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

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Message-ID: <CAADnVQLkNFDvmzmxEzVBo=C1UfcxYsO_sSaTkvwg8LCswypvWw@mail.gmail.com>
Date: Tue, 20 Jan 2026 17:43:47 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Menglong Dong <menglong8.dong@...il.com>, Alexei Starovoitov <ast@...nel.org>, Eduard <eddyz87@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v6 1/2] bpf, x86: inline bpf_get_current_task()
 for x86_64

On Tue, Jan 20, 2026 at 5:24 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> 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?

I think it needs to be arch specific, since 'current' is arch
specific. x86 is different from arm64.
Though both JITs support percpu pseudo insn, it doesn't help
to make get_current inlining generic.
One has to analyze each arch individually.