linux-kernel - Re: [PATCH 1/2] riscv: introduce unified static key mechanism for ISA extensions

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Date:   Sun, 22 May 2022 22:56:25 +0800
From:   Jisheng Zhang <jszhang@...nel.org>
To:     Atish Patra <atishp@...shpatra.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] riscv: introduce unified static key mechanism for
 ISA extensions

On Sun, May 22, 2022 at 12:33:12AM -0700, Atish Patra wrote:
> On Tue, May 17, 2022 at 11:53 AM Jisheng Zhang <jszhang@...nel.org> wrote:
> >
> > Currently, riscv has several extensions which may not be supported on
> > all riscv platforms, for example, FPU and so on. To support unified
> > kernel Image style, we need to check whether the feature is supported
> 
> /s/suportted/supported

Thanks, will fix it in v2 soon.
> 
> > or not. If the check sits at hot code path, then performance will be
> > impacted a lot. static key can be used to solve the issue. In the past
> > FPU support has been converted to use static key mechanism. I believe
> > we will have similar cases in the future.
> >
> > this patch tries to add an unified mechanism to use static keys for
> > some ISA extensions by implementing an array of default-false static keys
> > and enabling them when detected.
> >
> > Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> > ---
> >  arch/riscv/include/asm/hwcap.h | 40 ++++++++++++++++++++++++++++++++++
> >  arch/riscv/kernel/cpufeature.c |  7 ++++++
> >  2 files changed, 47 insertions(+)
> >
> > diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> > index 0734e42f74f2..b0433d2b880d 100644
> > --- a/arch/riscv/include/asm/hwcap.h
> > +++ b/arch/riscv/include/asm/hwcap.h
> > @@ -12,6 +12,7 @@
> >  #include <uapi/asm/hwcap.h>
> >
> >  #ifndef __ASSEMBLY__
> > +#include <linux/jump_label.h>
> >  /*
> >   * This yields a mask that user programs can use to figure out what
> >   * instruction set this cpu supports.
> > @@ -55,6 +56,16 @@ enum riscv_isa_ext_id {
> >         RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
> >  };
> >
> > +/*
> > + * This enum represents the logical ID for each RISC-V ISA extension static
> > + * keys. We can use static key to optimize code path if some ISA extensions
> > + * are available.
> > + */
> > +enum riscv_isa_ext_key {
> > +       RISCV_ISA_EXT_KEY_FPU,          /* For 'F' and 'D' */
> > +       RISCV_ISA_EXT_KEY_MAX,
> > +};
> > +
> >  struct riscv_isa_ext_data {
> >         /* Name of the extension displayed to userspace via /proc/cpuinfo */
> >         char uprop[RISCV_ISA_EXT_NAME_LEN_MAX];
> > @@ -62,6 +73,35 @@ struct riscv_isa_ext_data {
> >         unsigned int isa_ext_id;
> >  };
> >
> > +extern struct static_key_false riscv_isa_ext_keys[RISCV_ISA_EXT_KEY_MAX];
> > +
> > +static __always_inline int riscv_isa_ext2key(int num)
> > +{
> > +       switch (num) {
> > +       case RISCV_ISA_EXT_f:
> > +               return RISCV_ISA_EXT_KEY_FPU;
> > +       case RISCV_ISA_EXT_d:
> > +               return RISCV_ISA_EXT_KEY_FPU;
> > +       default:
> > +               return -EINVAL;
> > +       }
> > +}
> > +
> > +/*
> > + * @num must be a compile-time constant.
> > + */
> > +static __always_inline bool riscv_isa_have_key_extension(int num)
> > +{
> > +       if (RISCV_ISA_EXT_ID_MAX <= num)
> > +               return false;
> > +
> > +       num = riscv_isa_ext2key(num);
>  > +       if (RISCV_ISA_EXT_KEY_MAX <= num || num < 0)
> > +               return false;
> > +
> 
> Why do you need the additional check in the hot path ?
> riscv_isa_ext_keys array can be directly accessed at the caller
> instead of calling this function.

directly accessing the keys can make the code simpler, thanks for
the hint.