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

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Message-Id: <20170424.115118.1652158849030310645.davem@davemloft.net>
Date:   Mon, 24 Apr 2017 11:51:18 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     jslaby@...e.cz
Cc:     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

From: Jiri Slaby <jslaby@...e.cz>
Date: Mon, 24 Apr 2017 17:41:06 +0200

> 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?

I said I'm not OK with the alignment, so personally I am not
with how these macros work and what they will do to the code
generated for BPF packet accesses.

But I'll defer to Alexei on this because I don't have the time
nor the energy to fight this.

Thanks.