linux-kernel - Re: [PATCH bpf-next v6 1/2] bpf, x86: inline bpf_get_current_task() for x86

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Message-ID: <CAEf4BzYjbP4KwuVNT_pgjfXy=+7saEezmp1ck2L5txwtik5WYw@mail.gmail.com>
Date: Tue, 20 Jan 2026 17:23:51 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: ast@...nel.org, eddyz87@...il.com, davem@...emloft.net, dsahern@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	song@...nel.org, yonghong.song@...ux.dev, john.fastabend@...il.com, 
	kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	netdev@...r.kernel.org, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v6 1/2] bpf, x86: inline bpf_get_current_task()
 for x86_64

On Mon, Jan 19, 2026 at 11:06 PM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> Inline bpf_get_current_task() and bpf_get_current_task_btf() for x86_64
> to obtain better performance.
>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> Acked-by: Eduard Zingerman <eddyz87@...il.com>
> ---
> v5:
> - don't support the !CONFIG_SMP case
>
> v4:
> - handle the !CONFIG_SMP case
>
> v3:
> - implement it in the verifier with BPF_MOV64_PERCPU_REG() instead of in
>   x86_64 JIT.
> ---
>  kernel/bpf/verifier.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9de0ec0c3ed9..c4e2ffadfb1f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -17739,6 +17739,10 @@ static bool verifier_inlines_helper_call(struct bpf_verifier_env *env, s32 imm)
>         switch (imm) {
>  #ifdef CONFIG_X86_64
>         case BPF_FUNC_get_smp_processor_id:
> +#ifdef CONFIG_SMP
> +       case BPF_FUNC_get_current_task_btf:
> +       case BPF_FUNC_get_current_task:
> +#endif

Does this have to be x86-64 specific inlining? With verifier inlining
and per_cpu instruction support it should theoretically work across
all architectures that do support per-cpu instruction, no?

Eduard pointed out [0] to me for why we have that x86-64 specific
check. But looking at do_misc_fixups(), we have that early
bpf_jit_inlines_helper_call(insn->imm)) check, so if some JIT has more
performant inlining implementation, we will just do that.

So it seems like we can just drop all that x86-64 specific logic and
claim all three of these functions as inlinable, no?

And even more. We can drop rather confusing
verifier_inlines_helper_call() that duplicates the decision of which
helpers can be inlined or not, and have:

if (env->prog->jit_requested && bpf_jit_supports_percpu_insn() {
    switch (insn->imm) {
    case BPF_FUNC_get_smp_processor_id:
        ...
        break;
    case BPF_FUNC_get_current_task_btf:
    case BPF_FUNC_get_current_task_btf:
        ...
        break;
    default:
}

And the decision about inlining will live in one place.

Or am I missing some complications?

And with all that, should we mark get_current_task and
get_current_task_btf as __bpf_fastcall?


  [0] https://lore.kernel.org/all/20240722233844.1406874-4-eddyz87@gmail.com/

>                 return env->prog->jit_requested && bpf_jit_supports_percpu_insn();
>  #endif
>         default:
> @@ -23319,6 +23323,24 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>                         insn      = new_prog->insnsi + i + delta;
>                         goto next_insn;
>                 }
> +
> +               /* Implement bpf_get_current_task() and bpf_get_current_task_btf() inline. */
> +               if ((insn->imm == BPF_FUNC_get_current_task || insn->imm == BPF_FUNC_get_current_task_btf) &&
> +                   verifier_inlines_helper_call(env, insn->imm)) {
> +                       insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, (u32)(unsigned long)&current_task);
> +                       insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
> +                       insn_buf[2] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0);
> +                       cnt = 3;
> +
> +                       new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> +                       if (!new_prog)
> +                               return -ENOMEM;
> +
> +                       delta    += cnt - 1;
> +                       env->prog = prog = new_prog;
> +                       insn      = new_prog->insnsi + i + delta;
> +                       goto next_insn;
> +               }
>  #endif
>                 /* Implement bpf_get_func_arg inline. */
>                 if (prog_type == BPF_PROG_TYPE_TRACING &&
> --
> 2.52.0
>