SMP on x86/Linux is now an official feature and is not experimental.
Experimental SMP support for other architectures is underway.

Please view linux/Documentation/smp for more information about enabling SMP.

SMP support for Linux with up to 16 processors using the Intel MP
specification. 

To fix:

o       Clean up processor specific/independent split.
o       Document it all.        [PARTLY DONE]
o       Halting other CPUs on reset or panic doesn't always work.
o       Don't waste page at 4 K--don't need it now (watch the GDT code).
o       Dump bootup pages once booted somehow.
o       Clean up warnings/volatiles.
o       486 startup code.
o       How to handle mixed FPU/non FPU processors.
o       Test IO-APIC on various hardware.
o       Separate IO-APIC from SMP.