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Gmane

From: Colin Riley <colin <at> codeplay.com>
Subject: Re: ARM AARCH64 and AARCH32 in the same debugger
Newsgroups: gmane.comp.debugging.lldb.devel
Date: Tuesday 13th January 2015 09:41:10 UTC (over 3 years ago)
One of the issues here is the ability to capture the context of the 
current debugging situation, and it depends on a whole lot of variables.

With JTAG gdbservers it may not care that much about the difference in 
programs at the developer/process/OS level. It may simply provide static 
single thread per core. There is not enough information here, but, in 
this instance, it can be represented with a single target to be seen by 
LLDB. LLDB will have to do the heavy lifting at this point to switch 
archs and locate 'real' thread information which we have done in the 
past with success. In this mode, the user at least gets a single 
coherent view of what is being debugged. Single stepping from one 'arch' 
to another can be possible seamlessly, given a good implementation.

The other situation, which is mentioned before, uses multiple targets. 
This works correctly if you have multiple gdbservers for the separate 
bits of what is being debugged, although, from a developer standpoint, 
things get complicated:

1) There is no 'high level view' of the current debugging situation at 
any present time.
2) There is no cooperation between the targets, _even_ if they share 
resources like memory.
3) Single stepping from one target to the other is not seamless.
4) Events can be missed from one target whilst debugging the other.
5) goes on..

This is why last year I suggest we start preparing for some sort of 
higher-level platform which can manage multiple targets in a way which 
makes sense for a given architecture/soc/system/whatever. It can 
understand memory can be shared between targets, and therefore 
watchpoints can trigger events on different platforms. It can understand 
in a single stepping situation there may be a need to switch targets 
given a set of circumstances to allow for a seamless experience to 
developers.

At this point, whilst multiple target debugging works, it's value can be 
so much more. Especially given the targets LLDB is tending to support; 
parts of larger SoCs.

Colin



On 13/01/2015 00:45, Jason Molenda wrote:
> FWIW we were talking about this a little bit.  Assuming I'm correct that
you have one communication channel to the JTAG probe (one gdb-remote
connection), when you step from aarch32 user process code to aarch64
hypervisor code it's not just an architecture change -- we're changing to
an entirely different program (kernel versus user code).
>
> This genuinely does need to be represented in lldb as two separate
Targets that coexist simultaneously.  Only one of them will be executing at
any given time - so there would need to be some way for lldb to hand off
from userland Target to kernel Target.  That's definitely something new.
>
> If there are separate probes/gdb-remote communication channels for these
-- maybe the kernel is running on one core and the user process on another
-- then it's easy.
>
>
> The EFI example I mentioned earlier where a processor changes its mode
(16/32, then 64 bit) as it starts up is not a good comparison.  In that
case a single program is being debugged through the entire transition -
it's just the architecture that has changed mid-debug-session.
>
>
> J
>
>
>> On Jan 12, 2015, at 2:47 PM, Jason Molenda  wrote:
>>
>> I'm guessing the JTAG probe vends a single gdb-remote connection that is
either seeing the user-process aarch32 or the hypervisor aarch64 at any
given time.  So you're stepping along in aarch32 mode and then it syscalls
out to aarch64, all within a single Target.
>>
>> I think this is akin to the folks doing the low-level startup routines
on x86 machines - it changes its architecture from 32-bit to 64-bit as it
brings up the system.  From lldb's point of view this is a single Target
that changes architecture during the debug session.
>>
>>
>>> On Jan 12, 2015, at 2:36 PM, Greg Clayton  wrote:
>>>
>>> Yes you can: you can have one target for the 32 bit ARM and one for the
64 bit one. You currently can't have one target (process) be both 32 and 64
bit, but you can do this with two targets. I am guessing you would have one
target that is debugging the user space process and one for the 64 bit
hypervisor routine which I am assuming is the kernel?
>>>
>>> This is exactly why we developed LLDB with a modular plug-in based
architecture.
>>>
>>> Greg
>>>
>>>> On Jan 12, 2015, at 2:07 PM, Ted Woodward
 wrote:
>>>>
>>>> Here’s a scenario I’m being asked about:
>>>>
>>>> A CPU is executing in AARCH32. LLDB is talking to a JTAG probe via gdb
remote. The user stops the program, and it’s in an Android user space
application. Can we single step through it, and then an AARCH64 Hypervisor
routine, in the same LLDB instance? What would we need to set up to do
this?
>>>>
>>>> --
>>>> Qualcomm Innovation Center, Inc.
>>>> The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project
>>>>
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-- 
- Colin Riley
Games Technology Director

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