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SubjectRe: [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@kernel.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@kernel.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.

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