[PATCH v4] 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
v2 -> v3:
 Added the static branch back, cpu_relax() behavior is kept the same for
systems that do not support ZiHintPause
v3 -> v4:
 Adopted the newly added unified static keys for extensions
---
 arch/riscv/Makefile                     |  4 ++++
 arch/riscv/include/asm/hwcap.h          |  5 +++++
 arch/riscv/include/asm/vdso/processor.h | 21 ++++++++++++++++++---
 arch/riscv/kernel/cpu.c                 |  1 +
 arch/riscv/kernel/cpufeature.c          |  1 +
 5 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 34cf8a598617..6ddacc6f44b9 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 e48eebdd2631..dc47019a0b38 100644
--- a/arch/riscv/include/asm/hwcap.h
+++ b/arch/riscv/include/asm/hwcap.h
@@ -8,6 +8,7 @@
 #ifndef _ASM_RISCV_HWCAP_H
 #define _ASM_RISCV_HWCAP_H
 
+#include <asm/errno.h>
 #include <linux/bits.h>
 #include <uapi/asm/hwcap.h>
 
@@ -54,6 +55,7 @@ extern unsigned long elf_hwcap;
 enum riscv_isa_ext_id {
 	RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
 	RISCV_ISA_EXT_SVPBMT,
+	RISCV_ISA_EXT_ZIHINTPAUSE,
 	RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
 };
 
@@ -64,6 +66,7 @@ enum riscv_isa_ext_id {
  */
 enum riscv_isa_ext_key {
 	RISCV_ISA_EXT_KEY_FPU,		/* For 'F' and 'D' */
+	RISCV_ISA_EXT_KEY_ZIHINTPAUSE,
 	RISCV_ISA_EXT_KEY_MAX,
 };
 
@@ -83,6 +86,8 @@ static __always_inline int riscv_isa_ext2key(int num)
 		return RISCV_ISA_EXT_KEY_FPU;
 	case RISCV_ISA_EXT_d:
 		return RISCV_ISA_EXT_KEY_FPU;
+	case RISCV_ISA_EXT_ZIHINTPAUSE:
+		return RISCV_ISA_EXT_KEY_ZIHINTPAUSE;
 	default:
 		return -EINVAL;
 	}
diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
index 134388cbaaa1..1e4f8b4aef79 100644
--- a/arch/riscv/include/asm/vdso/processor.h
+++ b/arch/riscv/include/asm/vdso/processor.h
@@ -4,15 +4,30 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/jump_label.h>
 #include <asm/barrier.h>
+#include <asm/hwcap.h>
 
 static inline void cpu_relax(void)
 {
+	if (!static_branch_likely(&riscv_isa_ext_keys[RISCV_ISA_EXT_KEY_ZIHINTPAUSE])) {
 #ifdef __riscv_muldiv
-	int dummy;
-	/* In lieu of a halt instruction, induce a long-latency stall. */
-	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
+		int dummy;
+		/* In lieu of a halt instruction, induce a long-latency stall. */
+		__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
 #endif
+	} else {
+		/*
+		 * Reduce instruction retirement.
+		 * This assumes the PC changes.
+		 */
+#ifdef __riscv_zihintpause
+		__asm__ __volatile__ ("pause");
+#else
+		/* Encoding of the pause instruction */
+		__asm__ __volatile__ (".4byte 0x100000F");
+#endif
+	}
 	barrier();
 }
 
diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
index fba9e9f46a8c..a123e92b14dd 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -89,6 +89,7 @@ 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(svpbmt, RISCV_ISA_EXT_SVPBMT),
+	__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 1b3ec44e25f5..708df2c0bc34 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -198,6 +198,7 @@ void __init riscv_fill_hwcap(void)
 			} else {
 				SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
 				SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
+				SET_ISA_EXT_MAP("zihintpause", RISCV_ISA_EXT_ZIHINTPAUSE);
 			}
 #undef SET_ISA_EXT_MAP
 		}
-- 
2.25.1