Re: [PATCH bpf-next 1/2] bpf, x64: Propagate tailcall info only for tail_call_reachable subprogs

[Yonghong Song <yonghong.song@linux.dev>]

On 10/21/24 6:39 AM, Leon Hwang wrote:
> In the x86_64 JIT, when calling a function, tailcall info is propagated if
> the program is tail_call_reachable, regardless of whether the function is a
> subprog, helper, or kfunc. However, this propagation is unnecessary for
> not-tail_call_reachable subprogs, helpers, or kfuncs.
>
> The verifier can determine if a subprog is tail_call_reachable. Therefore,
> it can be optimized to only propagate tailcall info when the callee is
> subprog and the subprog is actually tail_call_reachable.
>
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---
>   arch/x86/net/bpf_jit_comp.c | 4 +++-
>   kernel/bpf/verifier.c       | 6 ++++++
>   2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 06b080b61aa57..6ad6886ecfc88 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -2124,10 +2124,12 @@ st:			if (is_imm8(insn->off))
>   
>   			/* call */
>   		case BPF_JMP | BPF_CALL: {
> +			bool pseudo_call = src_reg == BPF_PSEUDO_CALL;
> +			bool subprog_tail_call_reachable = dst_reg;
>   			u8 *ip = image + addrs[i - 1];
>   
>   			func = (u8 *) __bpf_call_base + imm32;
> -			if (tail_call_reachable) {
> +			if (pseudo_call && subprog_tail_call_reachable) {

Why we need subprog_tail_call_reachable? Does
	tail_call_reachable && psueudo_call
work the same way?

>   				LOAD_TAIL_CALL_CNT_PTR(bpf_prog->aux->stack_depth);
>   				ip += 7;
>   			}
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index f514247ba8ba8..6e7e42c7bc7b1 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -19990,6 +19990,12 @@ static int jit_subprogs(struct bpf_verifier_env *env)
>   			insn[0].imm = (u32)addr;
>   			insn[1].imm = addr >> 32;
>   		}
> +
> +		if (bpf_pseudo_call(insn))
> +			/* In the x86_64 JIT, tailcall information can only be
> +			 * propagated if the subprog is tail_call_reachable.
> +			 */
> +			insn->dst_reg = env->subprog_info[subprog].tail_call_reachable;
>   	}
>   
>   	err = bpf_prog_alloc_jited_linfo(prog);