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

[Luis Machado <luis.machado@arm.com>]

On 2/3/23 03:47, Thiago Jung Bauermann wrote:
> 
> 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?
> 

Given only AArch64 has this requirement at the moment, it may be easier to leave the process-wide tdesc up until the point we can safely/easily switch to thread-specific ones.

After that, AArch64 will have potentially distinct thread tdescs while other architectures will have threads sharing the same tdesc.