Does anybody have an opinion here? Should I just submit the simple "fix"
into gerrit and have the discussion there?
Tim
On Thu, Sep 22, 2016 at 12:25 PM, Tim Newsome <t...@sifive.com> wrote:
> I ran into the following problem, debugging this:
>
> 8000036c <just_before_read_loop>:
> 8000036c: 01000393 li t2,16
>
> 80000370 <read_loop>:
> 80000370: 00052303 lw t1,0(a0)
>
> Set a software breakpoint on just_before_read_loop, and run to it. Then
> set a hardware watchpoint on the address in a0. Tell gdb to continue.
> Gdb clears breakpoints and single steps to get past the software
> breakpoint. Then it sets all breakpoints again, and tells the target to
> resume. The target halts at read_loop because the watchpoint is hit, and
> reports a trap at 0x80000370. gdb however, then forcibly writes back the pc
> to 0x8000036c. The code for this behavior in gdb is in
> adjust_pc_after_break() (infrun.c). I didn’t follow that code in detail,
> but it seems to come down to gdb decrementing the PC because it’s not sure
> whether the software breakpoint or the watchpoint was hit.
>
> If OpenOCD tells gdb that it can tell the difference between the two, then
> this code is bypassed and everything works as I expect. OpenOCD can do that
> by adding swbreak+ to the reply to qSupported as follows:
>
> --- a/src/server/gdb_server.c
> +++ b/src/server/gdb_server.c
> @@ -2357,7 +2357,7 @@ static int gdb_query_packet(struct connection
> *connection,
> &buffer,
> &pos,
> &size,
> -
> "PacketSize=%x;qXfer:memory-map:read%c;qXfer:features:read%c;QStartNoAckMode+",
> +
> "PacketSize=%x;qXfer:memory-map:read%c;qXfer:features:read%c;QStartNoAckMode+;swbreak+",
> (GDB_BUFFER_SIZE - 1),
> ((gdb_use_memory_map == 1) && (flash_get_bank_count()
> > 0)) ? '+' : '-',
> (gdb_target_desc_supported == 1) ? '+' : '-');
>
> My question is: is this safe for all targets? If it is, this is an easy
> change. If not, I’ll have to invent some mechanism for targets to let the
> gdb_server layer know what should be returned. If that’s required, are
> there any guidelines I should follow?
>
> My target is RISC-V, which I’m working on implementing OpenOCD support
> for. I imagine this same issue exists on other targets as well.
>
> Thank you,
> Tim
>
>
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