Re: [PATCH v2 4/5] arch/riscv: Add bitrev.h file to support rev8 and brev8

[Jinjie Ruan]

On 5/7/2026 1:52 AM, Yury Norov wrote:
> From: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> 
> 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 as well as GENERIC_BITREVERSE,
> and provide <asm/bitrev.h> to utilize these instructions when
> the Zbkb extension is available at runtime via the alternatives
> mechanism.
> 
> [Yury: select the options conditionally on BITREVERSE]
> 
> Link: https://docs.riscv.org/reference/isa/unpriv/b-st-ext.html
> Suggested-by: David Laight <david.laight.linux@xxxxxxxxx>
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> Signed-off-by: Yury Norov <ynorov@xxxxxxxxxx>
> ---
>  arch/riscv/Kconfig              |  2 ++
>  arch/riscv/include/asm/bitrev.h | 51 +++++++++++++++++++++++++++++++++
>  2 files changed, 53 insertions(+)
>  create mode 100644 arch/riscv/include/asm/bitrev.h
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index d235396c4514..a708583f785d 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -104,6 +104,7 @@ config RISCV
>  	select FUNCTION_ALIGNMENT_8B if DYNAMIC_FTRACE_WITH_CALL_OPS
>  	select GENERIC_ARCH_TOPOLOGY
>  	select GENERIC_ATOMIC64 if !64BIT
> +	select GENERIC_BITREVERSE if HAVE_ARCH_BITREVERSE

Maybe 'select GENERIC_BITREVERSE if BITREVERSE' ?

>  	select GENERIC_CLOCKEVENTS_BROADCAST if SMP
>  	select GENERIC_CPU_DEVICES
>  	select GENERIC_CPU_VULNERABILITIES
> @@ -128,6 +129,7 @@ config RISCV
>  	select HAS_IOPORT if MMU
>  	select HAVE_ALIGNED_STRUCT_PAGE
>  	select HAVE_ARCH_AUDITSYSCALL
> +	select HAVE_ARCH_BITREVERSE if RISCV_ISA_ZBKB && BITREVERSE
>  	select HAVE_ARCH_HUGE_VMALLOC if HAVE_ARCH_HUGE_VMAP
>  	select HAVE_ARCH_HUGE_VMAP if MMU && 64BIT
>  	select HAVE_ARCH_JUMP_LABEL
> diff --git a/arch/riscv/include/asm/bitrev.h b/arch/riscv/include/asm/bitrev.h
> new file mode 100644
> index 000000000000..4b9b8d34cc3b
> --- /dev/null
> +++ b/arch/riscv/include/asm/bitrev.h
> @@ -0,0 +1,51 @@
> +/* 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;
> +
> +	if (!riscv_has_extension_likely(RISCV_ISA_EXT_ZBKB))
> +		return generic___bitrev32(x);
> +
> +	asm volatile(
> +		".option push\n"
> +		".option arch,+zbkb\n"
> +		"rev8 %0, %1\n"
> +		"brev8 %0, %0\n"
> +		".option pop"
> +		: "=r" (result) : "r" ((long)x)
> +	);
> +
> +	return result >> (__riscv_xlen - 32);
> +}
> +
> +static __always_inline __attribute_const__ u16 __arch_bitrev16(u16 x)
> +{
> +	return __arch_bitrev32(x) >> 16;
> +}
> +
> +static __always_inline __attribute_const__ u8 __arch_bitrev8(u8 x)
> +{
> +	unsigned long result;
> +
> +	if (!riscv_has_extension_likely(RISCV_ISA_EXT_ZBKB))
> +		return generic___bitrev8(x);
> +
> +	asm volatile(
> +		".option push\n"
> +		".option arch,+zbkb\n"
> +		"brev8 %0, %1\n"
> +		".option pop"
> +		: "=r" (result) : "r" ((long)x)
> +	);
> +
> +	return result;
> +}
> +#endif