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

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

* Pedro Alves <palves@redhat.com> [2018-09-17 13:54:38 +0100]:

> On 09/16/2018 01:13 AM, Craig Blackmore wrote:
> > 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 1 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 1 to match the recommended behaviour. If a target does not follow
> > this timing, then 'set riscv have_nonsteppable_watchpoint 0' 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'
> > 	(riscv_gdbarch_init): initialise gdbarch setting for
> > 	have_nonesteppable_watchpoint
> 
> This is something the target/stub knows, right?  I'd be much
> better to make this automatic, so that users wouldn't have to
> know to tweak anything.

Sure, you're thinking something like (to pick one at random) how the
'org.gnu.gdb.arm.neon' feature on ARM in the target description tells
GDB how to operate, right?  I totally agree.

... but.... we'd still probably want to keep the flag around (though
as an auto/on/off maybe) so the user could, if they wanted, override a
badly behaving target...

....and.... there's no current remote description support for RiscV at
all, so having implement that as a prerequisite seems a little steep
(to me).

My preference would be to allow this in basically as is, then make it
automatic once we have target description support in place.

Alternatively we could remove the control switch for now, and just go
with:

  set_gdbarch_have_nonsteppable_watchpoint (gdbarch, 1);

for everyone.  But if there's anyone out there not following the
recommendation that makes things a little harder for them in the short
term.

What do you think?

Thanks,
Andrew


> 
> Thanks,
> Pedro Alves