatile__ (						\
> +		prepend							\
> +		"	amocas" cas_sfx " %0, %z2, %1\n"		\
> +		append							\
> +		: "+&r" (r), "+A" (*(p))				\
> +		: "rJ" (n)						\
> +		: "memory");						\
> +end:									\
>  })
>  
>  #define __arch_cmpxchg(lr_sfx, sc_cas_sfx, prepend, append, r, p, co, o, n)	\

Unfortunately, this diff won't work e.g. when ZABHA is supported and
detected at boot while ZACAS is not supported; more thinking required...

  Andrea