Re: [PATCH bpf-next] arm64, bpf: add internal-only MOV instruction to resolve per-CPU addrs

[Will Deacon]

On Fri, Apr 05, 2024 at 09:17:07AM +0000, Puranjay Mohan wrote:
> Support an instruction for resolving absolute addresses of per-CPU
> data from their per-CPU offsets. This instruction is internal-only and
> users are not allowed to use them directly. They will only be used for
> internal inlining optimizations for now between BPF verifier and BPF
> JITs.
> 
> Since commit 7158627686f0 ("arm64: percpu: implement optimised pcpu
> access using tpidr_el1"), the per-cpu offset for the CPU is stored in
> the tpidr_el1/2 register of that CPU.
> 
> To support this BPF instruction in the ARM64 JIT, the following ARM64
> instructions are emitted:
> 
> mov dst, src		// Move src to dst, if src != dst
> mrs tmp, tpidr_el1/2	// Move per-cpu offset of the current cpu in tmp.
> add dst, dst, tmp	// Add the per cpu offset to the dst.
> 
> If CONFIG_SMP is not defined, then nothing is emitted if src == dst, and
> mov dst, src is emitted if dst != src.
> 
> To measure the performance improvement provided by this change, the
> benchmark in [1] was used:
> 
> Before:
> glob-arr-inc   :   23.597 ± 0.012M/s
> arr-inc        :   23.173 ± 0.019M/s
> hash-inc       :   12.186 ± 0.028M/s
> 
> After:
> glob-arr-inc   :   23.819 ± 0.034M/s
> arr-inc        :   23.285 ± 0.017M/s
> hash-inc       :   12.419 ± 0.011M/s
> 
> [1] https://github.com/anakryiko/linux/commit/8dec900975ef
> 
> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> ---
>  arch/arm64/include/asm/insn.h |  7 +++++++
>  arch/arm64/lib/insn.c         | 11 +++++++++++
>  arch/arm64/net/bpf_jit.h      |  6 ++++++
>  arch/arm64/net/bpf_jit_comp.c | 16 ++++++++++++++++
>  4 files changed, 40 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
> index db1aeacd4cd9..d16d68550c22 100644
> --- a/arch/arm64/include/asm/insn.h
> +++ b/arch/arm64/include/asm/insn.h
> @@ -135,6 +135,11 @@ enum aarch64_insn_special_register {
>  	AARCH64_INSN_SPCLREG_SP_EL2	= 0xF210
>  };
>  
> +enum aarch64_insn_system_register {
> +	AARCH64_INSN_SYSREG_TPIDR_EL1	= 0xC684,
> +	AARCH64_INSN_SYSREG_TPIDR_EL2	= 0xE682,
> +};

I think these constants should have bit 15 as 0...

> +
>  enum aarch64_insn_variant {
>  	AARCH64_INSN_VARIANT_32BIT,
>  	AARCH64_INSN_VARIANT_64BIT
> @@ -686,6 +691,8 @@ u32 aarch64_insn_gen_cas(enum aarch64_insn_register result,
>  }
>  #endif
>  u32 aarch64_insn_gen_dmb(enum aarch64_insn_mb_type type);
> +u32 aarch64_insn_gen_mrs(enum aarch64_insn_register result,
> +			 enum aarch64_insn_system_register sysreg);
>  
>  s32 aarch64_get_branch_offset(u32 insn);
>  u32 aarch64_set_branch_offset(u32 insn, s32 offset);
> diff --git a/arch/arm64/lib/insn.c b/arch/arm64/lib/insn.c
> index a635ab83fee3..b008a9b46a7f 100644
> --- a/arch/arm64/lib/insn.c
> +++ b/arch/arm64/lib/insn.c
> @@ -1515,3 +1515,14 @@ u32 aarch64_insn_gen_dmb(enum aarch64_insn_mb_type type)
>  
>  	return insn;
>  }
> +
> +u32 aarch64_insn_gen_mrs(enum aarch64_insn_register result,
> +			 enum aarch64_insn_system_register sysreg)
> +{
> +	u32 insn = aarch64_insn_get_mrs_value();
> +
> +	insn &= ~GENMASK(19, 0);
> +	insn |= sysreg << 5;

.. otherwise you're shifting into the opcode bits at the top. It works
out because bit 20 is 1, but I think it would be better to rework your
aarch64_insn_system_register values.

> +	return aarch64_insn_encode_register(AARCH64_INSN_REGTYPE_RT,
> +					    insn, result);
> +}
> diff --git a/arch/arm64/net/bpf_jit.h b/arch/arm64/net/bpf_jit.h
> index 23b1b34db088..b627ef7188c7 100644
> --- a/arch/arm64/net/bpf_jit.h
> +++ b/arch/arm64/net/bpf_jit.h
> @@ -297,4 +297,10 @@
>  #define A64_ADR(Rd, offset) \
>  	aarch64_insn_gen_adr(0, offset, Rd, AARCH64_INSN_ADR_TYPE_ADR)
>  
> +/* MRS */
> +#define A64_MRS_TPIDR_EL1(Rt) \
> +	aarch64_insn_gen_mrs(Rt, AARCH64_INSN_SYSREG_TPIDR_EL1)
> +#define A64_MRS_TPIDR_EL2(Rt) \
> +	aarch64_insn_gen_mrs(Rt, AARCH64_INSN_SYSREG_TPIDR_EL2)
> +
>  #endif /* _BPF_JIT_H */
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 76b91f36c729..e9ad9f257a18 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -877,6 +877,17 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
>  			emit(A64_ORR(1, tmp, dst, tmp), ctx);
>  			emit(A64_MOV(1, dst, tmp), ctx);
>  			break;
> +		} else if (insn_is_mov_percpu_addr(insn)) {
> +			if (dst != src)
> +				emit(A64_MOV(1, dst, src), ctx);
> +#ifdef CONFIG_SMP

CONFIG_SMP is always 'y' on arm64.

Will