Re: [PATCH] powerpc: Emulate most Book I instructions in emulate_step()

[Michael Ellerman <michael@ellerman.id.au>]

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On Thu, 2010-06-03 at 11:10 +1000, Matt Evans wrote:
> Paul Mackerras wrote:
> > [snip]
> > The second alternative -- emulating the lwarx/stwcx and all the
> > instructions in between -- sounds complicated but turns out to be
> > pretty straightforward in fact, since the code for each instruction is
> > pretty small, easy to verify that it's correct, and has little
> > interaction with other code.
> 
> Easy to verify -- visually or logically?
> 
> Having had a little experience with interpreters 'invisibly' operating
> behind the scenes I am all for very rigorous testing of these things.
> I have lost at least four of my nine lives to incorrect flag values,
> odd data problems and hideous heisenbugs etc. of such interpreters.
> Looked at another way, you'd be surprised how much one can break in an
> interpreter and still successfully run various programs.
> 
> Presumably your first pass is completely correct already, but I'm
> thinking that if any future changes are made to it 
> it would be good to include test code/modes alongside the interpreter
> so others can check alterations.  E.g. include the "run user program
> interpreted" test switch patch, or even better compare the interpreted
> state to real hardware execution.  There are other more directed test
> strategies (e.g. handwritten tests, random code) but these would be a
> good start.

Emphatic nod. We all trust Paulus to get this right, but I for one would
not be game to touch it without a test suite.

It's ripe territory for a boot time selftest IMHO.

cheers

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