powerpc/perf: hw breakpoints return ENOSPC

[Michael Neuling]

> > > > On this second syscall, fetch_bp_busy_slots() sets slots.pinned to be 1,
> > > > despite there being no breakpoint on this CPU.  This is because the call
> > > > the task_bp_pinned, checks all CPUs, rather than just the current CPU.
> > > > POWER7 only has one hardware breakpoint per CPU (ie. HBP_NUM=1), so we
> > > > return ENOSPC.
> > > 
> > > I think this comes from the ptrace legacy, we register a breakpoint on
> > > all cpus because when we migrate a task it cannot fail to migrate the
> > > breakpoint.
> > > 
> > > Its one of the things I hate most about the hwbp stuff as it relates to
> > > perf.
> > > 
> > > Frederic knows more...
> > 
> > Maybe I should wait for Frederic to respond but I'm not sure I
> > understand what you're saying.
> > 
> > I can see how using ptrace hw breakpoints and perf hw breakpoints at the
> > same time could be a problem, but I'm not sure how this would stop it.
> 
> ptrace uses perf for hwbp support so we're stuck with all kinds of
> stupid ptrace constraints.. or somesuch.

OK

> > Are you saying that we need to keep at least 1 slot free at all times,
> > so that we can use it for ptrace?
> 
> No, I'm saying perf-hwbp is weird because of ptrace, maybe the ptrace
> weirdness shouldn't live in perf-hwpb but in the ptrace-perf glue
> however..

OK.

> > Is "perf record -e mem:0x10000000 true" ever going to be able to work on
> > POWER7 with only one hw breakpoint resource per CPU?  
> 
> I think it should work... but I'm fairly sure it currently doesn't
> because of how things are done. 'perf record -ie mem:0x100... true'
> might just work.

Adding -i doesn't help. 

Mikey