[PATCH v4 03/13] riscv: Implement cmpxchg32/64() using Zacas

Alexandre Ghiti
2024-07-31T07:27:18Z

This adds runtime support for Zacas in cmpxchg operations.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
arch/riscv/Kconfig | 16 +++++++++++
arch/riscv/Makefile | 3 ++
arch/riscv/include/asm/cmpxchg.h | 48 +++++++++++++++++++++-----------
3 files changed, 50 insertions(+), 17 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 0f3cd7c3a436..d955c64d50c2 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -613,6 +613,22 @@ 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
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 6fe682139d2e..f1788131d5fe 100644
--- a/arch/riscv/Makefile
+++ b/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/')
diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index 9ba497ea18a5..c626fe0d08ae 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/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: \
--
2.39.2