The most important part is that it makes x86-64 compile again.
Without that 2.6 users won't be very happy.

It also works around a bug that allowed every user program to reboot the
system on B stepping K8.

Also update to match some recent i386 fixes.

Full ChangeLog:
 - Add acpi_pic_set_level_irq to make ACPI compile again
 - Work around compat mode K8 bug in IRET exception handling
 - Increase exception stack. The old 1k stack was too easy
   to overflow (from Jim Paradis, changed by me)
 - Replace safe_smp_processor_id with cpuid (needed for above)
 - When there is only one node always enable fake_node mode
 - Merge with i386 (NTP gettimeofday monoticity fix, irq nr_vectors change)
 - Fix compile problem for UP kernels in time/cpufreq
 - Set all nodes online at bootup
 - Define node_to_cpumask correctly