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Re: RISC-V: decr_pc_after_break causing problems
- From: John Baldwin <jhb at FreeBSD dot org>
- To: Palmer Dabbelt <palmer at sifive dot com>, Jim Wilson <jimw at sifive dot com>, andrew dot burgess at embecosm dot com
- Cc: gdb at sourceware dot org
- Date: Thu, 5 Jul 2018 15:54:32 -0700
- Subject: Re: RISC-V: decr_pc_after_break causing problems
- References: <mhng-c2172dd6-d7fb-4df6-87d4-a180afd408e3@palmer-si-x1c4>
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