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

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Date:   Mon, 24 Apr 2017 10:41:32 -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 08:45:11 +0200

> On 04/21/2017, 09:32 PM, Alexei Starovoitov wrote:
>> On Fri, Apr 21, 2017 at 04:12:43PM +0200, Jiri Slaby wrote:
>>> Do not use a custom macro FUNC for starts of the global functions, use
>>> ENTRY instead.
>>>
>>> And while at it, annotate also ends of the functions by ENDPROC.
>>>
>>> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
>>> Cc: "David S. Miller" <davem@...emloft.net>
>>> Cc: Alexey Kuznetsov <kuznet@....inr.ac.ru>
>>> Cc: James Morris <jmorris@...ei.org>
>>> Cc: Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
>>> Cc: Patrick McHardy <kaber@...sh.net>
>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>> Cc: Ingo Molnar <mingo@...hat.com>
>>> Cc: "H. Peter Anvin" <hpa@...or.com>
>>> Cc: x86@...nel.org
>>> Cc: netdev@...r.kernel.org
>>> ---
>>>  arch/x86/net/bpf_jit.S | 32 ++++++++++++++++++--------------
>>>  1 file changed, 18 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/arch/x86/net/bpf_jit.S b/arch/x86/net/bpf_jit.S
>>> index f2a7faf4706e..762c29fb8832 100644
>>> --- a/arch/x86/net/bpf_jit.S
>>> +++ b/arch/x86/net/bpf_jit.S
>>> @@ -23,16 +23,12 @@
>>>  	32 /* space for rbx,r13,r14,r15 */ + \
>>>  	8 /* space for skb_copy_bits */)
>>>  
>>> -#define FUNC(name) \
>>> -	.globl name; \
>>> -	.type name, @function; \
>>> -	name:
>>> -
>>> -FUNC(sk_load_word)
>>> +ENTRY(sk_load_word)
>>>  	test	%esi,%esi
>>>  	js	bpf_slow_path_word_neg
>>> +ENDPROC(sk_load_word)
>> 
>> this doens't look right.
>> It will add alignment nops in critical paths of these pseudo functions.
>> I'm also not sure whether it will still work afterwards.
>> Was it tested?
>> I'd prefer if this code kept as-is.
> 
> It cannot stay as-is simply because we want to know where the functions
> end to inject debuginfo properly. The code above does not warrant for
> any exception.

I totally and completely disagree.

> Executing a nop takes a little and having externally-callable functions
> aligned can actually help performance (no, I haven't measured nor tested
> the code). But sure, the tool is generic, so I can introduce a local
> macros to avoid alignments in the functions:

Not for this case, it's a bunch of entry points all packed together
intentionally so that SKB accesses of different access sizes (which is
almost always the case) from BPF programs use the smallest amount of
I-cache as possible.