Re: [PATCH v5] riscv: introduce asm/swab.h

[Ignacio Encinas Rubio <ignacio@iencinas.com>]

Hello Alex!

On 23/7/25 14:28, Alexandre Ghiti wrote:
> Hi Ignacio,
> 
> On 7/17/25 20:44, Ignacio Encinas wrote:
>> Implement endianness swap macros for RISC-V.
>>
>> Use the rev8 instruction when Zbb is available. Otherwise, rely on the
>> default mask-and-shift implementation.
>>
>> Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
>> Signed-off-by: Ignacio Encinas <ignacio@iencinas.com>
>> ---
>> Motivated by [1]. Tested with crc_kunit as pointed out here [2]. I can't
>> provide performance numbers as I don't have RISC-V hardware.
>>
>> [1] https://lore.kernel.org/all/20250302220426.GC2079@quark.localdomain/
>> [2] https://lore.kernel.org/all/20250216225530.306980-1-ebiggers@kernel.org/
>> ---
>> Changes in v5:
>> - Duplicate ___constant_swab helpers in arch/riscv/include/asm/swab.h to
>>    avoid delaying the patch as suggested by Alex in [3] (drop patch 1 and
>>    convert this into a 1-patch series)
>> - Link to v4: https://lore.kernel.org/r/20250426-riscv-swab-v4-0-64201404a68c@iencinas.com
>>
>> [3] https://lore.kernel.org/linux-riscv/7e22a448-3cee-4475-b69b-3dd45b57f168@ghiti.fr/
>>
>> Changes in v4:
>>
>> - Add missing include in the 1st patch, reported by
>>    https://lore.kernel.org/all/202504042300.it9RcOSt-lkp@intel.com/
>> - Rewrite the ARCH_SWAB macro as suggested by Arnd
>> - Define __arch_swab64 for CONFIG_32BIT (Ben)
>> - Link to v3: https://lore.kernel.org/r/20250403-riscv-swab-v3-0-3bf705d80e33@iencinas.com
>>
>> Changes in v3:
>>
>> PATCH 2:
>>    Use if(riscv_has_extension_likely) instead of asm goto (Eric). It
>>    looks like both versions generate the same assembly. Perhaps we should
>>    do the same change in other places such as arch/riscv/include/asm/bitops.h
>> - Link to v2: https://lore.kernel.org/r/20250319-riscv-swab-v2-0-d53b6d6ab915@iencinas.com
>>
>> Changes in v2:
>> - Introduce first patch factoring out the default implementation into
>>    asm-generic
>> - Remove blank line to make checkpatch happy
>> - Link to v1: https://lore.kernel.org/r/20250310-riscv-swab-v1-1-34652ef1ee96@iencinas.com
>> ---
>>   arch/riscv/include/asm/swab.h | 87 +++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 87 insertions(+)
>>
>> diff --git a/arch/riscv/include/asm/swab.h b/arch/riscv/include/asm/swab.h
>> new file mode 100644
>> index 0000000000000000000000000000000000000000..4f408f59fada7251d62f56d174ae76ff19f4a319
>> --- /dev/null
>> +++ b/arch/riscv/include/asm/swab.h
>> @@ -0,0 +1,87 @@
>> +/* SPDX-License-Identifier: GPL-2.0-only */
>> +#ifndef _ASM_RISCV_SWAB_H
>> +#define _ASM_RISCV_SWAB_H
>> +
>> +#include <linux/types.h>
>> +#include <linux/compiler.h>
>> +#include <asm/cpufeature-macros.h>
>> +#include <asm/hwcap.h>
>> +#include <asm-generic/swab.h>
>> +
>> +#if defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE)
> 
> 
> In order to fix kernel test robot report, we need to make sure the toolchain supports Zbb, the following diff fixes the issue for me:

Thank you very much for looking at this. I looked at the report but
figured something funny had happened. Somehow I missed this "basic"
thing early on... Oops!

> 
> diff --git a/arch/riscv/include/asm/swab.h b/arch/riscv/include/asm/swab.h
> index 4f408f59fada7..8faa293a9b841 100644
> --- a/arch/riscv/include/asm/swab.h
> +++ b/arch/riscv/include/asm/swab.h
> @@ -8,7 +8,7 @@
>  #include <asm/hwcap.h>
>  #include <asm-generic/swab.h>
> 
> -#if defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE)
> +#if defined(CONFIG_TOOLCHAIN_HAS_ZBB) && defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE)          \
> 
>  // Duplicated from include/uapi/linux/swab.h
>  #define ___constant_swab16(x) ((__u16)(                                \
> @@ -83,5 +83,5 @@ static __always_inline __u64 __arch_swab64(__u64 value)
> 
>  #undef ARCH_SWAB
> 
> -#endif /* defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE) */
> +#endif /* defined(CONFIG_TOOLCHAIN_HAS_ZBB) && defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE) */
>  #endif /* _ASM_RISCV_SWAB_H */
> 
> Would you mind squashing that into a v6 please?

Of course! I'll send it now. Thanks again.

> 
> Thanks,
> 
> Alex
> 
>> +
>> +// Duplicated from include/uapi/linux/swab.h
>> +#define ___constant_swab16(x) ((__u16)(                \
>> +    (((__u16)(x) & (__u16)0x00ffU) << 8) |            \
>> +    (((__u16)(x) & (__u16)0xff00U) >> 8)))
>> +
>> +#define ___constant_swab32(x) ((__u32)(                \
>> +    (((__u32)(x) & (__u32)0x000000ffUL) << 24) |        \
>> +    (((__u32)(x) & (__u32)0x0000ff00UL) <<  8) |        \
>> +    (((__u32)(x) & (__u32)0x00ff0000UL) >>  8) |        \
>> +    (((__u32)(x) & (__u32)0xff000000UL) >> 24)))
>> +
>> +#define ___constant_swab64(x) ((__u64)(                \
>> +    (((__u64)(x) & (__u64)0x00000000000000ffULL) << 56) |    \
>> +    (((__u64)(x) & (__u64)0x000000000000ff00ULL) << 40) |    \
>> +    (((__u64)(x) & (__u64)0x0000000000ff0000ULL) << 24) |    \
>> +    (((__u64)(x) & (__u64)0x00000000ff000000ULL) <<  8) |    \
>> +    (((__u64)(x) & (__u64)0x000000ff00000000ULL) >>  8) |    \
>> +    (((__u64)(x) & (__u64)0x0000ff0000000000ULL) >> 24) |    \
>> +    (((__u64)(x) & (__u64)0x00ff000000000000ULL) >> 40) |    \
>> +    (((__u64)(x) & (__u64)0xff00000000000000ULL) >> 56)))
>> +
>> +#define ARCH_SWAB(size, value)                        \
>> +({                                    \
>> +    unsigned long x = value;                    \
>> +                                    \
>> +    if (riscv_has_extension_likely(RISCV_ISA_EXT_ZBB)) {            \
>> +        asm volatile (".option push\n"                \
>> +                  ".option arch,+zbb\n"            \
>> +                  "rev8 %0, %1\n"                \
>> +                  ".option pop\n"                \
>> +                  : "=r" (x) : "r" (x));            \
>> +        x = x >> (BITS_PER_LONG - size);            \
>> +    } else {                                                        \
>> +        x = ___constant_swab##size(value);                      \
>> +    }                                \
>> +    x;                                \
>> +})
>> +
>> +static __always_inline __u16 __arch_swab16(__u16 value)
>> +{
>> +    return ARCH_SWAB(16, value);
>> +}
>> +
>> +static __always_inline __u32 __arch_swab32(__u32 value)
>> +{
>> +    return ARCH_SWAB(32, value);
>> +}
>> +
>> +#ifdef CONFIG_64BIT
>> +static __always_inline __u64 __arch_swab64(__u64 value)
>> +{
>> +    return ARCH_SWAB(64, value);
>> +}
>> +#else
>> +static __always_inline __u64 __arch_swab64(__u64 value)
>> +{
>> +    __u32 h = value >> 32;
>> +    __u32 l = value & ((1ULL << 32) - 1);
>> +
>> +    return ((__u64)(__arch_swab32(l)) << 32) | ((__u64)(__arch_swab32(h)));
>> +}
>> +#endif
>> +
>> +#define __arch_swab64 __arch_swab64
>> +#define __arch_swab32 __arch_swab32
>> +#define __arch_swab16 __arch_swab16
>> +
>> +#undef ___constant_swab16
>> +#undef ___constant_swab32
>> +#undef ___constant_swab64
>> +
>> +#undef ARCH_SWAB
>> +
>> +#endif /* defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE) */
>> +#endif /* _ASM_RISCV_SWAB_H */
>>
>> ---
>> base-commit: 155a3c003e555a7300d156a5252c004c392ec6b0
>> change-id: 20250307-riscv-swab-b81b94a9ac1b
>>
>> Best regards,