[ltt-dev] gdb-7.2 can't build with lttng-ust-0.12

Mathieu Desnoyers compudj@krystal.dyndns.org
Tue Apr 26 21:42:00 GMT 2011

* Tom Tromey (tromey@redhat.com) wrote:
> >>>>> "Dexuan" == Cui, Dexuan <dexuan.cui@intel.com> writes:
> Mathieu> Also, we should reopen the discussion on the way the UST Markers
> Mathieu> collect the registers for GDB, because the current way involves a
> Mathieu> _lot_ of ugly assembly code. It should be possible to only use a
> Mathieu> volatile inline asm to specify input constraints on the target marker
> Mathieu> parameters, and keep the instruction pointer address that corresponds
> Mathieu> to this inline asm in a section known by gdb (so gdb could use the
> Mathieu> drawf info to fetch data from registers/memory). If you can ensure
> Mathieu> that this would fit gdb's requirements, I could clean up the marker
> Mathieu> code and we could resync the APIs together. We could also provide
> Mathieu> this for UST Tracepoints in the same go, with pretty much the same
> Mathieu> interface as we'd use for UST Markers. I am aware that this would
> Mathieu> require change on the GDB side, but I think it's better to
> Mathieu> synchronise our effort rather than to shoot at different targets.
> I am not totally clear on the proposal here.
> If this means reusing the <sys/sdt.h> stuff, then there are gdb patches
> already submitted to fully support that; and updating the existing code
> to also use it should not be very difficult.  See gdb-patches from the
> last month or two.

Yes, the goal is to rely on sdt.h as soon as it supports a e.g.
STAP_PROBEV (with variable number of args). The Josh Stone found a neat
trick to do it, and I think he is preparing an updated version.

The only concern here is the packaging: if we depend on sdt.h, we have
to either ship it with UST, or have one more dependency on a package
available for all architectures that only contain this header. Thoughts ?

It's good to hear that GDB is already planning on supporting that.



> If instead you mean something else, it would be useful to have more
> information.  From your description it sounds like this may be a
> different design from SDT v3; but note that SDT v2 tried to use DWARF to
> access the parameters and ran into various problems.  I can get details
> on the failure modes if this is what you intend.
> Dexuan> Unluckily I'm pretty new to gdb and know few about this now.
> Dexuan> Let me Cc the gdb mailing list for more thoughts(hope this cross-posting
> Dexuan> wouldn't bother people).
> Cross-posting in cases like this is fine, even preferable.
> I don't know anything about the gdbserver bits here.  It seems to me
> that it would be reasonable to have gdbserver compile against various
> versions of UST.
> Tom

Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.

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