Re: [PATCH v4] arch/riscv: add Zihintpause support

Guo Ren
2022-07-05T23:47:54Z

Reviewed-by: Guo Ren <guoren@kernel.org>

On Wed, Jul 6, 2022 at 12:57 AM Dao Lu <daolu@rivosinc.com> wrote:
>
> ping
>
> On Mon, Jun 20, 2022 at 1:15 PM Dao Lu <daolu@rivosinc.com> wrote:
> >
> > 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
> >

--
Best Regards
Guo Ren