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

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

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.

>> 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.

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.

> 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.


-- 
Thiago