Re: Software breakpoint in kvmppc guest debug

[Hollis Blanchard <hollis_blanchard@mentor.com>]

On 11/09/2010 08:50 AM, Alexander Graf wrote:
> On 09.11.2010, at 17:36, Scott Wood wrote:
>> >  On Tue, 9 Nov 2010 13:14:47 +0100
>> >  Alexander Graf<agraf@suse.de>  wrote:
>>> >>  On 09.11.2010, at 04:40, Liu Yu-B13201 wrote:
>>>> >>>  Software breakpoint is a instruction which should make guest exit.
>>>> >>>  We replace guest code with software breakpoint instruction so that we can stop at anywhere we want.
>>>> >>>  
>>>> >>>  In my previous guest debug patches for e500, I used instruction (sc 64) as software breakpoint.
>>>> >>>  Seem this was not good, since (sc 64) maybe defined in future.
>>> >>  
>>> >>  That's always the case with overloading instructions :).
>>> >>  
>>>> >>>  also this instruction has uncertain effective on E.HV platform such as e500mc.
>>> >>  
>>> >>  Not sure if the hardware or other hypervisors break the spec, but according to 2.06 page 908 only LEV=0 and LEV=1 are defined.
>> >  
>> >  Does it say whether undefined values trap?  Maybe it only looks at the
>> >  low bit of LEV, or checks != 0, etc.
> It doesn't mentioned undefined values. It only mentions LEV=0 and LEV=1. But I don't like the idea of hard overloading a specific LEV value either way :). I'd rather use an undefined instruction for it.
Who's to say that undefined instruction will remain undefined in the 
future? It amounts to the same problem.

Although in other ways the ISA benefits from a great deal of foresight, 
as far as I remember this particular issue isn't addressed.

Hollis Blanchard
Mentor Graphics, Embedded Systems Division