[PATCH v2 0/2] arch/riscv: Add bitrev.h file to support rev8 and brev8

[PATCH v2 0/2] arch/riscv: Add bitrev.h file to support rev8 and brev8

[Jinjie Ruan]

Add bitrev.h file to support rev8 and brev8 for riscv.

Changes in v2:
- Define generic __bitrev8/16/32 for reuse in riscv.

Jinjie Ruan (2):
  bitops: Define generic __bitrev8/16/32 for reuse
  arch/riscv: Add bitrev.h file to support rev8 and brev8

 arch/riscv/Kconfig                    |  1 +
 arch/riscv/include/asm/bitrev.h       | 41 +++++++++++++++++++++++++++
 include/asm-generic/bitops/__bitrev.h | 22 ++++++++++++++
 include/linux/bitrev.h                | 20 +++----------
 4 files changed, 68 insertions(+), 16 deletions(-)
 create mode 100644 arch/riscv/include/asm/bitrev.h
 create mode 100644 include/asm-generic/bitops/__bitrev.h

-- 
2.34.1


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[PATCH v2 1/2] bitops: Define generic __bitrev8/16/32 for reuse

[Jinjie Ruan]

Define generic __bitrev8/16/32 using the implementation
in <linux/bitrev.h>, so they can be reused in <asm/bitrev.h>,
such as RISCV.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
---
 include/asm-generic/bitops/__bitrev.h | 22 ++++++++++++++++++++++
 include/linux/bitrev.h                | 20 ++++----------------
 2 files changed, 26 insertions(+), 16 deletions(-)
 create mode 100644 include/asm-generic/bitops/__bitrev.h

diff --git a/include/asm-generic/bitops/__bitrev.h b/include/asm-generic/bitops/__bitrev.h
new file mode 100644
index 000000000000..1b8c0f464d26
--- /dev/null
+++ b/include/asm-generic/bitops/__bitrev.h
@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS___BITREV_H_
+#define _ASM_GENERIC_BITOPS___BITREV_H_
+
+#include <asm/types.h>
+
+extern u8 const byte_rev_table[256];
+static __always_inline __attribute_const__ u8 generic___bitrev8(u8 byte)
+{
+	return byte_rev_table[byte];
+}
+
+static __always_inline __attribute_const__ u16 generic___bitrev16(u16 x)
+{
+	return (generic___bitrev8(x & 0xff) << 8) | generic___bitrev8(x >> 8);
+}
+
+static __always_inline __attribute_const__ u32 generic___bitrev32(u32 x)
+{
+	return (generic___bitrev16(x & 0xffff) << 16) | generic___bitrev16(x >> 16);
+}
+#endif /* _ASM_GENERIC_BITOPS___BITREV_H_ */
diff --git a/include/linux/bitrev.h b/include/linux/bitrev.h
index d35b8ec1c485..11620a70e776 100644
--- a/include/linux/bitrev.h
+++ b/include/linux/bitrev.h
@@ -12,22 +12,10 @@
 #define __bitrev8 __arch_bitrev8
 
 #else
-extern u8 const byte_rev_table[256];
-static inline u8 __bitrev8(u8 byte)
-{
-	return byte_rev_table[byte];
-}
-
-static inline u16 __bitrev16(u16 x)
-{
-	return (__bitrev8(x & 0xff) << 8) | __bitrev8(x >> 8);
-}
-
-static inline u32 __bitrev32(u32 x)
-{
-	return (__bitrev16(x & 0xffff) << 16) | __bitrev16(x >> 16);
-}
-
+#include <asm-generic/bitops/__bitrev.h>
+#define __bitrev32 generic___bitrev32
+#define __bitrev16 generic___bitrev16
+#define __bitrev8 generic___bitrev8
 #endif /* CONFIG_HAVE_ARCH_BITREVERSE */
 
 #define __bitrev8x4(x)	(__bitrev32(swab32(x)))
-- 
2.34.1


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

[Jinjie Ruan]

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

[David Laight]

On Wed, 15 Apr 2026 17:38:27 +0800
Jinjie Ruan <ruanjinjie@huawei.com> 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);
> +
> +	asm volatile(
> +		".option push\n"
> +		".option arch,+zbkb\n"
> +		"rev8 %0, %0\n"

It would be better to pass (long)x in for the source.
Might save the compiler doing a register-register move.

> +		"brev8 %0, %0\n"
> +		".option pop"
> +		: "+r" (result)
> +	);
> +
> +	if (__riscv_xlen == 64)
> +		return (u32)(result >> 32);

Is that right?
ACAICT __riscv_xlen is 32 for 32bit builds and 64 otherwise.
(No idea why riscv has its own private constant for that.)
I'm guessing that 'brev' is bit-reverse (or each byte) and 'rev'
a byteswap, the '8' suffix rather implies it acts on 8 bytes
which makes is 64bit only.
So does 'rev8' even compile for 32bit.
You are also likely to get a warning on 32bit for 'result >> 32'.

> +
> +	return (u32)result;
> +}

I'm not sure is is a good idea inline that into its callers.
But I can't think of a way to get either the instructions or
a call patched in at the call site.

> +
> +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;

That seems excessive when it could just be a 'brev' instruction.

Oh, and none of the casts on the function call parameters or results
are needed - they are all implied.

	David


> +}
> +#endif


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Re: [PATCH v2 1/2] bitops: Define generic __bitrev8/16/32 for reuse

[Yury Norov]

On Wed, Apr 15, 2026 at 05:38:26PM +0800, Jinjie Ruan wrote:
> Define generic __bitrev8/16/32 using the implementation
> in <linux/bitrev.h>, so they can be reused in <asm/bitrev.h>,
> such as RISCV.
> 
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>

Reviewed-by: Yury Norov <ynorov@nvidia.com>

I'll take it in -next for testing. That would help to catch some
probable warnings discussed in the #2.

> ---
>  include/asm-generic/bitops/__bitrev.h | 22 ++++++++++++++++++++++
>  include/linux/bitrev.h                | 20 ++++----------------
>  2 files changed, 26 insertions(+), 16 deletions(-)
>  create mode 100644 include/asm-generic/bitops/__bitrev.h
> 
> diff --git a/include/asm-generic/bitops/__bitrev.h b/include/asm-generic/bitops/__bitrev.h
> new file mode 100644
> index 000000000000..1b8c0f464d26
> --- /dev/null
> +++ b/include/asm-generic/bitops/__bitrev.h
> @@ -0,0 +1,22 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_GENERIC_BITOPS___BITREV_H_
> +#define _ASM_GENERIC_BITOPS___BITREV_H_
> +
> +#include <asm/types.h>
> +
> +extern u8 const byte_rev_table[256];
> +static __always_inline __attribute_const__ u8 generic___bitrev8(u8 byte)
> +{
> +	return byte_rev_table[byte];
> +}
> +
> +static __always_inline __attribute_const__ u16 generic___bitrev16(u16 x)
> +{
> +	return (generic___bitrev8(x & 0xff) << 8) | generic___bitrev8(x >> 8);
> +}
> +
> +static __always_inline __attribute_const__ u32 generic___bitrev32(u32 x)
> +{
> +	return (generic___bitrev16(x & 0xffff) << 16) | generic___bitrev16(x >> 16);
> +}
> +#endif /* _ASM_GENERIC_BITOPS___BITREV_H_ */
> diff --git a/include/linux/bitrev.h b/include/linux/bitrev.h
> index d35b8ec1c485..11620a70e776 100644
> --- a/include/linux/bitrev.h
> +++ b/include/linux/bitrev.h
> @@ -12,22 +12,10 @@
>  #define __bitrev8 __arch_bitrev8
>  
>  #else
> -extern u8 const byte_rev_table[256];
> -static inline u8 __bitrev8(u8 byte)
> -{
> -	return byte_rev_table[byte];
> -}
> -
> -static inline u16 __bitrev16(u16 x)
> -{
> -	return (__bitrev8(x & 0xff) << 8) | __bitrev8(x >> 8);
> -}
> -
> -static inline u32 __bitrev32(u32 x)
> -{
> -	return (__bitrev16(x & 0xffff) << 16) | __bitrev16(x >> 16);
> -}
> -
> +#include <asm-generic/bitops/__bitrev.h>
> +#define __bitrev32 generic___bitrev32
> +#define __bitrev16 generic___bitrev16
> +#define __bitrev8 generic___bitrev8
>  #endif /* CONFIG_HAVE_ARCH_BITREVERSE */
>  
>  #define __bitrev8x4(x)	(__bitrev32(swab32(x)))
> -- 
> 2.34.1

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