Re: [PATCH bpf-next v6 1/2] bpf, x86: inline bpf_get_current_task() for x86_64

[Menglong Dong <menglong.dong@linux.dev>]

On 2026/1/21 09:23 Andrii Nakryiko <andrii.nakryiko@gmail.com> write:
> On Mon, Jan 19, 2026 at 11:06 PM Menglong Dong <menglong8.dong@gmail.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@chinatelecom.cn>
> > Acked-by: Eduard Zingerman <eddyz87@gmail.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:

The verifier_inlines_helper_call() is confusing, but I think we can't
remove the x86-64 checking. For example, some architecture
don't support BPF_FUNC_get_current_task both in
bpf_jit_inlines_helper_call() and verifier_inlines_helper_call(), which
means it can't be inline.

> 
> 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?

As Alexei said, the implement of "current" is architecture specific,
and the per-cpu variable "current_task" only exist on x86_64.

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

I think it make sense, and the I saw bpf_get_smp_processor_id does
such operation:

const struct bpf_func_proto bpf_get_smp_processor_id_proto = {
	[...]
	.allow_fastcall	= true,
};

PS: I'm a little confused about the fast call. We inline many helper,
but it seems that bpf_get_smp_processor_id is the only one that
use the "allow_fastcall". Why? I'd better study harder.

Thanks!
Menglong Dong

> 
> 
>   [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
> >
>