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

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Message-ID: <08ab237ad7da8d1f6494cb434d9a5a46a599462c.camel@gmail.com>
Date: Mon, 05 Jan 2026 09:45:13 -0800
From: Eduard Zingerman <eddyz87@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>, ast@...nel.org
Cc: davem@...emloft.net, dsahern@...nel.org, daniel@...earbox.net, 
	andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
 yonghong.song@...ux.dev, 	john.fastabend@...il.com, kpsingh@...nel.org,
 sdf@...ichev.me, haoluo@...gle.com, 	jolsa@...nel.org, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, 	dave.hansen@...ux.intel.com,
 x86@...nel.org, hpa@...or.com, netdev@...r.kernel.org, 
	bpf@...r.kernel.org, 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, 2026-01-04 at 21:16 +0800, Menglong Dong 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;
> +}
> +
> +#define emit_ldx_percpu_r0(prog, variable)					\
> +	do {									\
> +		__verify_pcpu_ptr(&(variable));					\
> +		__emit_ldx_percpu_r0(&prog, (__force unsigned long)&(variable));\
> +	} while (0)
> +
>  static int emit_atomic_rmw(u8 **pprog, u32 atomic_op,
>  			   u32 dst_reg, u32 src_reg, s16 off, u8 bpf_size)
>  {
> @@ -2441,6 +2460,12 @@ st:			if (is_imm8(insn->off))
>  		case BPF_JMP | BPF_CALL: {
>  			u8 *ip = image + addrs[i - 1];
>  
> +			if (insn->src_reg == 0 && (insn->imm == BPF_FUNC_get_current_task ||
> +						   insn->imm == BPF_FUNC_get_current_task_btf)) {

I think this should be guarded by IS_ENABLED(CONFIG_SMP).
The current.h:get_current() used
arch/x86/include/asm/percpu.h:this_cpu_read_stable() that is unrolled
to __raw_cpu_read_stable(), which uses __force_percpu_arg(), which uses
__force_percpu_prefix, which is defined differently depending on CONFIG_SMP.

> +				emit_ldx_percpu_r0(prog, current_task);
> +				break;
> +			}
> +
>  			func = (u8 *) __bpf_call_base + imm32;
>  			if (src_reg == BPF_PSEUDO_CALL && tail_call_reachable) {
>  				LOAD_TAIL_CALL_CNT_PTR(stack_depth);
> @@ -4082,3 +4107,14 @@ bool bpf_jit_supports_timed_may_goto(void)
>  {
>  	return true;
>  }
> +
> +bool bpf_jit_inlines_helper_call(s32 imm)
> +{
> +	switch (imm) {
> +	case BPF_FUNC_get_current_task:
> +	case BPF_FUNC_get_current_task_btf:
> +		return true;
> +	default:
> +		return false;
> +	}
> +}