Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751797AbbHECFk (ORCPT ); Tue, 4 Aug 2015 22:05:40 -0400 Received: from szxga01-in.huawei.com ([58.251.152.64]:54431 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751471AbbHECFj (ORCPT ); Tue, 4 Aug 2015 22:05:39 -0400 Message-ID: <55C16F53.3070604@huawei.com> Date: Wed, 5 Aug 2015 10:05:07 +0800 From: "Wangnan (F)" User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: Alexei Starovoitov , He Kuang , pi3orama CC: "linux-kernel@vger.kernel.org" , Subject: Re: Cc llvmdev: Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event References: <1436522587-136825-1-git-send-email-hekuang@huawei.com> <1436522587-136825-4-git-send-email-hekuang@huawei.com> <55A042DC.6030809@plumgrid.com> <55A3404B.6020904@huawei.com> <20150713135223.GB9917@danjae.kornet> <4D441676-21A7-46EE-AAB0-EB529D408082@163.com> <20150713140915.GD9917@danjae.kornet> <55A46928.9090708@plumgrid.com> <55A4F869.1020705@huawei.com> <55A88085.8090407@plumgrid.com> <55A88137.7020609@huawei.com> <55A88449.3030008@plumgrid.com> <55B0D5FC.6050406@huawei.com> <55B1535E.8090406@plumgrid.com> <55B1AEE9.1080207@plumgrid.com> <55B1BC03.9020708@huawei.com> <55B35F42.70803@huawei.com> <55B6E685.30905@plumgrid.com> <55B89F04.5030304@huawei.com> <55B909B2.2080606@plumgrid.com> <55BB4B8A.5000207@huawei.com> <55BFC4A0.9060100@plumgrid.com> <55C07F5B.6030107@huawei.com> <55C16DC7.70408@huawei.com> In-Reply-To: <55C16DC7.70408@huawei.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.111.66.109] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7097 Lines: 249 Send again since llvmdev is moved to llvm-dev@lists.llvm.org On 2015/8/5 9:58, Wangnan (F) wrote: > > > On 2015/8/4 17:01, Wangnan (F) wrote: >> For people who in llvmdev: >> >> This mail is belong to a thread in linux kernel mailing list, the >> first message >> can be retrived from: >> >> http://lkml.kernel.org/r/55B1535E.8090406@plumgrid.com >> >> Our goal is to fild a way to make BPF program get an unique ID for >> each type >> so it can pass the ID to other part of kernel, then we can retrive >> the type and >> decode the structure using DWARF information. Currently we have two >> problem >> needs to solve: >> >> 1. Dwarf information generated by BPF backend lost all DW_AT_name field; >> >> 2. How to get typeid from local variable? I tried llvm.eh_typeid_for >> but it support global variable only. >> >> Following is my response to Alexei. >> >> On 2015/8/4 3:44, Alexei Starovoitov wrote: >>> On 7/31/15 3:18 AM, Wangnan (F) wrote: >>> >> >> [SNIP] >> >>> didn't have time to look at it. >>> from your llvm patches looks like you've got quite experienced >>> with it already :) >>> >>>> I'll post 2 LLVM patches by replying this mail. Please have a look and >>>> help me >>>> send them to LLVM if you think my code is correct. >>> >>> patch 1: >>> I don't quite understand the purpose of builtin_dwarf_cfa >>> returning R11. It's a special register seen inside llvm codegen >>> only. It doesn't have kernel meaning. >>> >> >> Kernel side verifier allows us to do arithmetic computation using two >> local variable >> address or local variable address and R11. Therefore, we can compute >> the location >> of a local variable using: >> >> mark = &my_var_a - __builtin_frame_address(0); >> >> If the stack allocation is fixed (if the location is never reused), >> the above 'mark' >> can be uniquely identify a local variable. That's why I'm interesting >> in it. However >> I'm not sure whether the prerequestion is hold. >> >>> patch 2: >>> do we really need to hack clang? >>> Can you just define a function that aliases to intrinsic, >>> like we do for ld_abs/ld_ind ? >>> void bpf_store_half(void *skb, u64 off, u64 val) >>> asm("llvm.bpf.store.half"); >>> then no extra patches necessary. >>> >>>> struct my_str { >>>> int x; >>>> int y; >>>> }; >>>> struct my_str __str_my_str; >>>> >>>> struct my_str2 { >>>> int x; >>>> int y; >>>> int z; >>>> }; >>>> struct my_str2 __str_my_str2; >>>> >>>> test_func(__builtin_bpf_typeid(&__str_my_str)); >>>> test_func(__builtin_bpf_typeid(&__str_my_str2)); >>>> mov r1, 1 >>>> call 4660 >>>> mov r1, 2 >>>> call 4660 >>> >>> this part looks good. I think it's usable. >>> >>> > 1. llvm.eh_typeid_for can be used on global variables only. So for >>> each >>> > output >>> > structure we have to define a global varable. >>> >>> why? I think it should work with local pointers as well. >>> >> >> It is defined by LLVM, in lib/CodeGen/Analysis.cpp: >> >> /// ExtractTypeInfo - Returns the type info, possibly bitcast, >> encoded in V. >> GlobalValue *llvm::ExtractTypeInfo(Value *V) { >> ... >> assert((GV || isa(V)) && >> "TypeInfo must be a global variable or NULL"); <-- we can >> use only constant pointers >> return GV; >> } >> >> So from llvm::Intrinsic::eh_typeid_for we can get type of global >> variables only. >> >> We may need a new intrinsic for that. >> >> >>> > 2. We still need to find a way to connect the fetchd typeid with >>> DWARF >>> > info. >>> > Inserting that ID into DWARF may workable? >>> >>> hmm, that id should be the same id we're seeing in dwarf, right? >> >> There's no 'typeid' field in dwarf originally. I'm still looking for >> a way >> to inject this ID into dwarf infromation. >> >>> I think it's used in exception handling which is reusing some of >>> the dwarf stuff for this, so the must be a way to connect that id >>> to actual type info. Though last time I looked at EH was >>> during g++ hacking days. No idea how llvm does it exactly, but >>> I'm assuming the logic for rtti should be similar. >>> >> >> I'm not sure whether RTTI use dwarf to deduce type information. I >> think not, >> because dwarf infos can be stripped out. >> > > Hi Alexei, > > Just found that llvm::Intrinsic::eh_typeid_for is function specific. > ID of same type in > different functions may be different. Here is an example: > > static int (*bpf_output_event)(unsigned long, void *buf, int size) = > (void *) 0x1234; > > struct my_str { > int x; > int y; > }; > struct my_str __str_my_str; > > struct my_str2 { > int x; > int y; > int z; > }; > struct my_str2 __str_my_str2; > > int func(int *ctx) > { > struct my_str var_a; > struct my_str2 var_b; > bpf_output_event(__builtin_bpf_typeid(&__str_my_str), &var_a, > sizeof(var_a)); > bpf_output_event(__builtin_bpf_typeid(&__str_my_str2), &var_b, > sizeof(var_b)); > return 0; > } > > int func2(int *ctx) > { > struct my_str var_a; > struct my_str2 var_b; > > /* change order here */ > bpf_output_event(__builtin_bpf_typeid(&__str_my_str2), &var_b, > sizeof(var_b)); > bpf_output_event(__builtin_bpf_typeid(&__str_my_str), &var_a, > sizeof(var_a)) > return 0; > } > > This program uses __builtin_bpf_typeid(llvm::Intrinsic::eh_typeid_for) > in func and func2 > for same two types but in different order. We expect same type get > same ID. > > Compiled with: > > $ clang -target bpf -S -O2 -c ./test_bpf_typeid.c > > The result is: > > .text > .globl func > .align 8 > func: # @func > # BB#0: # %entry > mov r2, r10 > addi r2, -8 > mov r1, 1 > mov r3, 8 > call 4660 > mov r2, r10 > addi r2, -24 > mov r1, 2 > mov r3, 12 > call 4660 > mov r0, 0 > ret > > .globl func2 > .align 8 > func2: # @func2 > # BB#0: # %entry > mov r2, r10 > addi r2, -24 > mov r1, 1 <--- we want 2 here. > mov r3, 12 > call 4660 > mov r2, r10 > addi r2, -8 > mov r1, 2 <--- we want 1 here. > mov r3, 8 > call 4660 > mov r0, 0 > ret > > .comm __str_my_str,8,4 # @__str_my_str > .comm __str_my_str2,12,4 # @__str_my_str2 > > > Conclusion: llvm::Intrinsic::eh_typeid_for is not on the right > direction... > > Thank you. -- 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/