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 [off-list ref] write:

On Sun, Jan 4, 2026 at 5:17 AM Menglong Dong [off-list ref] 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;
+}

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