linux-kernel - Re: [RFC PATCH v3 18/37] bpf tools: Record map accessing instructions for each program

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Date:	Mon, 25 May 2015 15:39:15 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.com>, <paulus@...ba.org>,
	<a.p.zijlstra@...llo.nl>, <mingo@...hat.com>, <acme@...nel.org>,
	<namhyung@...nel.org>, <jolsa@...nel.org>, <dsahern@...il.com>,
	<daniel@...earbox.net>, <brendan.d.gregg@...il.com>,
	<masami.hiramatsu.pt@...achi.com>
CC:	<lizefan@...wei.com>, <linux-kernel@...r.kernel.org>,
	<pi3orama@....com>
Subject: Re: [RFC PATCH v3 18/37] bpf tools: Record map accessing instructions
 for each program



On 2015/5/19 2:34, Alexei Starovoitov wrote:
> On 5/17/15 3:56 AM, Wang Nan wrote:
>> This patch records the indics of instructions which are needed to be
>> relocated. Those information are saved in 'reloc_desc' field in
>> 'struct bpf_program'. In loading phase (this patch takes effect in
>> opening phase), the collected instructions will be replaced by
>> map loading instructions.
>>
>> Since we are going to close the ELF file and clear all data at the end
>> of 'opening' phase, ELF information will no longer be valid in
>> 'loading' phase. We have to locate the instructions before maps are
>> loaded, instead of directly modifying the instruction.
>>
>> 'struct bpf_map_def' is introduce in this patch to let us know how many
>> maps defined in the object.
>>
>> Signed-off-by: Wang Nan <wangnan0@...wei.com>
> ...
>> +/*
>> + * packed attribute is unnecessary for 'bpf_map_def'.
>> + */
>> +struct bpf_map_def {
>> +    unsigned int type;
>> +    unsigned int key_size;
>> +    unsigned int value_size;
>> +    unsigned int max_entries;
>> +};
>
> the comment looks out of place. 'packed' is necessary somewhere else?
> What were you concerned about here?
>

I see a warning message:

  make: Entering directory `/home/w00229757/kernel-hydrogen/tools/lib/bpf'
    CC       libbpf.o
  In file included from libbpf.c:23:0:
  libbpf.h:31:1: error: packed attribute is unnecessary for 
‘bpf_map_def’ [-Werror=packed]
   } __attribute__((packed));
   ^
  cc1: all warnings being treated as errors

if I append __attribute__((packed)) here. This is because 
Makefile.include in tools/script
as '-Wpacked' warning message and it is inherited by perf. When 
compiling with perf this
problem breaks compiling. Therefore I don't use 'packed' here. However I 
think there should
be one. This comment is for that.










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