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

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Message-ID: <1cd452af-58cd-468c-9bb6-90f67711d0b0@ghiti.fr>
Date: Thu, 4 Jul 2024 18:36:26 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Andrea Parri <parri.andrea@...il.com>,
 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

On 27/06/2024 13:53, Andrea Parri wrote:
>> -#define __arch_cmpxchg_masked(sc_sfx, prepend, append, r, p, o, n)	\
>> +#define __arch_cmpxchg_masked(sc_sfx, cas_sfx, prepend, append, r, p, o, n)	\
>>   ({									\
>> +	__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:;								\
>>   	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)	\
>> @@ -133,6 +145,19 @@
>>   		: "memory");						\
>>   									\
>>   	r = (__typeof__(*(p)))((__retx & __mask) >> __s);		\
>> +	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");					\
>> +	}								\
>> +end:;									\
>>   })
> I admit that I found this all quite difficult to read; IIUC, this is
> missing an IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) check.

I'm not sure we need the zacas check here, since we could use a 
toolchain that supports zabha but not zacas, run this on a zabha/zacas 
platform and it would work.

>    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)

If that makes things clearer for you, sure, I can do that.

>
> Also, the patch presents the first occurrence of RISCV_ISA_EXT_ZABHA;
> perhaps worth moving the hwcap/cpufeature changes from patch #6 here?
>
>    Andrea
>
> 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)) {			\

But we need to keep this check, otherwise it would fail to compile on 
toolchains without Zabha support.

> -		__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)	\
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

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