Re: [PATCH v2 2/2] arch/riscv: Add bitrev.h file to support rev8 and brev8

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Apr 15, 2026 at 05:38:27PM +0800, Jinjie Ruan wrote:
> The RISC-V Bit-manipulation Extension for Cryptography (Zbkb) provides
> the 'brev8' instruction, which reverses the bits within each byte.
> Combined with the 'rev8' instruction (from Zbb or Zbkb), which reverses
> the byte order of a register, we can efficiently implement 16-bit,
> 32-bit, and (on RV64) 64-bit bit reversal.
> 
> This is significantly faster than the default software table-lookup
> implementation in lib/bitrev.c, as it replaces memory accesses and
> multiple arithmetic operations with just two or three hardware
> instructions.
> 
> Select HAVE_ARCH_BITREVERSE and provide <asm/bitrev.h> to utilize
> these instructions when the Zbkb extension is available at runtime
> via the alternatives mechanism.
> 
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> ---
>  arch/riscv/Kconfig              |  1 +
>  arch/riscv/include/asm/bitrev.h | 41 +++++++++++++++++++++++++++++++++
>  2 files changed, 42 insertions(+)
>  create mode 100644 arch/riscv/include/asm/bitrev.h
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 90c531e6abf5..05f2b2166a83 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -128,6 +128,7 @@ config RISCV
>  	select HAS_IOPORT if MMU
>  	select HAVE_ALIGNED_STRUCT_PAGE
>  	select HAVE_ARCH_AUDITSYSCALL
> +	select HAVE_ARCH_BITREVERSE if RISCV_ISA_ZBKB
>  	select HAVE_ARCH_HUGE_VMALLOC if HAVE_ARCH_HUGE_VMAP
>  	select HAVE_ARCH_HUGE_VMAP if MMU && 64BIT
>  	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
> diff --git a/arch/riscv/include/asm/bitrev.h b/arch/riscv/include/asm/bitrev.h
> new file mode 100644
> index 000000000000..9f205ac84796
> --- /dev/null
> +++ b/arch/riscv/include/asm/bitrev.h
> @@ -0,0 +1,41 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_BITREV_H
> +#define __ASM_BITREV_H
> +
> +#include <linux/types.h>
> +#include <asm/cpufeature-macros.h>
> +#include <asm/hwcap.h>
> +#include <asm-generic/bitops/__bitrev.h>
> +
> +static __always_inline __attribute_const__ u32 __arch_bitrev32(u32 x)
> +{
> +	unsigned long result = x;
> +
> +	if (!riscv_has_extension_likely(RISCV_ISA_EXT_ZBKB))
> +		return generic___bitrev32(x);

This breaks the build when CONFIG_HAVE_ARCH_BITREVERSE is set because
generic___bitrev32() ultimately calls generic___bitrev8(), which uses
byte_rev_table but that is only included in lib/bitrev.c when
CONFIG_HAVE_ARCH_BITREVERSE is not set. How was this tested? This seems
a pretty basic build problem that has showed up in a variety of
configurations (at least all the configurations that our CI tests).

  $ make -skj"$(nproc)" ARCH=riscv CROSS_COMPILE=riscv64-linux- mrproper defconfig all
  ERROR: modpost: "byte_rev_table" [lib/zlib_deflate/zlib_deflate.ko] undefined!
  ERROR: modpost: "byte_rev_table" [drivers/net/ethernet/spacemit/k1_emac.ko] undefined!
  ERROR: modpost: "byte_rev_table" [drivers/net/ethernet/stmicro/stmmac/stmmac.ko] undefined!

  https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/24529356842
  https://lore.kernel.org/177635154368.6552.7060101263009785041@8692ffc4d55e/

Yury, are you intending to send this series to Linus in the 7.1 merge
window? If not, it shouldn't be in -next at this point.

> +	asm volatile(
> +		".option push\n"
> +		".option arch,+zbkb\n"
> +		"rev8 %0, %0\n"
> +		"brev8 %0, %0\n"
> +		".option pop"
> +		: "+r" (result)
> +	);
> +
> +	if (__riscv_xlen == 64)
> +		return (u32)(result >> 32);
> +
> +	return (u32)result;
> +}
> +
> +static __always_inline __attribute_const__ u16 __arch_bitrev16(u16 x)
> +{
> +	return __arch_bitrev32((u32)x) >> 16;
> +}
> +
> +static __always_inline __attribute_const__ u8 __arch_bitrev8(u8 x)
> +{
> +	return __arch_bitrev32((u32)x) >> 24;
> +}
> +#endif
> -- 
> 2.34.1
> 

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