Re: [PATCH v3 4/7] riscv/runtime-const: Introduce runtime_const_mask_32()

[Guo Ren]

On Thu, Apr 2, 2026 at 7:39 PM K Prateek Nayak <kprateek.nayak@xxxxxxx> wrote:
>
> Futex hash computation requires a mask operation with read-only after
> init data that will be converted to a runtime constant in the subsequent
> commit.
>
> Introduce runtime_const_mask_32 to further optimize the mask operation
> in the futex hash computation hot path. GCC generates a:
>
>   lui   a0, 0x12346       # upper; +0x800 then >>12 for correct rounding
>   addi  a0, a0, 0x678     # lower 12 bits
>   and   a1, a1, a0        # a1 = a1 & a0
>
> pattern to tackle arbitrary 32-bit masks and the same was also suggested
> by Claude which is implemented here. The (__mask & val) operation is
> intentionally placed outside of asm block to allow compilers to further
> optimize it if possible.
>
> __runtime_fixup_ptr() already patches a "lui + addi" sequence which has
> been reused to patch the same sequence for __runtime_fixup_mask().
>
> Assisted-by: Claude:claude-sonnet-4-5
> Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> ---
> Changelog v2..v3:
>
> o Moved the "&" operation outside the inline asm block to allow for
>   compilers to further optimize it if possible. (Based on David's
>   comment on ARM64 bits).
> ---
>  arch/riscv/include/asm/runtime-const.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h
> index d766e2b9e6df..85efba8ecf12 100644
> --- a/arch/riscv/include/asm/runtime-const.h
> +++ b/arch/riscv/include/asm/runtime-const.h
> @@ -153,6 +153,22 @@
>         __ret;                                                  \
>  })
>
> +#define runtime_const_mask_32(val, sym)                                \
> +({                                                             \
> +       u32 __mask;                                             \
> +       asm_inline(".option push\n\t"                           \
> +               ".option norvc\n\t"                             \
> +               "1:\t"                                          \
> +               "lui    %[__mask],0x89abd\n\t"                  \
> +               "addi   %[__mask],%[__mask],-0x211\n\t"         \
Ref include/uapi/linux/reboot.h:
#define LINUX_REBOOT_CMD_CAD_ON 0x89ABCDEF

#define RUNTIME_MAGIC 0x89ABCDEF

"lui %[__mask], %%hi(RUNTIME_MAGIC)\n\t"
"addi %[__mask], %[__mask], %%lo(RUNTIME_MAGIC)\n\t"


> +               ".option pop\n\t"                               \
> +               ".pushsection runtime_mask_" #sym ",\"a\"\n\t"  \
> +               ".long 1b - .\n\t"                              \
> +               ".popsection"                                   \
> +               : [__mask] "=r" (__mask));                      \
> +       (__mask & val);                                         \
> +})
> +
>  #define runtime_const_init(type, sym) do {                     \
>         extern s32 __start_runtime_##type##_##sym[];            \
>         extern s32 __stop_runtime_##type##_##sym[];             \
> @@ -256,6 +272,12 @@ static inline void __runtime_fixup_shift(void *where, unsigned long val)
>         mutex_unlock(&text_mutex);
>  }
>
> +static inline void __runtime_fixup_mask(void *where, unsigned long val)
> +{
> +       __runtime_fixup_32(where, where + 4, val);
> +       __runtime_fixup_caches(where, 2);
> +}
> +
>  static inline void runtime_const_fixup(void (*fn)(void *, unsigned long),
>                                        unsigned long val, s32 *start, s32 *end)
>  {
> --
> 2.34.1
>
>


-- 
Best Regards
 Guo Ren