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Subject[PATCH] arch/riscv: Add Zihintpause extension support
Date
This patch:
1. Build with _zihintpause if the toolchain has support for it
2. Detects if the platform supports the extension
3. Use PAUSE for cpu_relax if both toolchain and the platform support it

Signed-off-by: Dao Lu <daolu@rivosinc.com>
---
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 | 7 +++++++
5 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 7d81102cffd4..900a8fda1a2d 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 0734e42f74f2..caa9ee5459b4 100644
--- a/arch/riscv/include/asm/hwcap.h
+++ b/arch/riscv/include/asm/hwcap.h
@@ -52,6 +52,7 @@ extern unsigned long elf_hwcap;
*/
enum riscv_isa_ext_id {
RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
+ 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..106b35ba8cac 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
+ } else {
+#ifdef __riscv_zihintpause
+ /*
+ * 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 ccb617791e56..89e563e9c4cc 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -88,6 +88,7 @@ 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(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 1b2d42d7f589..327c19507dbb 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -24,6 +24,8 @@ 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
@@ -192,6 +194,7 @@ void __init riscv_fill_hwcap(void)
set_bit(*ext - 'a', this_isa);
} else {
SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
+ SET_ISA_EXT_MAP("zihintpause", RISCV_ISA_EXT_ZIHINTPAUSE);
}
#undef SET_ISA_EXT_MAP
}
@@ -213,6 +216,10 @@ void __init riscv_fill_hwcap(void)

}

+ if (__riscv_isa_extension_available(riscv_isa, RISCV_ISA_EXT_ZIHINTPAUSE)) {
+ static_branch_enable(&riscv_pause_available);
+ }
+
/* 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)) {
--
2.36.0
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