Subject: Re: netfilter 07/41: arp_tables: unfold two critical loops in arp_packet_match()

From: Eric Dumazet <[email-masked]>
Date: Tue, 24 Mar 2009 22:06:50 +0100

David Miller a écrit :
> From: Patrick McHardy <[email-masked]>
> Date: Tue, 24 Mar 2009 15:03:16 +0100 (MET)
> 
>> +/*
>> + * Unfortunatly, _b and _mask are not aligned to an int (or long int)
>> + * Some arches dont care, unrolling the loop is a win on them.
>> + */
>> +static unsigned long ifname_compare(const char *_a, const char *_b, const char *_mask)
>> +{
>> +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
>> +	const unsigned long *a = (const unsigned long *)_a;
>> +	const unsigned long *b = (const unsigned long *)_b;
> 
> I think we can at least give some help for the platforms which
> require alignment.
> 
> We can, for example, assume 16-bit alignment and thus loop
> over u16's

Right. How about this incremental patch ?

Thanks

[PATCH] arp_tables: ifname_compare() can assume 16bit alignment

Arches without efficient unaligned access can still perform a loop
assuming 16bit alignment in ifname_compare()

Signed-off-by: Eric Dumazet <[email-masked]>

diff --git a/net/ipv4/netfilter/arp_tables.c b/net/ipv4/netfilter/arp_tables.c
index 64a7c6c..84b9c17 100644
--- a/net/ipv4/netfilter/arp_tables.c
+++ b/net/ipv4/netfilter/arp_tables.c
@@ -76,6 +76,7 @@ static inline int arp_devaddr_compare(const struct arpt_devaddr_info *ap,
 /*
  * Unfortunatly, _b and _mask are not aligned to an int (or long int)
  * Some arches dont care, unrolling the loop is a win on them.
+ * For other arches, we only have a 16bit alignement.
  */
 static unsigned long ifname_compare(const char *_a, const char *_b, const char *_mask)
 {
@@ -95,10 +96,13 @@ static unsigned long ifname_compare(const char *_a, const char *_b, const char *
 	BUILD_BUG_ON(IFNAMSIZ > 4 * sizeof(unsigned long));
 #else
 	unsigned long ret = 0;
+	const u16 *a = (const u16 *)_a;
+	const u16 *b = (const u16 *)_b;
+	const u16 *mask = (const u16 *)_mask;
 	int i;
 
-	for (i = 0; i < IFNAMSIZ; i++)
-		ret |= (_a[i] ^ _b[i]) & _mask[i];
+	for (i = 0; i < IFNAMSIZ/sizeof(u16); i++)
+		ret |= (a[i] ^ b[i]) & mask[i];
 #endif
 	return ret;
 }

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