arch/riscv: add Zihintpause support

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@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Dao Lu <daolu@rivosinc.com>
[Palmer: Some minor merge conflicts.]
Link: https://lore.kernel.org/all/20220620201530.3929352-1-daolu@rivosinc.com/
Link: https://lore.kernel.org/all/20220811053356.17375-1-palmer@rivosinc.com/
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Makefile

@@ -60,6 +60,10 @@ riscv-march-$(toolchain-need-zicsr-zifencei) := $(riscv-march-y)_zicsr_zifencei
 toolchain-supports-zicbom := $(call cc-option-yn, -march=$(riscv-march-y)_zicbom)
 riscv-march-$(toolchain-supports-zicbom) := $(riscv-march-y)_zicbom
 
+# 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)
 

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>
 
@@ -55,6 +56,7 @@ enum riscv_isa_ext_id {
 	RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
 	RISCV_ISA_EXT_SVPBMT,
 	RISCV_ISA_EXT_ZICBOM,
+	RISCV_ISA_EXT_ZIHINTPAUSE,
 	RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
 };
 
@@ -65,6 +67,7 @@ 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,
 };
 
@@ -84,6 +87,8 @@ 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;
 	}

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();
 }
 

arch/riscv/kernel/cpu.c

@@ -94,6 +94,7 @@ 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(zicbom, RISCV_ISA_EXT_ZICBOM),
+	__RISCV_ISA_EXT_DATA(zihintpause, RISCV_ISA_EXT_ZIHINTPAUSE),
 	__RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
 };
 

arch/riscv/kernel/cpufeature.c

@@ -202,6 +202,7 @@ void __init riscv_fill_hwcap(void)
 				SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
 				SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
 				SET_ISA_EXT_MAP("zicbom", RISCV_ISA_EXT_ZICBOM);
+				SET_ISA_EXT_MAP("zihintpause", RISCV_ISA_EXT_ZIHINTPAUSE);
 			}
 #undef SET_ISA_EXT_MAP
 		}