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

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Date:   Mon, 24 Apr 2017 17:55:07 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Jiri Slaby <jslaby@...e.cz>
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


* 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!

Thanks,

	Ingo