linux-kernel - Re: [PATCH v3 07/29] x86: bpf

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Date:   Mon, 24 Apr 2017 18:02:51 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     David Miller <davem@...emloft.net>, alexei.starovoitov@...il.com,
        mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        x86@...nel.org, jpoimboe@...hat.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, daniel@...earbox.net, edumazet@...gle.com
Subject: Re: [PATCH v3 07/29] x86: bpf_jit, use ENTRY+ENDPROC

On 04/24/2017, 05:55 PM, Ingo Molnar wrote:
> * Jiri Slaby <jslaby@...e.cz> wrote:
> 
>> On 04/24/2017, 05:08 PM, David Miller wrote:
>>> If you align the entry points, then the code sequence as a whole is
>>> are no longer densely packed.
>>
>> Sure.
>>
>>> Or do I misunderstand how your macros work?
>>
>> Perhaps. So the suggested macros for the code are:
>> #define BPF_FUNC_START_LOCAL(name) \
>> 		SYM_START(name, SYM_V_LOCAL, SYM_A_NONE)
>> #define BPF_FUNC_START(name) \
>> 		SYM_START(name, SYM_V_GLOBAL, SYM_A_NONE)
>>
>> and they differ from the standard ones:
>> #define SYM_FUNC_START_LOCAL(name)                      \
>>         SYM_START(name, SYM_V_LOCAL, SYM_A_ALIGN)
>> #define SYM_FUNC_START(name)                            \
>>         SYM_START(name, SYM_V_GLOBAL, SYM_A_ALIGN)
>>
>>
>> The difference is SYM_A_NONE vs. SYM_A_ALIGN, which means:
>> #define SYM_A_ALIGN                             ALIGN
>> #define SYM_A_NONE                              /* nothing */
>>
>> Does it look OK now?
> 
> No, the patch changes alignment which is undesirable, it needs to preserve the 
> existing (non-)alignment of the symbols!

OK, so I am not expressing myself explicitly enough, it seems.

So, correct, the patch v3 adds alignments. I suggested in the discussion
the macros above. They do not add alignments. If everybody is OK with
that, v4 of the patch won't add alignments. OK?

thanks,
-- 
js
suse labs