the AltiVec compatibility could be included in kernel, by on the fly catching the illegal instruction fault, and emulating; this is unnecessary bloat, IMNSHO, and probably quite slow. Or you can install a SIGILL handler, and perform emulation yourself out of the handler, if it triggers; this is not as easy as it seems, and is probably cheaper to test the presence of AltiVec (using SIGILL handler once, e.g.) and switching to non-altivec implementation on a higher level (i.e. not per failed instruction)