linux-kernel - Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

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Message-ID: <55BFC4A0.9060100@plumgrid.com>
Date:	Mon, 3 Aug 2015 12:44:32 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	"Wangnan (F)" <wangnan0@...wei.com>, He Kuang <hekuang@...wei.com>,
	pi3orama <pi3orama@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce
 function for outputing data to perf event

On 7/31/15 3:18 AM, Wangnan (F) wrote:
>
>     However with the newest llvm + clang the DWARF info is still incorrect:
>
> $ objdump  --dwarf=info ./out.o
> ...
>   <1><3f>: Abbrev Number: 3 (DW_TAG_structure_type)
>      <40>   DW_AT_name        : (indirect string, offset: 0x0): clang
> version 3.8.0 (http://llvm.org/git/clang.git
> f0fcd3432cbed83500df70c18f275d8affb89e5e) (http://llvm.org/git/llvm.git
> c8ccd78d31d4949fa1c14e954ccb06253e18cf37)
>      <44>   DW_AT_byte_size   : 8
>      <45>   DW_AT_decl_file   : 1
>      <46>   DW_AT_decl_line   : 4
>   <2><47>: Abbrev Number: 4 (DW_TAG_member)
>      <48>   DW_AT_name        : (indirect string, offset: 0x0): clang
> version 3.8.0 (http://llvm.org/git/clang.git
> f0fcd3432cbed83500df70c18f275d8affb89e5e) (http://llvm.org/git/llvm.git
> c8ccd78d31d4949fa1c14e954ccb06253e18cf37)
>      <4c>   DW_AT_type        : <0x60>
>      <50>   DW_AT_decl_file   : 1
>      <51>   DW_AT_decl_line   : 5
>      <52>   DW_AT_data_member_location: 0
>   <2><53>: Abbrev Number: 4 (DW_TAG_member)
>      <54>   DW_AT_name        : (indirect string, offset: 0x0): clang
> version 3.8.0 (http://llvm.org/git/clang.git
> f0fcd3432cbed83500df70c18f275d8affb89e5e) (http://llvm.org/git/llvm.git
> c8ccd78d31d4949fa1c14e954ccb06253e18cf37)
>      <58>   DW_AT_type        : <0x60>
>      <5c>   DW_AT_decl_file   : 1
>      <5d>   DW_AT_decl_line   : 6
>      <5e>   DW_AT_data_member_location: 4
> ...
>
> The DW_AT_name is missing.

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.

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.

 > 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?
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.

btw, for any deep llvm stuff you may need to move the thread to llvmdev.
May be folks there will have other ideas.

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