Hi Colin,
Multiple target debugging is a massive interest to us at CSR. We design
chips with various processor types (e.g. kalimba, XAP, 8051, ARM etc)
and on several of our chips we have multiple-processors. There are lots
of combinations of setups that we have either already done, or are
actively experimenting on. Generally, we have heterogenous setups (e.g.
XAP+8051, or 4*XAP+kalimba+8051) etc.
I see that lldb already supports the concept of a target list, an active
target and manual switching between current targets. However, as Colin
alludes, there are several features associated with multiple-target
which require control from a higher-level.
What we currently have in our existing debuggers is options of the form,
"I'm debugging targets A and B, if A stops do I want B stop as well?".
The answer to that question is very much specific to that user's current
debug scenario. Of course, getting B to stop if A does, is best
implemented in the hardware, and typically a register will be available
as a mechanism to configure this feature. In our (CSRs) world probably
one of the processors will have access to the associated hardware block,
and our debugger will talk to this target to access the feature.
So, of course, if non-active target(s) stops whilst stepping/running the
active one, some notification needs to be passed up, informing the debug
session controller of this, and determining whether or not to switch
active target.
Greg and Jim both mentioned using the Platform class as the place to
implement this kind of thing. However, does the Platform not only deal
in homogenous entities? Is it correct to use this concept to control
different processor families. With my limited lldb architectural
knowledge, I would have thought that the most likely candidate to
control this is the Debugger object itself.
Matt
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
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