netdev - [RFC] skb align patch

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Date:	Sun, 20 Sep 2009 14:22:12 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Jesse Brandeburg <jesse.brandeburg@...il.com>,
	Jesper Dangaard Brouer <hawk@...u.dk>
Cc:	netdev@...r.kernel.org
Subject: [RFC] skb align patch

Based on the Intel suggestion that PCI-express overhead is
a significant cost.

Would people doing performance please measure the impact of
changing SKB alignment (64 bit only).


--- a/arch/x86/include/asm/system.h	2009-09-20 14:08:40.922346912 -0700
+++ b/arch/x86/include/asm/system.h	2009-09-20 14:14:37.012371200 -0700
@@ -455,4 +455,14 @@ static inline void rdtsc_barrier(void)
 	alternative(ASM_NOP3, "lfence", X86_FEATURE_LFENCE_RDTSC);
 }
 
+#ifndef CONFIG_X86_32
+/*
+ * DMA to unaligned address is more expensive than the the
+ * overhead of unaligned CPU access.
+ */
+#define NET_IP_ALIGN	0
+#define NET_SKB_PAD	L1_CACHE_BYTES
+#endif
+
+
 #endif /* _ASM_X86_SYSTEM_H */
--
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