[lldb-dev] Simultaneous multiple target debugging
Colin Riley
colin at codeplay.com
Thu Aug 28 08:30:59 PDT 2014
Yeah, I've been thinking about the different streams you'd need. A
difficult one is a call stack on a platform, with frames on different
ABIs/Targets representing cross-architecture calls.
If anyone here has ever used the Cell PPU/SPU gdb, which had this
feature, I think you'll agree it's absolute gold in terms of value.
Sadly I can't attend this years meeting, but I'll write a BOF proposal
and send it in anyway with one of the other Codeplayers there to host.
Colin
On 28/08/2014 15:00, Todd Fiala wrote:
> It might be nice to mock up just the debugger command streams we think
> are needed/wanted to handle several common usages of the heterogeneous
> processor debugging scenarios on this thread before putting any code
> behind it. That way we can talk through it a bit with concrete
> examples to further illuminate the kinds of changes/support we'll need.
>
>
> On Wed, Aug 27, 2014 at 11:21 PM, Matthew Gardiner <mg11 at csr.com
> <mailto:mg11 at csr.com>> wrote:
>
> Yes, a "device" abstraction seems to be the correct controlling
> entity. In fact, from an embedded debugging perspective it is
> __the__ logical entity which groups "debuggable opportunities"
> together. However, when Jim mentions "this coordinating entity
> should not be restricted to different devices " and alludes to
> control over different targets (which are in __some_way__
> associated in the debugging user's mind), but may be running on
> different machines etc.; then I think that conceptually we are
> still talking about the same thing, but the name "device" then
> becomes questionable. I can only really think of something a tad
> wooly like "DebugScenario", "DebugSession" or
> "DeploymentScenario"... :-(
>
> Yes, the Platform should remain just being the Platform.
>
> Regarding "debuggable opportunities" - solely these are just the
> "Target" objects that we already have? (In fact Colin's original
> post does in fact just state "that can take multiple targets
> together and understand how they operate...").
>
> What's really tricky, I think, is how to make the device/scenario
> controlling entity look very generic on the outside, but within be
> able to coordinate very target specific activities. It seems that
> the debuggable_oppurtunity/target would require some way of
> communicating the kind of multi-target features it can support e.g.
>
> CanStopOthers
> CanBeUnselectedAsActive
> ...and so on...
>
> Interested to see how things pan out.
>
> Matt
>
>
> jingham at apple.com <mailto:jingham at apple.com> wrote:
>
> Right, it seems to me clear that you need two entities.
>
> One knows what targets can be created in a given debugging
> scenario, and how to hook up to them. Then you need another
> to manage picking some subset of these targets, and
> coordinating the processes running in each of them.
>
> The Platform seemed the logical place to do the first job.
> However, Matthew is right that at present the Platforms are
> homogenous, and more deal with OS details. So maybe it would
> be better to keep the Platform more about OS details, then we
> could add a "device" abstraction that represents composite
> entities with multiple debuggable opportunities, and then each
> of these "debuggable opportunities" would have a Platform to
> represent the OS like features of this opportunity (need some
> good word for this.) That might be a better way to go. Note
> that the "debuggable opportunities" are more general than just
> different devices on a board. For instance, you could imagine
> debugging the kernel, and a user-space process running on that
> OS, and coordinating those just as you would a main processor
> and a co-processor... To make matters a little confusing, a
> "device" might represent all the processes running on a single
> OS, since that's not formally different from the more
> straightforward device scenario. So in some ways a platform
> IS also a device in this sense. Maybe the abstraction is more
> a target provider, and the Platform is a homogenous target
> provider, in addition to its OS duties, and a device is a
> heterogenous target provider?
>
> But in either case, once you've chosen to attach to several
> debug sockets, there's the whole business that Matthew
> mentions of coordinating the sessions. That is clearly a
> whole different kettle of fish from just "what can I attach to".
>
> BTW, this coordinating entity should not be restricted to
> different devices. At that level, of course, it is really
> about coordinating targets & their process regardless of where
> they come from. For instance you'd want to be able to use the
> same structure to coordinate debugging message passing or
> socket traffic, etc on two user space processes on the same or
> different systems. It would also be interesting to model this
> coordination in a way that could also be extended to threads
> in a single process. Right now, each thread's behavior is
> programmed using the ThreadPlans which work only on a per
> thread basis and don't make any attempt to coordinate
> threads. But it would be useful (and more so when you start
> doing keep alive debugging) to have some way to program "when
> thread A does X, wait for thread B to do Y..." That isn't
> formally different from two processes or several
> co-processors. Be interesting to see how much of the
> coordination we could make very general.
>
> Jim
>
>
>
> On Aug 27, 2014, at 7:55 AM, Todd Fiala <tfiala at google.com
> <mailto:tfiala at google.com>> wrote:
>
> Greg and Jim both mentioned using the Platform class
> as the place to implement this kind of thing.
>
> I think Jim later mentioned a higher-level concept is
> needed to do some of the orchestration that we'd want to
> enable, IIRC.
>
>
> On Tue, Aug 26, 2014 at 11:18 PM, Matthew Gardiner
> <mg11 at csr.com <mailto:mg11 at csr.com>> wrote:
> 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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- Colin Riley
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