riscv: Implement cmpxchg32/64() using Zacas

This adds runtime support for Zacas in cmpxchg operations.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20241103145153.105097-4-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -632,6 +632,22 @@ config RISCV_ISA_ZAWRS
 	  use of these instructions in the kernel when the Zawrs extension is
 	  detected at boot.
 
+config TOOLCHAIN_HAS_ZACAS
+	bool
+	default y
+	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zacas)
+	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zacas)
+	depends on AS_HAS_OPTION_ARCH
+
+config RISCV_ISA_ZACAS
+	bool "Zacas extension support for atomic CAS"
+	depends on TOOLCHAIN_HAS_ZACAS
+	depends on RISCV_ALTERNATIVE
+	default y
+	help
+	  Enable the use of the Zacas ISA-extension to implement kernel atomic
+	  cmpxchg operations when it is detected at boot.
+
 	  If you don't know what to do here, say Y.
 
 config TOOLCHAIN_HAS_ZBB

arch/riscv/Makefile

@@ -82,6 +82,9 @@ else
 riscv-march-$(CONFIG_TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI) := $(riscv-march-y)_zicsr_zifencei
 endif
 
+# Check if the toolchain supports Zacas
+riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZACAS) := $(riscv-march-y)_zacas
+
 # Remove F,D,V from isa string for all. Keep extensions between "fd" and "v" by
 # matching non-v and non-multi-letter extensions out with the filter ([^v_]*)
 KBUILD_CFLAGS += -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)fd([^v_]*)v?/\1\2/')

arch/riscv/include/asm/cmpxchg.h

@@ -12,6 +12,7 @@
 #include <asm/fence.h>
 #include <asm/hwcap.h>
 #include <asm/insn-def.h>
+#include <asm/cpufeature-macros.h>
 
 #define __arch_xchg_masked(sc_sfx, prepend, append, r, p, n)		\
 ({									\
@@ -137,24 +138,37 @@
 	r = (__typeof__(*(p)))((__retx & __mask) >> __s);		\
 })
 
-#define __arch_cmpxchg(lr_sfx, sc_sfx, prepend, append, r, p, co, o, n)	\
+#define __arch_cmpxchg(lr_sfx, sc_cas_sfx, prepend, append, r, p, co, o, n)	\
 ({									\
-	register unsigned int __rc;					\
+	if (IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) &&			\
+	    riscv_has_extension_unlikely(RISCV_ISA_EXT_ZACAS)) {	\
+		r = o;							\
 									\
-	__asm__ __volatile__ (						\
-		prepend							\
-		"0:	lr" lr_sfx " %0, %2\n"				\
-		"	bne  %0, %z3, 1f\n"				\
-		"	sc" sc_sfx " %1, %z4, %2\n"			\
-		"	bnez %1, 0b\n"					\
-		append							\
-		"1:\n"							\
-		: "=&r" (r), "=&r" (__rc), "+A" (*(p))			\
-		: "rJ" (co o), "rJ" (n)					\
-		: "memory");						\
+		__asm__ __volatile__ (					\
+			prepend						\
+			"	amocas" sc_cas_sfx " %0, %z2, %1\n"	\
+			append						\
+			: "+&r" (r), "+A" (*(p))			\
+			: "rJ" (n)					\
+			: "memory");					\
+	} else {							\
+		register unsigned int __rc;				\
+									\
+		__asm__ __volatile__ (					\
+			prepend						\
+			"0:	lr" lr_sfx " %0, %2\n"			\
+			"	bne  %0, %z3, 1f\n"			\
+			"	sc" sc_cas_sfx " %1, %z4, %2\n"		\
+			"	bnez %1, 0b\n"				\
+			append						\
+			"1:\n"						\
+			: "=&r" (r), "=&r" (__rc), "+A" (*(p))		\
+			: "rJ" (co o), "rJ" (n)				\
+			: "memory");					\
+	}								\
 })
 
-#define _arch_cmpxchg(ptr, old, new, sc_sfx, prepend, append)		\
+#define _arch_cmpxchg(ptr, old, new, sc_cas_sfx, prepend, append)	\
 ({									\
 	__typeof__(ptr) __ptr = (ptr);					\
 	__typeof__(*(__ptr)) __old = (old);				\
@@ -164,15 +178,15 @@
 	switch (sizeof(*__ptr)) {					\
 	case 1:								\
 	case 2:								\
-		__arch_cmpxchg_masked(sc_sfx, prepend, append,		\
+		__arch_cmpxchg_masked(sc_cas_sfx, prepend, append,	\
 					__ret, __ptr, __old, __new);	\
 		break;							\
 	case 4:								\
-		__arch_cmpxchg(".w", ".w" sc_sfx, prepend, append,	\
+		__arch_cmpxchg(".w", ".w" sc_cas_sfx, prepend, append,	\
 				__ret, __ptr, (long), __old, __new);	\
 		break;							\
 	case 8:								\
-		__arch_cmpxchg(".d", ".d" sc_sfx, prepend, append,	\
+		__arch_cmpxchg(".d", ".d" sc_cas_sfx, prepend, append,	\
 				__ret, __ptr, /**/, __old, __new);	\
 		break;							\
 	default:							\