how to check for "optional" ppc chip features (MSR_BE)

I've been looking at PowerISA_Public.pdf that I downloaded from some ppc
site.  It describes various things as "need not be supported on all
implementations", for example the MSR_BE bit.  Is there a generic way to
detect if such a feature is supported, or a known table of models that
support features, or what?

Right now I'm considering MSR_BE (branch tracing).  I have a patch to use
this (arch_has_block_step, enabling a PTRACE_SINGLEBLOCK).  The only
machine handy to test is a Mac G5 (PPC970FX, 3.0 (pvr 003c 0300)).  I know
this chip supports MSR_BE.  But that's only because I wrote an affirmative
test case and tried it and saw it work right.  

Before submitting the kernel changes, I want to get the CPU model
conditionalization correct (a runtime check on some feature bit mask is
fine here, if CONFIG_* alone does not indicate for sure).


Thanks,
Roland