Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752344AbbETAOh (ORCPT ); Tue, 19 May 2015 20:14:37 -0400 Received: from mail-pa0-f46.google.com ([209.85.220.46]:34529 "EHLO mail-pa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751894AbbETAOf (ORCPT ); Tue, 19 May 2015 20:14:35 -0400 Message-ID: <555BD1E9.5000000@plumgrid.com> Date: Tue, 19 May 2015 17:14:33 -0700 From: Alexei Starovoitov User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Andy Lutomirski CC: "David S. Miller" , Ingo Molnar , Daniel Borkmann , Michael Holzheu , Zi Shen Lim , Linux API , Network Development , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper References: <1432079946-9878-1-git-send-email-ast@plumgrid.com> <1432079946-9878-3-git-send-email-ast@plumgrid.com> In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1456 Lines: 50 On 5/19/15 5:11 PM, Andy Lutomirski wrote: > On Tue, May 19, 2015 at 4:59 PM, Alexei Starovoitov wrote: >> bpf_tail_call() arguments: >> ctx - context pointer >> jmp_table - one of BPF_MAP_TYPE_PROG_ARRAY maps used as the jump table >> index - index in the jump table >> >> In this implementation x64 JIT bypasses stack unwind and jumps into the >> callee program after prologue, so the callee program reuses the same stack. >> >> The logic can be roughly expressed in C like: >> >> u32 tail_call_cnt; >> >> void *jumptable[2] = { &&label1, &&label2 }; >> >> int bpf_prog1(void *ctx) >> { >> label1: >> ... >> } >> >> int bpf_prog2(void *ctx) >> { >> label2: >> ... >> } >> >> int bpf_prog1(void *ctx) >> { >> ... >> if (tail_call_cnt++ < MAX_TAIL_CALL_CNT) >> goto *jumptable[index]; ... and pass my 'ctx' to callee ... >> >> ... fall through if no entry in jumptable ... >> } >> > > What causes the stack pointer to be right? Is there some reason that > the stack pointer is the same no matter where you are in the generated > code? that's why I said 'it's _roughly_ expressed in C' this way. Stack pointer doesn't change. It uses the same stack frame. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/