Re: [PATCH bpf-next v2 2/2] bpf, arm64: inline bpf_get_smp_processor_id() helper

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Apr 24, 2024 at 10:36 AM Puranjay Mohan <puranjay@kernel.org> wrote:
>
> As ARM64 JIT now implements BPF_MOV64_PERCPU_REG instruction, inline
> bpf_get_smp_processor_id().
>
> ARM64 uses the per-cpu variable cpu_number to store the cpu id.
>
> Here is how the BPF and ARM64 JITed assembly changes after this commit:
>
>                                          BPF
>                                         =====
>               BEFORE                                       AFTER
>              --------                                     -------
>
> int cpu = bpf_get_smp_processor_id();           int cpu = bpf_get_smp_processor_id();
> (85) call bpf_get_smp_processor_id#229032       (18) r0 = 0xffff800082072008
>                                                 (bf) r0 = r0

nit: hmm, you are probably using a bit outdated bpftool, it should be
emitted as:

(bf) r0 = &(void __percpu *)(r0)

>                                                 (61) r0 = *(u32 *)(r0 +0)
>
>                                       ARM64 JIT
>                                      ===========
>
>               BEFORE                                       AFTER
>              --------                                     -------
>
> int cpu = bpf_get_smp_processor_id();      int cpu = bpf_get_smp_processor_id();
> mov     x10, #0xfffffffffffff4d0           mov     x7, #0xffff8000ffffffff
> movk    x10, #0x802b, lsl #16              movk    x7, #0x8207, lsl #16
> movk    x10, #0x8000, lsl #32              movk    x7, #0x2008
> blr     x10                                mrs     x10, tpidr_el1
> add     x7, x0, #0x0                       add     x7, x7, x10
>                                            ldr     w7, [x7]
>
> Performance improvement using benchmark[1]
>
>              BEFORE                                       AFTER
>             --------                                     -------
>
> glob-arr-inc   :   23.817 ± 0.019M/s      glob-arr-inc   :   24.631 ± 0.027M/s
> arr-inc        :   23.253 ± 0.019M/s      arr-inc        :   23.742 ± 0.023M/s
> hash-inc       :   12.258 ± 0.010M/s      hash-inc       :   12.625 ± 0.004M/s
>
> [1] https://github.com/anakryiko/linux/commit/8dec900975ef
>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> ---
>  kernel/bpf/verifier.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>

Besides the nits, lgtm.

Acked-by: Andrii Nakryiko <andrii@kernel.org>

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9715c88cc025..3373be261889 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -20205,7 +20205,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>                         goto next_insn;
>                 }
>
> -#ifdef CONFIG_X86_64
> +#if defined(CONFIG_X86_64) || defined(CONFIG_ARM64)

I think you can drop this, we are protected by
bpf_jit_supports_percpu_insn() check and newly added inner #if/#elif
checks?

>                 /* Implement bpf_get_smp_processor_id() inline. */
>                 if (insn->imm == BPF_FUNC_get_smp_processor_id &&
>                     prog->jit_requested && bpf_jit_supports_percpu_insn()) {
> @@ -20214,11 +20214,20 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>                          * changed in some incompatible and hard to support
>                          * way, it's fine to back out this inlining logic
>                          */
> +#if defined(CONFIG_X86_64)
>                         insn_buf[0] = BPF_MOV32_IMM(BPF_REG_0, (u32)(unsigned long)&pcpu_hot.cpu_number);
>                         insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
>                         insn_buf[2] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
>                         cnt = 3;
> +#elif defined(CONFIG_ARM64)
> +                       struct bpf_insn cpu_number_addr[2] = { BPF_LD_IMM64(BPF_REG_0, (u64)&cpu_number) };
>

this &cpu_number offset is not guaranteed to be within 4GB on arm64?

> +                       insn_buf[0] = cpu_number_addr[0];
> +                       insn_buf[1] = cpu_number_addr[1];
> +                       insn_buf[2] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
> +                       insn_buf[3] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
> +                       cnt = 4;
> +#endif
>                         new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
>                         if (!new_prog)
>                                 return -ENOMEM;
> --
> 2.40.1
>