Re: Implementation of different software breakpoint kinds in gdb server

[Pedro Alves <palves at redhat dot com>]

On 10/18/2012 11:28 AM, Yao Qi wrote:
> On 10/18/2012 06:08 PM, Pedro Alves wrote:
>> Offhand, the main issues with tracepoints on ARM are:
>>
>> #1 - gdbserver needs to know how to step over the tracepoints, without gdb's intervention.
>>
>> #2 - ARM can't hw single-step, so that needs to be done the hard way, with breakpoints
>>      (a.k.a., software single-step).
>>      All the logic to do that is in gdb.  This conflicts with #1.
>>
>> So we'd need to teach gdbserver to software single-step.  Maybe it's possible
>> to tell offline all the possible destinations of an instruction, so we could still
>> leave that logic in gdb, but I suspect not.
> 
> I am wondering that it might be relatively easier to implement fast tracepoint in which step-over is not needed, if finding a jumppad is not a problem :)

Maybe.  :-)

The "move out of jumppad" logic uses single-stepping.  I guess we could come up
with something clever to avoid it.  Related, linux_fast_tracepoint_collecting logic
assumes it can identify a thread from its tls base / thread area.  And this assumes
you can retrieve this address without an infcall.  This is possible on x86.  Don't know
about ARM (well, in an ABI-stable way).

-- 
Pedro Alves