[PATCH v2] arch/riscv: add Zihintpause support

[Dao Lu]

Implement support for the ZiHintPause extension.

The PAUSE instruction is a HINT that indicates the current hart’s rate of
instruction retirement should be temporarily reduced or paused.

Reviewed-by: Heiko Stuebner <heiko at sntech.de>
Tested-by: Heiko Stuebner <heiko at sntech.de>
Signed-off-by: Dao Lu <daolu at rivosinc.com>
---

v1 -> v2:
 Remove the usage of static branch, use PAUSE if toolchain supports it

 arch/riscv/Makefile                     | 4 ++++
 arch/riscv/include/asm/hwcap.h          | 1 +
 arch/riscv/include/asm/vdso/processor.h | 8 +++++++-
 arch/riscv/kernel/cpu.c                 | 1 +
 arch/riscv/kernel/cpufeature.c          | 2 ++
 5 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 7d81102cffd4..900a8fda1a2d 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -56,6 +56,10 @@ riscv-march-$(CONFIG_RISCV_ISA_C)	:= $(riscv-march-y)c
 toolchain-need-zicsr-zifencei := $(call cc-option-yn, -march=$(riscv-march-y)_zicsr_zifencei)
 riscv-march-$(toolchain-need-zicsr-zifencei) := $(riscv-march-y)_zicsr_zifencei
 
+# Check if the toolchain supports Zihintpause extension
+toolchain-supports-zihintpause := $(call cc-option-yn, -march=$(riscv-march-y)_zihintpause)
+riscv-march-$(toolchain-supports-zihintpause) := $(riscv-march-y)_zihintpause
+
 KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
 KBUILD_AFLAGS += -march=$(riscv-march-y)
 
diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
index 0734e42f74f2..caa9ee5459b4 100644
--- a/arch/riscv/include/asm/hwcap.h
+++ b/arch/riscv/include/asm/hwcap.h
@@ -52,6 +52,7 @@ extern unsigned long elf_hwcap;
  */
 enum riscv_isa_ext_id {
 	RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
+	RISCV_ISA_EXT_ZIHINTPAUSE,
 	RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
 };
 
diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
index 134388cbaaa1..4de911a25051 100644
--- a/arch/riscv/include/asm/vdso/processor.h
+++ b/arch/riscv/include/asm/vdso/processor.h
@@ -8,7 +8,13 @@
 
 static inline void cpu_relax(void)
 {
-#ifdef __riscv_muldiv
+#ifdef __riscv_zihintpause
+	/*
+	 * Reduce instruction retirement.
+	 * This assumes the PC changes.
+	 */
+	__asm__ __volatile__ ("pause");
+#elif __riscv_muldiv
 	int dummy;
 	/* In lieu of a halt instruction, induce a long-latency stall. */
 	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
index ccb617791e56..89e563e9c4cc 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -88,6 +88,7 @@ int riscv_of_parent_hartid(struct device_node *node)
  */
 static struct riscv_isa_ext_data isa_ext_arr[] = {
 	__RISCV_ISA_EXT_DATA(sscofpmf, RISCV_ISA_EXT_SSCOFPMF),
+	__RISCV_ISA_EXT_DATA(zihintpause, RISCV_ISA_EXT_ZIHINTPAUSE),
 	__RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
 };
 
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 1b2d42d7f589..37ff06682ae6 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -25,6 +25,7 @@ static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
 __ro_after_init DEFINE_STATIC_KEY_FALSE(cpu_hwcap_fpu);
 #endif
 
+
 /**
  * riscv_isa_extension_base() - Get base extension word
  *
@@ -192,6 +193,7 @@ void __init riscv_fill_hwcap(void)
 				set_bit(*ext - 'a', this_isa);
 			} else {
 				SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
+				SET_ISA_EXT_MAP("zihintpause", RISCV_ISA_EXT_ZIHINTPAUSE);
 			}
 #undef SET_ISA_EXT_MAP
 		}
-- 
2.36.0