[lldb-dev] GDB RSPs non-stop mode capability in v5.0

Ramana via lldb-dev lldb-dev at lists.llvm.org
Mon Apr 2 07:00:06 PDT 2018

On Thu, Mar 29, 2018 at 11:37 PM, Jim Ingham <jingham at apple.com> wrote:

> > On Mar 29, 2018, at 10:40 AM, Greg Clayton via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >
> >
> >
> >> On Mar 29, 2018, at 10:36 AM, Frédéric Riss <friss at apple.com> wrote:
> >>
> >>
> >>
> >>> On Mar 29, 2018, at 9:27 AM, Greg Clayton <clayborg at gmail.com> wrote:
> >>>
> >>>
> >>>
> >>>> On Mar 29, 2018, at 9:10 AM, Frédéric Riss <friss at apple.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>>> On Mar 29, 2018, at 7:32 AM, Greg Clayton via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>>> On Mar 29, 2018, at 2:08 AM, Ramana via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >>>>>>
> >>>>>> Hi,
> >>>>>>
> >>>>>> It appears that the lldb-server, as of v5.0, did not implement the
> GDB RSPs non-stop mode (https://sourceware.org/gdb/
> onlinedocs/gdb/Remote-Non_002dStop.html#Remote-Non_002dStop). Am I wrong?
> >>>>>>
> >>>>>> If the support is actually not there, what needs to be changed to
> enable the same in lldb-server?
> >>>>>
> >>>>> As Pavel said, adding support into lldb-server will be easy. Adding
> support to LLDB will be harder. One downside of enabling this mode will be
> a performance loss in the GDB remote packet transfer. Why? IIRC this mode
> requires a read thread where one thread is always reading packets and
> putting them into a packet buffer. Threads that want to send a packet an
> get a reply must not send the packet then use a condition variable + mutex
> to wait for the response. This threading overhead really slows down the
> packet transfers. Currently we have a mutex on the GDB remote communication
> where each thread that needs to send a packet will take the mutex and then
> send the packet and wait for the response on the same thread. I know the
> performance differences are large on MacOS, not sure how they are on other
> systems. If you do end up enabling this, please run the "process plugin
> packet speed-test" command which is available only when debugging with
> ProcessGDBRemote. It will send an receive various packets of various sizes
> and report speed statistics back to you.
> >>>>>>
> >>>>>> Also, in lldb at least I see some code relevant to non-stop mode,
> but is non-stop mode fully implemented in lldb or there is only partial
> support?
> >>>>>
> >>>>> Everything in LLDB right now assumes a process centric debugging
> model where when one thread stops all threads are stopped. There will be
> quite a large amount of changes needed for a thread centric model. The
> biggest issue I know about is breakpoints. Any time you need to step over a
> breakpoint, you must stop all threads, disable the breakpoint, single step
> the thread and re-enable the breakpoint, then start all threads again. So
> even the thread centric model would need to start and stop all threads many
> times.
> >>>>
> >>>> If we work on this, that’s not the way we should approach breakpoints
> in non-stop mode (and it’s not how GDB does it). I’m not sure why Ramana is
> interested in it, but I think one of the main motivations to add it to GDB
> was systems where stopping all some threads for even a small amount of time
> would just break things. You want a way to step over breakpoints without
> disrupting the other threads.
> >>>>
> >>>> Instead of removing the breakpoint, you can just teach the debugger
> to execute the code that has been patched in a different context. You can
> either move the code someplace else and execute it there or emulate it.
> Sometimes you’ll need to patch it if it is PC-relative. IIRC, GDB calls
> this displaced stepping. It’s relatively simple and works great.
> >>>
> >>> This indeed is one of the changes we would need to do for non-stop
> mode. We have the EmulateInstruction class in LLDB that is designed just
> for this kind of thing. You can give the emulator function a read/write
> memory and read/write register callbacks and a baton and it can execute the
> instruction and read/write memory and regisrters as needed through the
> context. It would be very easy to have the read register callback know to
> take the PC of the original instruction and return it if the PC is
> requested.
> >>>
> >>> We always got push back in the past about adding full instruction
> emulation support as Chris Lattner wanted it to exist in LLVM in the
> tablegen tables, but no one ever got around to doing that part. So we added
> prologue instruction parsing and any instructions that can modify the PC
> (for single stepping) to the supported emulated instructions.
> >>>
> >>> So yes, emulating instructions without removing them from the code is
> one of the things required for this feature. Not impossible, just very time
> consuming to be able to emulate every instruction out of place. I would
> _love_ to see that go in and would be happy to review patches for anyone
> wanting to take this on. Though the question still remains: does this
> happen in LLVM or in LLDB. Emulating instruction in LLVM might provide some
> great testing that could happen in the LLVM layers.
> >>
> >> In my porting experience, emulation is actually rarely needed. Of
> course, if LLVM had a readily available emulation library we could just use
> that, but it’s not the case. Most of the time, just copying the instruction
> to some scratch space and executing it there is enough (you potentially
> need to patch offsets if the instruction uses PC-relative addressing).
> >>
> >
> > That is true but that involves starting and stopping the thread one time
> which can be time consuming. It is easier to do it this way, but the
> starting and stopping of a thread is very costly. It would be better to try
> and emulate all the instructions we can and then fall back to emulating the
> instruction at another address if needed. Of course, you might be able to
> emulate the instruction and have a branch that branches to the next real
> instruction so we just have to start the process again without having to
> stop it. That would be a nice approach.
> The really cool trick would be to insert the breakpoint as a branch to a
> landing pad we insert.  Then the landing pad could look like:
> if (thread_is_supposed_to_stop_at_breakpoints() && breakpoint_condition())
>   __builtin_trap();
> emulate_instructions_you_needed_to_remove();
> jump_back_to_next_instruction();
> Then we could support some threads that NEVER stop, and also run
> breakpoint conditions locally which would make them really cheap.  You
> could even squirrel away some state in the target that told you that a
> NEVER stop breakpoint hit the breakpoint, so your accounting would still be
> correct.

In fact, we also do something similar on these lines and it is pretty
straight forward.

> >
> >> Fred
> >>
> >>>>
> >>>> I’ve been interested in displaced stepping for different reasons. If
> we had that capability, it would become much easier to patch code. I’d love
> to use this to have breakpoint conditions injected and evaluated without
> round tripping to the debugger when the condition returns false.
> >>>
> >>> Agreed!
> >>>
> >>>>
> >>>> Fred
> >>>>
> >>>>> Be sure to speak with myself, Jim Ingham and Pavel in depth before
> undertaking this task as there will be many changes required.
> >>>>>
> >>>>> Greg
> >>>>>>
> >>>>>> Thanks,
> >>>>>> Ramana
> >>>>>>
> >>>>>>
> >>>>>> _______________________________________________
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> >>>>>> lldb-dev at lists.llvm.org
> >>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
> >>>>>
> >>>>> _______________________________________________
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> >
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