Re: [PATCH] powerpc: Disable DAWR on POWER9

[Michael Neuling <mikey@neuling.org>]

On Fri, 2018-03-16 at 17:53 -0300, Pedro Franco de Carvalho wrote:
> Michael Neuling <mikey@neuling.org> writes:
> 
> > For ptrace, we now return an error if run on a POWER9. GDB gives this
> > error when you attempt to run a program with a breakpoint set on
> > POWER9:
> >   "Unexpected error setting breakpoint or watchpoint: No such device."
> 
> Thanks a lot for implementing this!
> 
> Would it also make sense to have PPC_PTRACE_GETHWDBGINFO return
> dbginfo.num_data_bps = 0 if !breakpoint_available()? In this case GDB
> would work in a friendlier way, because it would transparently switch to
> software watchpoints.

If I do that, it looks like GDB falls back to software emulation (very sloooow)
of the watchpoint rather than failing with the error I gave before. 

That's probably the better solution, so I'll do that... Thanks!

> Not sure what to do about .features in this case. I assume currently it
> only has PPC_DEBUG_FEATURE_DATA_BP_RANGE set and not the DAWR feature
> flag (since the cpu feature is disabled).

Yeah, currently you'll get PPC_DEBUG_FEATURE_DATA_BP_RANGE and no
PPC_DEBUG_FEATURE_DATA_BP_DAWR. 

If I set .features = 0 when !breakpoint_enabled() (as well as setting
num_data_bps = 0 above), GDB seems to not like it when I run the program on P9:

    (gdb) r
    Starting program: /bin/true 
    Warning:
    Could not insert hardware watchpoint 1.
    Could not insert hardware breakpoints:
    You may have requested too many hardware breakpoints/watchpoints.
    (gdb)

So I think I'll leave this bits as I had in version 1 of the patch.

Thanks for the catch.

Mikey