RFC: Debugging interface (gdb)

Chris Jones jones.chris.g at gmail.com
Wed May 22 22:09:13 PDT 2013


On Wed, May 22, 2013 at 7:51 PM, Robert O'Callahan <robert at ocallahan.org>wrote:

> I'm pretty sure ptrace stepi will update the instructions-retired counter
> correctly. We'll see...
>

I'm uneasy about several interactions, but I share your "pretty sure"
sentiment ;).


> Handling the case where a signal needs to be delivered at that instruction
> sounds tricky.
>

AFAICT this is the same problem as knowing to context-switch at insn i
while stepping.


> No idea what to do for breakpoints. Theoretically, inserting int3
> breakpoints could cause divergence. Probably doesn't matter. Same is true
> for debug registers I suppose, but matters even less. I guess just using
> int3s is simple and gives you unlimited breakpoints --- and we don't have
> to worry about threads racing.
>

Yeah, I lean toward SW-only breakpoints too.  I haven't thought of
anything problematic yet, but I also haven't thought very hard yet.


> In my mind I would like gdb support to be good enough to be useful, but
> rather than making it really great it might be better to invest in stuff
> like checkpointing and then Chronicle/Chronomancer style
> recording/debugging of segments of the trace. My ultimate goal is to have
> something a lot better than anything gdb (even with reverse execution) can
> provide :-).
>

Yep, agreed again.  Before we can really do reverse-* anything, gdb or
chronicle/chronomancer, we're going to need some fancy new technology like
checkpointing.  So we can't really think of those things before the new
tech.  Down the road though, I could see gdb reverse-continue playing
really well with something like chronicle/chronomancer, to quickly zoom in
on the segment you want to hyper-record.  (Need a better name than that
;).)

Cheers,
Chris



>
> Rob
> --
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