[lldb-dev] Simultaneous multiple target debugging
colin at codeplay.com
Tue Aug 26 09:02:44 PDT 2014
In fact, would any folk be interested in a BOF session on this topic at
the next meeting?
On 26/08/2014 16:47, Colin Riley wrote:
> Has anybody done any work on integrating features into LLDB to allow
> for 'meaningful' simultaneous multiple target debugging? There are
> various scenarios in which this is a very valuable feature:
> 1) coprocessor debugging, in single-process systems (i.e, embedded DSP
> alongside say a host CPU core)
> 2) graphical debugging, e.g. games: ideally you want to be able to
> debug the CPU code alongside any GPU workgroups, and have a single
> interface to any shared resources such as memory.
> We've done work like this in the past to LLDB, it's not been
> contributed back because we couldn't do so for commercial reasons (and
> it's not in a state to contribute back, either). However in the future
> I think this will become a 'killer app' feature for LLDB and we should
> be planning to support it.
> At the moment we can have multiple targets, processes etc running in
> an LLDB session. However I am failing to see any system for
> communication and interpretation of multiple targets as a whole. If we
> take the DSP/CPU situation, I may be watching a CPU memory location
> whilst at the same time single-stepping through the DSP. It's
> currently undefined and a bit unknown as to how this situation would
> work in LLDB as stands. From what I can see, it's quite hard to use
> the current independent target framework to achieve a meaningful
> debugging session.
> It's as though we'd want some sort of session object, that can take
> multiple targets together and understand how they operate as to
> achieve some sort of well-defined behaviour in how it's debugged. I.e,
> in the DSP/CPU scenario, the session object would understand the DSP
> has access to the CPU memory, and as such, if we're currently on the
> DSP single stepping, it would allow a CPU watchpoint event through to
> the DSP session, with an ability to switch target.
> There are many more items we'd need to allow communication between. A
> quick example, we have an LLDB version here that supports non-stop
> mode debugging (see
> - and we _will_ contribute this back). At the moment stepping through
> one thread and a breakpoint happens in another is a bit nasty: LLDB
> simply switches to whatever thread id is greater. When this sort of
> usability issue exists in a single-target fashion, we may need to look
> at extracting this out into some sort of policy system that targets
> (and, these theoretical session objects) can use to decide how to
> handle certain event situations.
> Apologies if this is a bit of a brain dump. It's quite a complex
> concept, which is why I think dialogue needs to start now as it's
> something as I've mentioned we are actively doing at Codeplay, but
> when the time comes to push upstream, want to do so in a way the
> community thinks is valuable. There may be other viewpoints, like
> 'super debugservers' that can manage multiple targets and spoof a
> single target to LLDB, for example.
> Any other opinions or thoughts out there? :)
- Colin Riley
Games Technology Director
Codeplay Software Ltd
45 York Place, Edinburgh, EH1 3HP
Tel: 0131 466 0503
Fax: 0131 557 6600
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