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

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Date: Thu, 27 Jun 2024 13:53:25 +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

> -#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.  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)

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)) {			\
-		__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)	\