Re: [PATCH v3 07/29] x86: bpf_jit, use ENTRY+ENDPROC

From: Jiri Slaby <jslaby@suse.cz>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: mingo@redhat.com, tglx@linutronix.de, hpa@zytor.com,
	x86@kernel.org, jpoimboe@redhat.com,
	linux-kernel@vger.kernel.org,
	"David S. Miller" <davem@davemloft.net>,
	netdev@vger.kernel.org, Daniel Borkmann <daniel@iogearbox.net>,
	Eric Dumazet <edumazet@google.com>
Subject: Re: [PATCH v3 07/29] x86: bpf_jit, use ENTRY+ENDPROC
Date: Mon, 24 Apr 2017 08:45:11 +0200	[thread overview]
Message-ID: <697947f4-0a2c-1480-0995-9919556dc020@suse.cz> (raw)
In-Reply-To: <20170421193213.GB91454@ast-mbp.thefacebook.com>

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@suse.cz>
>> Cc: "David S. Miller" <davem@davemloft.net>
>> Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
>> Cc: James Morris <jmorris@namei.org>
>> Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
>> Cc: Patrick McHardy <kaber@trash.net>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: x86@kernel.org
>> Cc: netdev@vger.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.

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:

#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)

#define BPF_FUNC_END(name) SYM_FUNC_END(name)

thanks,
-- 
js
suse labs