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

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Message-ID: <CAADnVQLrV+0RB8REtcN9x+ub_S-DCrRqTj4s+QtX_ROrA=OwBw@mail.gmail.com>
Date: Mon, 5 Jan 2026 10:04:16 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Menglong Dong <menglong8.dong@...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 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.

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