Re: [PATCH] RISC-V: enable have_nonsteppable_watchpoint by default

[Andrew Burgess <andrew.burgess@embecosm.com>]

* Joel Brobecker <brobecker@adacore.com> [2018-10-03 15:37:03 -0700]:

> Hi Craig, hi Andrew,
> 
> > ---
> > 
> > The RISC-V debug spec 0.13 recommends that write triggers fire before the
> > write is committed. If the target follows this behaviour, then
> > have_nonsteppable_watchpoint needs to be set to 'on' so that GDB will step
> > over the watchpoint before checking if the value has changed.
> > 
> > This patch adds a setshow for have_nonsteppable_watchpoint which defaults
> > to 'on' to match the recommended behaviour. If a target does not follow
> > this timing, then 'set riscv have_nonsteppable_watchpoint off' will need
> > to be issued on the command line.
> > 
> > gdb/ChangeLog:
> > 
> > 	* riscv-tdep.c (set_have_nonsteppable_watchpoint): Add callback
> > 	for 'set riscv have_nonsteppable_watchpoint'.
> > 	(show_have_nonsteppable_watchpoint): Add callback for
> > 	'show riscv have_nonsteppable_watchpoint'.
> > 	(riscv_gdbarch_init): Initialise gdbarch setting for
> > 	have_nonesteppable_watchpoint.
> 
> I assume this patch is waiting for review from Andrew?

No, I was hoping for some feedback from Pedro, he commented in this
post:
   https://sourceware.org/ml/gdb-patches/2018-09/msg00570.html

that he wasn't happy with the approach Craig took. He would like to
see the switching done automatically from the target description.  In
this post:
   https://sourceware.org/ml/gdb-patches/2018-09/msg00572.html

I agree with Pedro, but take the position that as riscv doesn't
currently have any target description support (I hope to work on some
of this soon) then I'd like this patch to go in as it is and improve
on it later.

However, as Pedro is a global maintainer, I don't feel I can OK the
patch with his negative feedback outstanding...

Thanks,
Andrew





> 
> I took a look, since I am interested in it as well. Here are my
> comments.
> 
> You forgot to document the introduction of riscv_have_nonsteppable_watchpoint
> in your ChangeLog above; eg:
> 
>         * riscv-tdep.c (riscv_have_nonsteppable_watchpoint): New static
>         global.
> 
> You also forgot to document that you're adding new subcommands.
> 
> The rest looks good to me.
> 
> > ---
> > 
> > diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
> > index 254914c..857c5d1 100644
> > --- a/gdb/riscv-tdep.c
> > +++ b/gdb/riscv-tdep.c
> > @@ -226,6 +226,36 @@ show_use_compressed_breakpoints (struct ui_file *file, int from_tty,
> >  		      "to %s%s.\n"), value, additional_info);
> >  }
> >  
> > +/* Controls whether the debugger should step over hardware watchpoints before
> > +   checking if the watched variable has changed.  If true, then the debugger
> > +   will step over the watchpoint.  */
> > +
> > +static int riscv_have_nonsteppable_watchpoint = 1;
> > +
> > +/* The set callback for 'set riscv have-nonsteppable-watchpoint'.  */
> > +
> > +static void
> > +set_have_nonsteppable_watchpoint (const char *args, int from_tty,
> > +				  struct cmd_list_element *c)
> > +{
> > +  struct gdbarch *gdbarch = target_gdbarch ();
> > +
> > +  set_gdbarch_have_nonsteppable_watchpoint (gdbarch,
> > +					    riscv_have_nonsteppable_watchpoint);
> > +}
> > +
> > +/* The show callback for 'show riscv have-nonsteppable-watchpoint'.  */
> > +
> > +static void
> > +show_have_nonsteppable_watchpoint (struct ui_file *file, int from_tty,
> > +				   struct cmd_list_element *c,
> > +				   const char *value)
> > +{
> > +  fprintf_filtered (file,
> > +		    _("Debugger must step over hardware watchpoints is set to "
> > +		      "%s.\n"), value);
> > +}
> > +
> >  /* The set and show lists for 'set riscv' and 'show riscv' prefixes.  */
> >  
> >  static struct cmd_list_element *setriscvcmdlist = NULL;
> > @@ -2736,6 +2766,8 @@ riscv_gdbarch_init (struct gdbarch_info info,
> >    set_gdbarch_return_value (gdbarch, riscv_return_value);
> >    set_gdbarch_breakpoint_kind_from_pc (gdbarch, riscv_breakpoint_kind_from_pc);
> >    set_gdbarch_sw_breakpoint_from_kind (gdbarch, riscv_sw_breakpoint_from_kind);
> > +  set_gdbarch_have_nonsteppable_watchpoint (gdbarch,
> > +					    riscv_have_nonsteppable_watchpoint);
> >  
> >    /* Register architecture.  */
> >    set_gdbarch_num_regs (gdbarch, RISCV_LAST_REGNUM + 1);
> > @@ -2980,4 +3012,20 @@ can be used."),
> >  				show_use_compressed_breakpoints,
> >  				&setriscvcmdlist,
> >  				&showriscvcmdlist);
> > +
> > +  add_setshow_boolean_cmd ("have-nonsteppable-watchpoint", no_class,
> > +			   &riscv_have_nonsteppable_watchpoint,
> > +			   _("\
> > +Set whether debugger must step over hardware watchpoints"),
> > +			   _("\
> > +Show whether debugger must step over hardware watchpoints"),
> > +			   _("\
> > +The RISC-V debug spec recommends that hardware write watchpoints fire before\n\
> > +the write is committed, in which case, GDB must step over the watchpoint\n\
> > +before checking the old and new values.  Set this option to 'on' (default)\n\
> > +for targets that follow this behaviour, otherwise set to 'off'."),
> > +			   set_have_nonsteppable_watchpoint,
> > +			   show_have_nonsteppable_watchpoint,
> > +			   &setriscvcmdlist,
> > +			   &showriscvcmdlist);
> >  }
> > 
> 
> -- 
> Joel