Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752140AbbETAM1 (ORCPT ); Tue, 19 May 2015 20:12:27 -0400 Received: from mail-la0-f47.google.com ([209.85.215.47]:36671 "EHLO mail-la0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751080AbbETAMV (ORCPT ); Tue, 19 May 2015 20:12:21 -0400 MIME-Version: 1.0 In-Reply-To: <1432079946-9878-3-git-send-email-ast@plumgrid.com> References: <1432079946-9878-1-git-send-email-ast@plumgrid.com> <1432079946-9878-3-git-send-email-ast@plumgrid.com> From: Andy Lutomirski Date: Tue, 19 May 2015 17:11:59 -0700 Message-ID: Subject: Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper To: Alexei Starovoitov Cc: "David S. Miller" , Ingo Molnar , Daniel Borkmann , Michael Holzheu , Zi Shen Lim , Linux API , Network Development , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1249 Lines: 47 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? --Andy -- 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/