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Re: RISC-V: decr_pc_after_break causing problems


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.


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