Re: [PATCH v3 4/8] gdbserver/linux-aarch64: When thread stops, update its target description

Thiago Jung Bauermann via Gdb-patches
2023-02-03T03:47:59Z

Simon Marchi <simark@simark.ca> writes:

> On 2/1/23 21:54, Thiago Jung Bauermann wrote:
>>
>> Luis Machado <luis.machado@arm.com> writes:
>>
>>> On 2/1/23 16:21, Simon Marchi wrote:
>>>>
>>>>>> diff --git a/gdbserver/linux-low.h b/gdbserver/linux-low.h
>>>>>> index 221de85aa2ee..b52eb23cc444 100644
>>>>>> --- a/gdbserver/linux-low.h
>>>>>> +++ b/gdbserver/linux-low.h
>>>>>> @@ -604,6 +604,12 @@ class linux_process_target : public process_stratum_target
>>>>>> /* Architecture-specific setup for the current thread. */
>>>>>> virtual void low_arch_setup () = 0;
>>>>>> + /* Allows arch-specific code to set the thread's target description when the
>>>>>> + inferior stops. Returns nullptr if no thread-specific target description
>>>>>> + is necessary. */
>>>>>> + virtual const struct target_desc *
>>>>>> + get_thread_tdesc (const thread_info *thread);
>>>>>
>>>>> I think the comment for this function is not correct. The function does
>>>>> not SET the thread's target description, but just GETS a target
>>>>> description suitable for `thread`. It's the caller's job to do the
>>>>> setting.
>>>> This comment also gave me pause. How does a getter set something. I
>>>> then understood that it allowed the arch-specific code to provide a
>>>> thread-specific tdesc. I would suggest just:
>>>
>>> FWIW, I read it as "the functions *allows* arch-specific code to set".
>>> So it doesn't set on its own, but it does allow something else to do
>>> it.
>>
>> Yes, that's what was in my mind when I wrote the comment. But I agree
>> it's unclear, and I adopted Simon's suggested version.
>>
>>>> The other thought I had while re-reading the patch is why do we need to
>>>> return and store nullptr if the thread target description is the same as
>>>> the main one for the process. get_thread_tdesc could just return
>>>> process_info->tdesc if we don't need a separate tdesc, and we would
>>>> store that same pointer in thread_info->tdesc.
>>
>> We don't need to return and store nullptr if the thread target
>> description is the same as the main one for the process. Things will
>> work fine if we do as you suggest. IIRC my private branch worked liked
>> that for a while, before I changed it to the current version.
>>
>> I changed it because I thought it was a clearer mental model if
>> thread_info->tdesc is nullptr when there's not thread-specific target
>> description. I can make the get_thread_tdesc method always return a
>> valid target description if you think it's better that way.
>
> Either way works.

Ok, if there's no preference I suggest leaving it as it is now (unless
we decide moving away from process_info->tdesc).

>>>> And get_thread_tdesc would just return that (in fact,
>>>> get_thread_tdesc might not be necessary then). Perhaps it makes some
>>>> things more complicated down the road, but I can't think of anything.
>>
>> Sorry, I don't understand this part. get_thread_tdesc is necessary
>> because it's the hook that allows arch-specific code to provide a target
>> description for the thread. I don't see how it can become unnecessary.
>>
>> Perhaps you mean the get_thread_target_desc function? Sorry about the
>> names being so similar, I spent some time trying to think of a better
>> name for either the method or the function but failed.
>
> Err yeah, I meant the free function that returns the process' tdesc if
> the thread doesn't have one.
>
>> In any case, it wouldn't be possible to make get_thread_target_desc just
>> return thread_info->tdesc because at least the way these patches are
>> currently written, when the inferior starts or a new thread of the
>> inferior is spawned thread_info->tdesc is nullptr. gdbserver will only
>> call get_thread_tdesc after the first stop (in get_thread_regcache, in
>> the process of obtaining the pc register), so we will need to cope with
>> that situation.
>
> Ok. Would it work if a new thread initially inherited the tdesc from
> its process?

Yes, it would. Version 1 of these patches didn't work exactly like that
because I removed process_info->tdesc, but the new thread inherited from
the parent thread's tdesc. This happened in
linux_process_target::handle_extended_wait where the clone and fork
events are handled.

>>> Sounds reasonable.
>>>
>>> Moving towards thread-specific target descriptions/gdbarch would be a positive thing
>>> given
>>> the SVE precedent. The process-wide target description/gdbarch no
>>> longer reflects the correct settings for each thread on AArch64's with SVE support.
>>
>> In the first version of these patches I removed the process-wide target
>> description and moved it to thread_info, but it was a big patch that
>> touched many targets. I can bring it back if you think it's worth it.
>
> At least for the register description, if we decide it's now a
> per-thread thing, what does it mean to have a process-wide description
> anyway? I think it would make sense to get rid of it, that could help
> confirm our model works (and it would remove the chance of using the
> wrong tdesc by mistake for a thread that has its own tdesc). It's
> probably something that can be done incrementally though.

Would it be done incrementally by keeping process_info->tdesc but
changing each target in turn to use thread_info->tdesc and ignore the
process_info one?

--
Thiago