[PATCH] Unaligend accesses nulldevname

[PATCH] Unaligend accesses nulldevname

[Kurt Garloff]

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Hi,

I found an excessive amount of unaligned accesses on my AXP workstation
and tracked it down to ip_packet_match() in the ip_tables module.
indev and outdev are not properly aligned if set to nulldevname in
ipt_do_table().
This destroys the benefits of comparing names in units of (long) and
on architectures with expensive unaligned accesses (such as ia64 or
alpha), it even hurts a lot.

Find attached a patch against 2.6.0. A similar patch is needed for 2.4,
also attached.

Please consider merging them.

Looking at ip_packet_match(), I have two more thoughts:
* It should not be inlined. It's too large to benefit from inlining,
  IMHO. (OTOH, it's only called from one place, so it does not
  really matter.)
* There's a comment about the compiler being able to unroll the 2/4
  (64/32bit) iter loop which is not completely appropriate: We don't
  pass -funroll-loops, so gcc does not do it :-(
  It would be beneficial though.

Regards,
-- 
Kurt Garloff  <garloff@suse.de>                            Cologne, DE 
SUSE LINUX AG, Nuernberg, DE                          SUSE Labs (Head)

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--- linux-2.6.0.ix86/net/ipv4/netfilter/ip_tables.c.orig	2003-12-18 03:58:28.000000000 +0100
+++ linux-2.6.0.ix86/net/ipv4/netfilter/ip_tables.c	2004-01-07 23:49:29.000000000 +0100
@@ -260,7 +260,7 @@
 	     struct ipt_table *table,
 	     void *userdata)
 {
-	static const char nulldevname[IFNAMSIZ];
+	static const char nulldevname[IFNAMSIZ] __attribute__((aligned(sizeof(long))));
 	u_int16_t offset;
 	struct iphdr *ip;
 	u_int16_t datalen;

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--- linux-2.4.19/net/ipv4/netfilter/ip_tables.c	2002-02-25 20:38:14.000000000 +0100
+++ linux-2.4.19.AXP/net/ipv4/netfilter/ip_tables.c	2004-01-09 01:55:01.000000000 +0100
@@ -259,7 +264,7 @@
 	     struct ipt_table *table,
 	     void *userdata)
 {
-	static const char nulldevname[IFNAMSIZ] = { 0 };
+	static const char nulldevname[IFNAMSIZ] __attribute__((aligned(sizeof(long)))) = { 0 };
 	u_int16_t offset;
 	struct iphdr *ip;
 	void *protohdr;

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Re: [PATCH] Unaligend accesses nulldevname

[Harald Welte]

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On Thu, Jan 08, 2004 at 12:00:11AM +0100, Kurt Garloff wrote:
> Hi,
> 
> I found an excessive amount of unaligned accesses on my AXP workstation
> and tracked it down to ip_packet_match() in the ip_tables module.
> indev and outdev are not properly aligned if set to nulldevname in
> ipt_do_table().
> This destroys the benefits of comparing names in units of (long) and
> on architectures with expensive unaligned accesses (such as ia64 or
> alpha), it even hurts a lot.

Thanks for submitting your patch, this is the same bug report as
https://bugzilla.netfilter.org/cgi-bin/bugzilla/show_bug.cgi?id=84

> Find attached a patch against 2.6.0. A similar patch is needed for 2.4,
> also attached.

the fix is already in our patch-o-matic CVS tree, and I'm going to
submit this to davem together with some other fixes I have pending.

> Looking at ip_packet_match(), I have two more thoughts:
> * It should not be inlined. It's too large to benefit from inlining,
>   IMHO. (OTOH, it's only called from one place, so it does not
>   really matter.)

That is the idea.  It's just split in two functions to make it more
readable.  It's never intended to be called from somewhere else.

> * There's a comment about the compiler being able to unroll the 2/4
>   (64/32bit) iter loop which is not completely appropriate: We don't
>   pass -funroll-loops, so gcc does not do it :-(
>   It would be beneficial though.

I'm not a compiler geek.  Thanks for pointing this out, I will do some
testing and look at the compiler output to see what is happening.

> Regards,
> -- 
> Kurt Garloff  <garloff@suse.de>                            Cologne, DE 
-- 
- Harald Welte <laforge@netfilter.org>             http://www.netfilter.org/
============================================================================
  "Fragmentation is like classful addressing -- an interesting early
   architectural error that shows how much experimentation was going
   on while IP was being designed."                    -- Paul Vixie

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