[RFC] Removing in process agent (IPA)

[Andrew Burgess]

Luis Machado <luis.machado@arm.com> writes:

> On 6/14/24 14:48, Simon Marchi wrote:
>> 
>> 
>> On 2024-06-14 04:08, Andrew Burgess wrote:
>>> "Terekhov, Mikhail" <Mikhail.Terekhov@dell.com> writes:
>>>
>>>>> -----Original Message-----
>>>>> From: Luis Machado <luis.machado@arm.com>
>>>>>
>>>>> Similarly, I also think it might be time to consider dropping the tracepoints
>>>>> (mainly static and fast) machinery from gdbserver, as I suspect that is another
>>>>> mechanism that is not being used very often.
>>>>>
>>>> We use tracepoints in our stub from the last millennia I guess (back from EMC days)
>>>> and plan to do so in the future.
>>>>
>>>>> I recall trying to use tracepoints for practical purposes (back in the day), and it
>>>>> wasn't up to the task, being too cumbersome to use, having bad failure modes
>>>>
>>>> Putting some UI/GUI on top of it makes it relatively simple to use.
>>>>
>>>>> and having other security implications when trying to debug stuff in
>>>>> production platforms. The remote protocol side of it is OK, but again, I bet no
>>>>> debugging stubs are using that anymore.
>>>>
>>>> We do use them but not in production though.
>>>
>>> Just so I'm clear, you're talking about tracepoints here either in
>>> gdbserver, or at least (if you have your own stub) in GDB and the remote
>>> protocol, right?
>>>
>>> This doesn't require gdb to continue shipping the IPA right?  Or have I
>>> failed to understand your position?
>> 
>> Just to clarify one thing: the IPA is only needed for the fast
>> tracepoints mechanism, which allows recording trace data all in process,
>> without hitting a trap and the control going back to GDBserver.  We can
>> still use regular trap-based tracepoints without the IPA.
>> 
>> The possible levels of removal are:
>> 
>> 1. Remove the IPA, thus removing the support for fast tracepoints in
>>    GDBserver (you could still use regular trap-based tracepoints)
>> 2. Remove the support for tracepoints in GDBserver altogether (you could
>>    still use the tracepoints feature in GDB, when connecting to another
>>    RSP implementation than GDBserver, although the tracepoints feature
>>    in GDB would become untested code)
>> 3. Remove the support for tracepoints in GDB and GDBserver completely
>>    (which I would not consider since Mikhail says they use that).
>
> To be clear, I'm only proposing removing (1) at the moment. (2) might be worth
> removing in the future if nobody is still using it (drops tracepoint support in
> gdbserver). (3) is the more generic layer, and I suppose there are still a few
> users out there.

While there are users of (3) I would suggest we should keep (2) so that
we (GDB project) can test (3).

My initial question was only about (1), dropping IPA and everything that
implies.  Again, I have no strong feelings either way, I asked only
because the idea was floated in some replies in other threads and I
thought the question should be asked in its own thread.

Thanks,
Andrew