linux-kernel - Re: [PATCH bpf-next v2 1/2] bpf, x86: inline bpf_get_current_task() for x86

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Message-ID: <6226983.lOV4Wx5bFT@7940hx>
Date: Tue, 06 Jan 2026 09:52:36 +0800
From: Menglong Dong <menglong.dong@...ux.dev>
To: Menglong Dong <menglong8.dong@...il.com>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: 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 v2 1/2] bpf,
 x86: inline bpf_get_current_task() for x86_64

On 2026/1/6 02:04 Alexei Starovoitov <alexei.starovoitov@...il.com> write:
> On Sun, Jan 4, 2026 at 5:17 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. The instruction we use here is:
> >
> >   65 48 8B 04 25 [offset] // mov rax, gs:[offset]
> >
> > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > ---
> > v2:
> > - check the variable type in emit_ldx_percpu_r0 with __verify_pcpu_ptr
> > - remove the usage of const_current_task
> > ---
> >  arch/x86/net/bpf_jit_comp.c | 36 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 36 insertions(+)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index e3b1c4b1d550..f5ff7c77aad7 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1300,6 +1300,25 @@ static void emit_st_r12(u8 **pprog, u32 size, u32 dst_reg, int off, int imm)
> >         emit_st_index(pprog, size, dst_reg, X86_REG_R12, off, imm);
> >  }
> >
> > +static void __emit_ldx_percpu_r0(u8 **pprog, __force unsigned long ptr)
> > +{
> > +       u8 *prog = *pprog;
> > +
> > +       /* mov rax, gs:[ptr] */
> > +       EMIT2(0x65, 0x48);
> > +       EMIT2(0x8B, 0x04);
> > +       EMIT1(0x25);
> > +       EMIT((u32)ptr, 4);
> > +
> > +       *pprog = prog;
> > +}
> 
> Why asm?
> Let's use BPF_MOV64_PERCPU_REG() similar to the way
> BPF_FUNC_get_smp_processor_id inlining is handled.

Ah, this is a good point. I didn't know the existing of
BPF_MOV64_PERCPU_REG :/

So we can inline it directly in the verifier instead of the
arch.

I'll use it in the next version.

Thanks!
Menglong Dong

> 
> pw-bot: cr
>