[PATCH v2] arch/riscv: add Zihintpause support

[Dao Lu]

Thanks Heiko and Palmer, will address the comments, rebase and send a v3.

On Tue, Jun 14, 2022 at 9:44 AM Heiko Stübner <heiko at sntech.de> wrote:
>
> Am Dienstag, 24. Mai 2022, 23:19:50 CEST schrieb Dao Lu:
> > 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
> >
> >  arch/riscv/Makefile                     | 4 ++++
> >  arch/riscv/include/asm/hwcap.h          | 1 +
> >  arch/riscv/include/asm/vdso/processor.h | 8 +++++++-
> >  arch/riscv/kernel/cpu.c                 | 1 +
> >  arch/riscv/kernel/cpufeature.c          | 2 ++
> >  5 files changed, 15 insertions(+), 1 deletion(-)
> >
> > 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,
>
> svpbmt got merged meanwhile, so this patch needs a rebase
> onto 5.19-rc.
>
> One more nit below
>
> > +     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..4de911a25051 100644
> > --- a/arch/riscv/include/asm/vdso/processor.h
> > +++ b/arch/riscv/include/asm/vdso/processor.h
> > @@ -8,7 +8,13 @@
> >
> >  static inline void cpu_relax(void)
> >  {
> > -#ifdef __riscv_muldiv
> > +#ifdef __riscv_zihintpause
> > +     /*
> > +      * Reduce instruction retirement.
> > +      * This assumes the PC changes.
> > +      */
> > +     __asm__ __volatile__ ("pause");
> > +#elif __riscv_muldiv
> >       int dummy;
> >       /* In lieu of a halt instruction, induce a long-latency stall. */
> >       __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> > 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..37ff06682ae6 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -25,6 +25,7 @@ static DECLARE_BITMAP(riscv_isa, RISCV_ISA_EXT_MAX) __read_mostly;
> >  __ro_after_init DEFINE_STATIC_KEY_FALSE(cpu_hwcap_fpu);
> >  #endif
> >
> > +
>
> this is an unrelated change and also is adding an unneeded extra empty line.
>
>
> Heiko
>
> >  /**
> >   * riscv_isa_extension_base() - Get base extension word
> >   *
> > @@ -192,6 +193,7 @@ 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
> >               }
> >
>
>
>
>