RISC-V: decr_pc_after_break causing problems

[Palmer Dabbelt]

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.