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

[Menglong Dong]

On 2026/1/6 02:04 Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> write:
> On Sun, Jan 4, 2026 at 5:17 AM Menglong Dong <menglong8.dong@xxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > ---
> > 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
>