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>