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

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Date:	Wed, 5 Aug 2015 16:59:01 +0800
From:	He Kuang <hekuang@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.com>,
	"Wangnan (F)" <wangnan0@...wei.com>, pi3orama <pi3orama@....com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<llvm-dev@...ts.llvm.org>
Subject: Re: [LLVMdev] Cc llvmdev: Re: llvm bpf debug info. Re: [RFC PATCH v4
 3/3] bpf: Introduce function for outputing data to perf event

Hi, Alexei

On 2015/7/30 1:13, Alexei Starovoitov wrote:
> On 7/29/15 2:38 AM, He Kuang wrote:
>> Hi, Alexei
>>
>> On 2015/7/28 10:18, Alexei Starovoitov wrote:
>>> On 7/25/15 3:04 AM, He Kuang wrote:
>>>> I noticed that for 64-bit elf format, the reloc sections have
>>>> 'Addend' in the entry, but there's no 'Addend' info in bpf elf
>>>> file(64bit). I think there must be something wrong in the process
>>>> of .s -> .o, which related to 64bit/32bit. Anyway, we can parse out the
>>>> AT_name now, DW_AT_LOCATION still missed and need your help.
>>>

Another thing about DW_AT_name, we've already found that the name
string is stored indirectly and needs relocation which is
architecture specific, while the e_machine info in bpf obj file
is "unknown", both objdump and libdw cannot parse DW_AT_name
correctly.

Should we just use a known architeture for bpf object file
instead of "unknown"? If so, we can use the existing relocation
codes in libdw and get DIE name by simply invoking
dwarf_diename(). The drawback of this method is that, e.g. we
use "x86-64" instead, is hard to distinguish bpf obj file with
x86-64 elf file. Do you think this is ok?

Otherwise, for not touching libdw, we should reimplement the
relocation codes already in libdw for bpf elf file with "unknown"
machine info specially in perf. I wonder whether it is worth doing
this and what's your opinion?

Thank you.

>> index directly:
>>
>>    __bpf_trace_output_data(__builtin_dwarf_type(myvar_a), &myvar_a, size);
>>
>
> probably both A and B won't really work when programs get bigger
> and optimizations will start moving lines around.
> the builtin_dwarf_type idea is actually quite interesting.
> Potentially that builtin can stringify type name and later we can
> search it in dwarf. Please take a look how to add such builtin.
> There are few similar builtins that deal with exception handling
> and need type info. May be they can be reused. Like:
> int_eh_typeid_for and int_eh_dwarf_cfa
>
>

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