From: Paul Mackerras <paulus@samba.org>

On machines with Altivec (i.e.  G4 and G5 processors), the altivec
floating-point instructions can operate in two modes: one where
denormalized inputs or outputs are truncated to zero, and one where they
aren't.  In the latter mode the processor can take an exception when it
encounters denormalized floating-point inputs or outputs rather than
dealing with them in hardware.

This patch adds code to deal properly with the exception, by emulating the
instruction that caused the exception.  Previously the kernel just switched
the altivec unit into the truncate-to-zero mode, which works but is a bit
gross.  Fortunately there are only a limited set of altivec instructions
which can generate the assist exception, so we don't have to emulate the
whole altivec instruction set.

This patch also makes sure that we always have a handler for the altivec
unavailable exception.  Without this, if you run a kernel that is not
configured for altivec support on a machine with altivec, it works fine
until a user process tries to execute an altivec instruction.  At that
point the kernel thinks it has taken an unknown exception and panics.  With
this patch it sends a SIGILL to the process instead.