[lldb-dev] Simultaneous multiple target debugging

Colin Riley 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 
> 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

Codeplay Software Ltd
45 York Place, Edinburgh, EH1 3HP
Tel: 0131 466 0503
Fax: 0131 557 6600
Website: http://www.codeplay.com
Twitter: https://twitter.com/codeplaysoft

This email and any attachments may contain confidential and /or privileged information and is for use by the addressee only. If you are not the intended recipient, please notify Codeplay Software Ltd immediately and delete the message from your computer. You may not copy or forward it,or use or disclose its contents to any other person. Any views or other information in this message which do not relate to our business are not authorized by Codeplay software Ltd, nor does this message form part of any contract unless so stated.
As internet communications are capable of data corruption Codeplay Software Ltd does not accept any responsibility for any changes made to this message after it was sent. Please note that Codeplay Software Ltd does not accept any liability or responsibility for viruses and it is your responsibility to scan any attachments.
Company registered in England and Wales, number: 04567874
Registered office: 81 Linkfield Street, Redhill RH1 6BY

More information about the lldb-dev mailing list