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>

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 42d7ff8..3fa8ef3 100644 (file)
--- a/arch/riscv/Makefile
+++ b/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)
 

diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
index ed4045e..3c8a5ca 100644 (file)
--- 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>
 
@@ -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;
        }

diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
index 134388c..1e4f8b4 100644 (file)
--- 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 76a2a22..a77c380 100644 (file)
--- a/arch/riscv/kernel/cpu.c
+++ b/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),
 };
 

diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index f914e8d..c233fbc 100644 (file)
--- a/arch/riscv/kernel/cpufeature.c
+++ b/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
                }