linux-kernel - Re: [PATCH bpf-next v4 1/2] bpf, x86: inline bpf_get_current_task() for x86

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Message-ID: <CAADnVQ+Q2m19+rMLXbq98uobL6Zy5yKceDiw-PAmrmCSvvjHaw@mail.gmail.com>
Date: Tue, 13 Jan 2026 17:24:11 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Menglong Dong <menglong.dong@...ux.dev>
Cc: Menglong Dong <menglong8.dong@...il.com>, Alexei Starovoitov <ast@...nel.org>, Eduard <eddyz87@...il.com>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v4 1/2] bpf, x86: inline bpf_get_current_task()
 for x86_64

On Tue, Jan 13, 2026 at 5:19 PM Menglong Dong <menglong.dong@...ux.dev> wrote:
>
> On 2026/1/14 01:50 Alexei Starovoitov <alexei.starovoitov@...il.com> write:
> > On Mon, Jan 12, 2026 at 2:45 AM 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.
> > >
> > > In !CONFIG_SMP case, the percpu variable is just a normal variable, and
> > > we can read the current_task directly.
> > >
> > > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > > ---
> > > 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 | 29 +++++++++++++++++++++++++++++
> > >  1 file changed, 29 insertions(+)
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 3d44c5d06623..12e99171afd8 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -17688,6 +17688,8 @@ 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:
> > > +       case BPF_FUNC_get_current_task_btf:
> > > +       case BPF_FUNC_get_current_task:
> > >                 return env->prog->jit_requested && bpf_jit_supports_percpu_insn();
> > >  #endif
> > >         default:
> > > @@ -23273,6 +23275,33 @@ 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)) {
> >
> > Though verifier_inlines_helper_call() gates this with CONFIG_X86_64,
> > I think we still need explicit:
> > #if defined(CONFIG_X86_64) && !defined(CONFIG_UML)
> >
> > just like we did for BPF_FUNC_get_smp_processor_id.
> > Please check. I suspect UML will break without it.
>
> Do you mean that we need to use
> #if defined(CONFIG_X86_64) && !defined(CONFIG_UML)
> here?
>
> The whole code is already within it. You can have a look up:
>
> #if defined(CONFIG_X86_64) && !defined(CONFIG_UML)
>                 /* Implement bpf_get_smp_processor_id() inline. */
>                 if (insn->imm == BPF_FUNC_get_smp_processor_id &&
>                     verifier_inlines_helper_call(env, insn->imm)) {
> [......]
>                 /* 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)) {
> [......]
> #endif

oh. I see. I misread it as '+#endif' (with a +) and assumed
it's part of new code.

>
> >
> > > +#ifdef CONFIG_SMP
> > > +                       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);
> > > +#else
> > > +                       struct bpf_insn ld_current_addr[2] = {
> > > +                               BPF_LD_IMM64(BPF_REG_0, (unsigned long)&current_task)
> > > +                       };
> > > +                       insn_buf[0] = ld_current_addr[0];
> > > +                       insn_buf[1] = ld_current_addr[1];
> > > +                       insn_buf[2] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0);
> > > +#endif
> >
> > I wouldn't bother with !SMP.
> > If we need to add defined(CONFIG_X86_64) && !defined(CONFIG_UML)
> > I would add && defined(CONFIG_SMP) to it.
>
> OK, let's skip the !SMP case to make the code more clear.

Similar thoughts about your other patch where you introduce
decl_tag to deal with different configs.
For bpf CI we don't need to do such things.
The kernel has to be configured with selftest/bpf/config.
So doing extra work in test_progs to recognize !SMP looks like overkill.