[lldb-dev] Simultaneous multiple target debugging

[Colin Riley]

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 
> https://sourceware.org/gdb/current/onlinedocs/gdb/Non_002dStop-Mode.html 
> - 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
>

-- 
- Colin Riley
Games Technology Director

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