[PATCH v3] 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 unchanged if either the
system or the toolchain does not support ZiHintPause
---
 arch/riscv/Makefile                     |  4 ++++
 arch/riscv/include/asm/hwcap.h          |  1 +
 arch/riscv/include/asm/vdso/processor.h | 19 ++++++++++++++++---
 arch/riscv/kernel/cpu.c                 |  1 +
 arch/riscv/kernel/cpufeature.c          |  8 ++++++++
 5 files changed, 30 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 4e2486881840..f24f4f8c9144 100644
--- a/arch/riscv/include/asm/hwcap.h
+++ b/arch/riscv/include/asm/hwcap.h
@@ -53,6 +53,7 @@ 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,
 };
 
diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
index 134388cbaaa1..314ec17c40d8 100644
--- a/arch/riscv/include/asm/vdso/processor.h
+++ b/arch/riscv/include/asm/vdso/processor.h
@@ -4,15 +4,28 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/jump_label.h>
 #include <asm/barrier.h>
+#include <asm/hwcap.h>
 
+extern struct static_key_false riscv_pause_available;
 static inline void cpu_relax(void)
 {
+	if (!static_branch_likely(&riscv_pause_available)) {
 #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
+#ifdef __riscv_zihintpause
+	} else {
+		/*
+		 * Reduce instruction retirement.
+		 * This assumes the PC changes.
+		 */
+		__asm__ __volatile__ ("pause");
+#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 a6f62a6d1edd..78c284c487e8 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -30,6 +30,8 @@ static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
 #ifdef CONFIG_FPU
 __ro_after_init DEFINE_STATIC_KEY_FALSE(cpu_hwcap_fpu);
 #endif
+DEFINE_STATIC_KEY_FALSE(riscv_pause_available);
+EXPORT_SYMBOL_GPL(riscv_pause_available);
 
 /**
  * riscv_isa_extension_base() - Get base extension word
@@ -199,6 +201,7 @@ 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
 		}
@@ -219,6 +222,11 @@ void __init riscv_fill_hwcap(void)
 			bitmap_and(riscv_isa, riscv_isa, this_isa, RISCV_ISA_EXT_MAX);
 	}
 
+#ifdef __riscv_zihintpause
+	if (__riscv_isa_extension_available(riscv_isa, RISCV_ISA_EXT_ZIHINTPAUSE))
+		static_branch_enable(&riscv_pause_available);
+#endif
+
 	/* We don't support systems with F but without D, so mask those out
 	 * here. */
 	if ((elf_hwcap & COMPAT_HWCAP_ISA_F) && !(elf_hwcap & COMPAT_HWCAP_ISA_D)) {
-- 
2.25.1