Re: RISC-V: decr_pc_after_break causing problems

[Andrew Burgess <andrew dot burgess at embecosm dot com>]

* John Baldwin <jhb@FreeBSD.org> [2018-07-05 15:54:32 -0700]:

> On 7/3/18 5:35 PM, Palmer Dabbelt wrote:
> > On Tue, 03 Jul 2018 17:17:04 PDT (-0700), Jim Wilson wrote:
> >> On Mon, Jun 25, 2018 at 7:54 PM, Jim Wilson <jimw@sifive.com> wrote:
> >>> The RISC-V port in the riscv-tdep.c file has
> >>>   set_gdbarch_decr_pc_after_break (gdbarch, (has_compressed_isa ? 2 : 4));
> >>
> >> I'm still hoping to get a response to this.  I need to make
> >> coordinated fixes to both gdb and the linux kernel to get breakpoints
> >> working correctly.
> > 
> > Andrew: I think this materialized itself when you submitted the GDB patches, 
> > probably because we have this in our Linux code:
> > 
> >     asmlinkage void do_trap_break(struct pt_regs *regs)
> >     {
> >     #ifdef CONFIG_GENERIC_BUG
> >             if (!user_mode(regs)) {
> >                     enum bug_trap_type type;
> >     
> >                     type = report_bug(regs->sepc, regs);
> >                     switch (type) {
> >                     case BUG_TRAP_TYPE_NONE:
> >                             break;
> >                     case BUG_TRAP_TYPE_WARN:
> >                             regs->sepc += sizeof(bug_insn_t);
> >                             return;
> >                     case BUG_TRAP_TYPE_BUG:
> >                             die(regs, "Kernel BUG");
> >                     }
> >             }
> >     #endif /* CONFIG_GENERIC_BUG */
> >     
> >             force_sig_fault(SIGTRAP, TRAP_BRKPT, (void __user *)(regs->sepc), current);
> >             regs->sepc += 0x4;
> >     }
> > 
> > There's at least one bug in the Linux port here: we can enter a breakpoint trap 
> > via either ebreak (a 4-byte instruction) or c.ebreak (a 2-byte instruction).  
> > c.ebreak is necessary for a sane debugger so we need to support it.  Our 
> > options are:
> > 
> > * Handle c.ebreak in Linux and leave this as it stands.
> > * Remove both the Linux PC adjustment and the GDB PC adjustment.
> > 
> > I'm inclined to take the second option as it's less code.  I suppose 
> > technically it's an ABI break, but since it's broken anyway then I'm happy with 
> > taking it.
> > 
> > Is there something I'm missing?  If not Jim will submit a Linux patch and then 
> > we'll pull the trigger on this one.
> 
> FWIW, my preference would be for the decr_after_pc_break match the hardware
> which from my understanding of the thread means it should always be
> zero.

I agree this solution is the correct approach and GDB should be
changed.

Thanks,
Andrew