[PATCH v3] arch/riscv: add Zihintpause support

[Dao Lu]

Will look at rebasing to use the unified static key mechanism for
the next version.

Regards,
Dao

On Thu, Jun 16, 2022 at 8:49 PM Samuel Holland <samuel at sholland.org> wrote:
>
> On 6/15/22 4:14 PM, Dao Lu 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 at sntech.de>
> > Tested-by: Heiko Stuebner <heiko at sntech.de>
> > Signed-off-by: Dao Lu <daolu at 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 unchanged if either the
> > system or the toolchain does not support ZiHintPause
> > ---
> >  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          |  8 ++++++++
> >  5 files changed, 30 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 4e2486881840..f24f4f8c9144 100644
> > --- a/arch/riscv/include/asm/hwcap.h
> > +++ b/arch/riscv/include/asm/hwcap.h
> > @@ -53,6 +53,7 @@ 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,
> >  };
> >
> > diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
> > index 134388cbaaa1..314ec17c40d8 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
> > +#ifdef __riscv_zihintpause
> > +     } else {
> > +             /*
> > +              * 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 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 a6f62a6d1edd..78c284c487e8 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -30,6 +30,8 @@ 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);
>
> This patch should probably be rebased to use the unified static key
> mechanism[1], since that patch has now been merged.
>
> Regards,
> Samuel
>
> [1]: https://lore.kernel.org/linux-riscv/20220522153543.2656-2-jszhang@kernel.org/