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

Pedro Alves
2023-02-06T20:26:56Z

On 2023-02-02 2:54 a.m., Thiago Jung Bauermann via Gdb-patches 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.
>
>>> 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.

Please drop the "get_" prefix from the class method, it doesn't really
add value, and we typically don't add it. Most GDB getter/setters are
in foo() / set_foo() pair style, rather than get_foo() / set_foo().

A "get_" prefix is however typically used for global getter functions.

FWIW, I have the same thought as Simon while reading this.

>
> 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.
>
>