This option enables kernel support for the Altivec extensions to the
PowerPC processor. The kernel currently supports saving and restoring
altivec registers, and turning on the 'altivec enable' bit so user
processes can execute altivec instructions.

This option is only usefully if you have a processor that supports
altivec (G4, otherwise known as 74xx series), but does not have
any affect on a non-altivec cpu (it does, however add code to the
kernel).

If in doubt, say Y here.

source code:

arch/powerpc/lib/vmx-helper.c