Subject: Re: [PATCH bpf-next v2 3/6] bpf, arm64: Add 64-bit bitops kfuncs support

From: Puranjay Mohan <[email-masked]>
Date: Thu, 19 Feb 2026 15:10:27 +0000

> Implement JIT inlining of the 64-bit bitops kfuncs on arm64.
>
> bpf_clz64(), bpf_ffs64(), bpf_fls64(), and bpf_bitrev64() are always
> inlined using mandatory ARMv8 CLZ/RBIT instructions. bpf_ctz64() is
> inlined via RBIT + CLZ, or via the native CTZ instruction when
> FEAT_CSSC is available. bpf_rol64() and bpf_ror64() are always inlined
> via RORV.
>
> bpf_popcnt64() is not inlined as the native population count instruction
> requires NEON/SIMD registers, which should not be touched from BPF
> programs. It therefore falls back to a regular function call.
>
> Signed-off-by: Leon Hwang <[email-masked]>
> ---
>  arch/arm64/net/bpf_jit_comp.c | 123 ++++++++++++++++++++++++++++++++++
>  1 file changed, 123 insertions(+)
>
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 7a530ea4f5ae..f03f732063d9 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -1192,6 +1192,127 @@ static int add_exception_handler(const struct bpf_insn *insn,
>  	return 0;
>  }
>  
> +static inline u32 a64_clz64(u8 rd, u8 rn)
> +{
> +	/*
> +	 * Arm Architecture Reference Manual for A-profile architecture
> +	 * (Document number: ARM DDI 0487)
> +	 *
> +	 *   A64 Base Instruction Descriptions
> +	 *   C6.2 Alphabetical list of A64 base instructions
> +	 *
> +	 *   C6.2.91 CLZ
> +	 *
> +	 *     Count leading zeros
> +	 *
> +	 *     This instruction counts the number of consecutive binary zero bits,
> +	 *     starting from the most significant bit in the source register,
> +	 *     and places the count in the destination register.
> +	 */
> +	/* CLZ Xd, Xn */
> +	return 0xdac01000 | (rn << 5) | rd;
> +}
> +
> +static inline u32 a64_ctz64(u8 rd, u8 rn)
> +{
> +	/*
> +	 * Arm Architecture Reference Manual for A-profile architecture
> +	 * (Document number: ARM DDI 0487)
> +	 *
> +	 *   A64 Base Instruction Descriptions
> +	 *   C6.2 Alphabetical list of A64 base instructions
> +	 *
> +	 *   C6.2.144 CTZ
> +	 *
> +	 *     Count trailing zeros
> +	 *
> +	 *     This instruction counts the number of consecutive binary zero bits,
> +	 *     starting from the least significant bit in the source register,
> +	 *     and places the count in the destination register.
> +	 *
> +	 *     This instruction requires FEAT_CSSC.
> +	 */
> +	/* CTZ Xd, Xn */
> +	return 0xdac01800 | (rn << 5) | rd;
> +}
> +
> +static inline u32 a64_rbit64(u8 rd, u8 rn)
> +{
> +	/*
> +	 * Arm Architecture Reference Manual for A-profile architecture
> +	 * (Document number: ARM DDI 0487)
> +	 *
> +	 *   A64 Base Instruction Descriptions
> +	 *   C6.2 Alphabetical list of A64 base instructions
> +	 *
> +	 *   C6.2.320 RBIT
> +	 *
> +	 *     Reverse bits
> +	 *
> +	 *     This instruction reverses the bit order in a register.
> +	 */
> +	/* RBIT Xd, Xn */
> +	return 0xdac00000 | (rn << 5) | rd;
> +}

Instead of hardcoding the instructions with the above functions, do it the
proper way something like the following patch (not compile tested):

-- >8 --

diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
index 18c7811774d3..b2696af0b817 100644
--- a/arch/arm64/include/asm/insn.h
+++ b/arch/arm64/include/asm/insn.h
@@ -221,6 +221,9 @@ enum aarch64_insn_data1_type {
 	AARCH64_INSN_DATA1_REVERSE_16,
 	AARCH64_INSN_DATA1_REVERSE_32,
 	AARCH64_INSN_DATA1_REVERSE_64,
+	AARCH64_INSN_DATA1_RBIT,
+	AARCH64_INSN_DATA1_CLZ,
+	AARCH64_INSN_DATA1_CTZ,
 };

 enum aarch64_insn_data2_type {
@@ -389,6 +392,9 @@ __AARCH64_INSN_FUNCS(rorv,	0x7FE0FC00, 0x1AC02C00)
 __AARCH64_INSN_FUNCS(rev16,	0x7FFFFC00, 0x5AC00400)
 __AARCH64_INSN_FUNCS(rev32,	0x7FFFFC00, 0x5AC00800)
 __AARCH64_INSN_FUNCS(rev64,	0x7FFFFC00, 0x5AC00C00)
+__AARCH64_INSN_FUNCS(rbit,	0x7FFFFC00, 0x5AC00000)
+__AARCH64_INSN_FUNCS(clz,      0x7FFFFC00, 0x5AC01000)
+__AARCH64_INSN_FUNCS(ctz,      0x7FFFFC00, 0x5AC01800)
 __AARCH64_INSN_FUNCS(and,      0x7F200000, 0x0A000000)
 __AARCH64_INSN_FUNCS(bic,      0x7F200000, 0x0A200000)
 __AARCH64_INSN_FUNCS(orr,      0x7F200000, 0x2A000000)
diff --git a/arch/arm64/lib/insn.c b/arch/arm64/lib/insn.c
index 4e298baddc2e..2229ab596cda 100644
--- a/arch/arm64/lib/insn.c
+++ b/arch/arm64/lib/insn.c
@@ -1008,6 +1008,15 @@ u32 aarch64_insn_gen_data1(enum aarch64_insn_register dst,
 		}
 		insn = aarch64_insn_get_rev64_value();
 		break;
+	case AARCH64_INSN_DATA1_CLZ:
+		insn = aarch64_insn_get_clz_value();
+		break;
+	case AARCH64_INSN_DATA1_RBIT:
+		insn = aarch64_insn_get_rbit_value();
+		break;
+	case AARCH64_INSN_DATA1_CTZ:
+		insn = aarch64_insn_get_ctz_value();
+		break;
 	default:
 		pr_err("%s: unknown data1 encoding %d\n", __func__, type);
 		return AARCH64_BREAK_FAULT;
diff --git a/arch/arm64/net/bpf_jit.h b/arch/arm64/net/bpf_jit.h
index bbea4f36f9f2..af806c39dadb 100644
--- a/arch/arm64/net/bpf_jit.h
+++ b/arch/arm64/net/bpf_jit.h
@@ -248,6 +248,12 @@
 #define A64_REV16(sf, Rd, Rn) A64_DATA1(sf, Rd, Rn, REVERSE_16)
 #define A64_REV32(sf, Rd, Rn) A64_DATA1(sf, Rd, Rn, REVERSE_32)
 #define A64_REV64(Rd, Rn)     A64_DATA1(1, Rd, Rn, REVERSE_64)
+/* Rd = RBIT(Rn) */
+#define A64_RBIT(sf, Rd, Rn) A64_DATA1(sf, Rd, Rn, RBIT)
+/* Rd = CLZ(Rn) */
+#define A64_CLZ(sf, Rd, Rn) A64_DATA1(sf, Rd, Rn, CLZ)
+/* Rd = CTZ(Rn) */
+#define A64_CTZ(sf, Rd, Rn) A64_DATA1(sf, Rd, Rn, CTZ)

 /* Data-processing (2 source) */
 /* Rd = Rn OP Rm */

-- 8< --

Thanks,
Puranjay