Re: [RFC PATCH 2/4] riscv: add compile-time test into is_compat_task()

[Guo Ren]

On Fri, Dec 22, 2023 at 5:02 PM Leonardo Bras <leobras@xxxxxxxxxx> wrote:
>
> Currently several places will test for CONFIG_COMPAT before testing
> is_compat_task(), probably in order to avoid a run-time test into the task
> structure.
>
> Since is_compat_task() is an inlined function, it would be helpful to add a
> compile-time test of CONFIG_COMPAT, making sure it always returns zero when
> the option is not enabled during the kernel build.
>
> With this, the compiler is able to understand in build-time that
> is_compat_task() will always return 0, and optimize-out some of the extra
> code introduced by the option.
>
> This will also allow removing a lot #ifdefs that were introduced, and make
> the code more clean.
>
> Signed-off-by: Leonardo Bras <leobras@xxxxxxxxxx>
> ---
>  arch/riscv/include/asm/compat.h    | 3 +++
>  arch/riscv/include/asm/elf.h       | 4 ----
>  arch/riscv/include/asm/pgtable.h   | 6 ------
>  arch/riscv/include/asm/processor.h | 4 ++--
>  4 files changed, 5 insertions(+), 12 deletions(-)
>
> diff --git a/arch/riscv/include/asm/compat.h b/arch/riscv/include/asm/compat.h
> index 2ac955b51148f..91517b51b8e27 100644
> --- a/arch/riscv/include/asm/compat.h
> +++ b/arch/riscv/include/asm/compat.h
> @@ -14,6 +14,9 @@
>
>  static inline int is_compat_task(void)
>  {
> +       if (!IS_ENABLED(CONFIG_COMPAT))
> +               return 0;
> +
>         return test_thread_flag(TIF_32BIT);
>  }
>
> diff --git a/arch/riscv/include/asm/elf.h b/arch/riscv/include/asm/elf.h
> index 59a08367fddd7..2e88257cafaea 100644
> --- a/arch/riscv/include/asm/elf.h
> +++ b/arch/riscv/include/asm/elf.h
> @@ -53,13 +53,9 @@ extern bool compat_elf_check_arch(Elf32_Ehdr *hdr);
>  #define ELF_ET_DYN_BASE                ((DEFAULT_MAP_WINDOW / 3) * 2)
>
>  #ifdef CONFIG_64BIT
> -#ifdef CONFIG_COMPAT
>  #define STACK_RND_MASK         (is_compat_task() ? \
>                                  0x7ff >> (PAGE_SHIFT - 12) : \
>                                  0x3ffff >> (PAGE_SHIFT - 12))
> -#else
> -#define STACK_RND_MASK         (0x3ffff >> (PAGE_SHIFT - 12))
> -#endif
>  #endif
>
>  /*
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 1d472b31e0cfe..ea5b269be223a 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -127,16 +127,10 @@
>  #define VA_USER_SV48 (UL(1) << (VA_BITS_SV48 - 1))
>  #define VA_USER_SV57 (UL(1) << (VA_BITS_SV57 - 1))
>
> -#ifdef CONFIG_COMPAT
>  #define MMAP_VA_BITS_64 ((VA_BITS >= VA_BITS_SV48) ? VA_BITS_SV48 : VA_BITS)
>  #define MMAP_MIN_VA_BITS_64 (VA_BITS_SV39)
>  #define MMAP_VA_BITS (is_compat_task() ? VA_BITS_SV32 : MMAP_VA_BITS_64)
>  #define MMAP_MIN_VA_BITS (is_compat_task() ? VA_BITS_SV32 : MMAP_MIN_VA_BITS_64)
> -#else
> -#define MMAP_VA_BITS ((VA_BITS >= VA_BITS_SV48) ? VA_BITS_SV48 : VA_BITS)
> -#define MMAP_MIN_VA_BITS (VA_BITS_SV39)
> -#endif /* CONFIG_COMPAT */
> -
>  #else
>  #include <asm/pgtable-32.h>
>  #endif /* CONFIG_64BIT */
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index f19f861cda549..ed32e53e55999 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -22,7 +22,7 @@
>  ({                                                             \
>         unsigned long mmap_end;                                 \
>         typeof(addr) _addr = (addr);                            \
> -       if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && is_compat_task())) \
> +       if ((_addr) == 0 || is_compat_task())                   \
>                 mmap_end = STACK_TOP_MAX;                       \
>         else if ((_addr) >= VA_USER_SV57)                       \
>                 mmap_end = STACK_TOP_MAX;                       \
> @@ -39,7 +39,7 @@
>         typeof(addr) _addr = (addr);                            \
>         typeof(base) _base = (base);                            \
>         unsigned long rnd_gap = DEFAULT_MAP_WINDOW - (_base);   \
> -       if ((_addr) == 0 || (IS_ENABLED(CONFIG_COMPAT) && is_compat_task())) \
> +       if ((_addr) == 0 || is_compat_task())                   \
>                 mmap_base = (_base);                            \
>         else if (((_addr) >= VA_USER_SV57) && (VA_BITS >= VA_BITS_SV57)) \
>                 mmap_base = VA_USER_SV57 - rnd_gap;             \
> --
> 2.43.0
>
Reviewed-by: Guo Ren <guoren@xxxxxxxxxx>

-- 
Best Regards
 Guo Ren