RISC-V: decr_pc_after_break causing problems

[John Baldwin]

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.

-- 
John Baldwin