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Subject[PATCH v4] arch/riscv: add Zihintpause support
Date
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>
---

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
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