x86: Align skb w/ start of cacheline on newer core 2/Xeon Arch

x86 architectures can handle unaligned accesses in hardware, and it has
been shown that unaligned DMA accesses can be expensive on Nehalem
architectures. As such we should overwrite NET_IP_ALIGN to resolve
this issue.

Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: [e-mail]
Signed-off-by: Alexander Duyck <[e-mail]>
Signed-off-by: Jeff Kirsher <[e-mail]>
Acked-by: H. Peter Anvin <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

arch/x86/include/asm/system.h

···
 	alternative(ASM_NOP3, "lfence", X86_FEATURE_LFENCE_RDTSC);
 }
 
+#ifdef CONFIG_MCORE2
+/*
+ * We handle most unaligned accesses in hardware.  On the other hand
+ * unaligned DMA can be quite expensive on some Nehalem processors.
+ *
+ * Based on this we disable the IP header alignment in network drivers.
+ */
+#define NET_IP_ALIGN	0
+#endif
 #endif /* _ASM_X86_SYSTEM_H */