Re: [PATCH bpf-next v2 1/2] bpf, x86: inline bpf_get_current_task() for x86_64

On 2026/1/6 01:45 Eduard Zingerman [off-list ref] write:

On Sun, 2026-01-04 at 21:16 +0800, Menglong Dong wrote:

quoted

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 <redacted>
---
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.

Yeah, I missed this part. I'll use BPF_MOV64_PERCPU_REG() in
the next version, which should avoid this problem.

Thanks!
Menglong Dong

quoted

+				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;
+	}
+}