[PATCH 2/7] riscv: Implement cmpxchg8/16() using Zabha

[Alexandre Ghiti]

This adds runtime support for Zabha in cmpxchg8/16 operations.

Signed-off-by: Alexandre Ghiti <alexghiti at rivosinc.com>
---
 arch/riscv/Kconfig               | 16 ++++++++++++++++
 arch/riscv/Makefile              | 10 ++++++++++
 arch/riscv/include/asm/cmpxchg.h | 26 ++++++++++++++++++++++++--
 3 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index b443def70139..05597719bb1c 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -579,6 +579,22 @@ config RISCV_ISA_V_PREEMPTIVE
 	  preemption. Enabling this config will result in higher memory
 	  consumption due to the allocation of per-task's kernel Vector context.
 
+config TOOLCHAIN_HAS_ZABHA
+	bool
+	default y
+	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zabha)
+	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zabha)
+	depends on AS_HAS_OPTION_ARCH
+
+config RISCV_ISA_ZABHA
+	bool "Zabha extension support for atomic byte/half-word operations"
+	depends on TOOLCHAIN_HAS_ZABHA
+	default y
+	help
+	  Adds support to use atomic byte/half-word operations in the kernel.
+
+	  If you don't know what to do here, say Y.
+
 config TOOLCHAIN_HAS_ZACAS
 	bool
 	default y
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index d5b60b87998c..f58ac921dece 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -89,6 +89,16 @@ else
 riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZACAS) := $(riscv-march-y)_zacas
 endif
 
+# Check if the toolchain supports Zabha
+ifdef CONFIG_AS_IS_LLVM
+# Support for experimental Zabha was merged in LLVM 19.
+KBUILD_CFLAGS += -menable-experimental-extensions
+KBUILD_AFLAGS += -menable-experimental-extensions
+riscv-march-y := $(riscv-march-y)_zabha1p0
+else
+riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZABHA) := $(riscv-march-y)_zabha
+endif
+
 # Remove F,D,V from isa string for all. Keep extensions between "fd" and "v" by
 # matching non-v and non-multi-letter extensions out with the filter ([^v_]*)
 KBUILD_CFLAGS += -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)fd([^v_]*)v?/\1\2/')
diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index 1c50b4821ac8..65de9771078e 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -103,8 +103,14 @@
  * indicated by comparing RETURN with OLD.
  */
 
-#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__ zabha, end;						\
+									\
+	asm goto(ALTERNATIVE("nop", "j %[zabha]", 0,			\
+			     RISCV_ISA_EXT_ZABHA, 1)			\
+			: : : : zabha);					\
+									\
 	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)	\
@@ -131,6 +137,17 @@
 		: "memory");						\
 									\
 	r = (__typeof__(*(p)))((__retx & __mask) >> __s);		\
+	goto end;							\
+									\
+zabha:									\
+	__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)	\
@@ -175,8 +192,13 @@ end:									\
 									\
 	switch (sizeof(*__ptr)) {					\
 	case 1:								\
+		__arch_cmpxchg_masked(sc_sfx, ".b" sc_sfx,		\
+					prepend, append,		\
+					__ret, __ptr, __old, __new);    \
+		break;							\
 	case 2:								\
-		__arch_cmpxchg_masked(sc_sfx, prepend, append,		\
+		__arch_cmpxchg_masked(sc_sfx, ".h" sc_sfx,		\
+					prepend, append,		\
 					__ret, __ptr, __old, __new);	\
 		break;							\
 	case 4:								\
-- 
2.39.2