Re: [PATCH bpf-next v4 1/2] bpf, x86: inline bpf_get_current_task() for x86_64

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

On 2026/1/14 01:50 Alexei Starovoitov <alexei.starovoitov@gmail.com> write:
> On Mon, Jan 12, 2026 at 2:45 AM 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.
> >
> > 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@chinatelecom.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

> 
> > +#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.

Thanks!
Menglong Dong

> 
> pw-bot: cr
>