[PATCH] skip sync_arb_IDs on P4/Xeon

The "Synchronize Arbitration IDs with APIC IDs" IPI does not make sense
on Pentium 4 and Xeon because they don't have an APIC bus where
arbitration IDs would be needed. 

The Intel Developers manual (Vol.  3, Ch.  8.6.1) says this IPI is
unsupported on P4/Xeon.  Using it may, according to Intel, lead to
unpredictable results.
Signed-off-by: Martin Wilck <martin.wilck@fujitsu-siemens.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/apic.c

@@ -300,6 +300,10 @@ int __init verify_local_APIC(void)
 
 void __init sync_Arb_IDs(void)
 {
+	/* Unsupported on P4 - see Intel Dev. Manual Vol. 3, Ch. 8.6.1 */
+	unsigned int ver = GET_APIC_VERSION(apic_read(APIC_LVR));
+	if (ver >= 0x14)	/* P4 or higher */
+		return;
 	/*
 	 * Wait for idle.
 	 */