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From: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
To: Simon Marchi <simark@simark.ca>
Cc: Luis Machado <luis.machado@arm.com>,
	Andrew Burgess <aburgess@redhat.com>,
	Thiago Jung Bauermann via Gdb-patches
	<gdb-patches@sourceware.org>
Subject: Re: [PATCH v3 4/8] gdbserver/linux-aarch64: When thread stops, update its target description
Date: Fri, 03 Feb 2023 03:47:31 +0000	[thread overview]
Message-ID: <871qn79zqk.fsf@linaro.org> (raw)
In-Reply-To: <e6c0ec52-d8d3-decf-7363-8e7d81e1f882@simark.ca>


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

  reply	other threads:[~2023-02-03  3:47 UTC|newest]

Thread overview: 94+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-01-30  4:45 [PATCH v3 0/8] gdbserver improvements for AArch64 SVE support Thiago Jung Bauermann
2023-01-30  4:45 ` [PATCH v3 1/8] gdbserver: Add assert in find_register_by_number Thiago Jung Bauermann
2023-01-31 17:05   ` Simon Marchi
2023-01-31 19:49     ` Thiago Jung Bauermann
2023-02-01 15:43       ` Simon Marchi
2023-01-30  4:45 ` [PATCH v3 2/8] gdbserver: Add PID parameter to linux_get_auxv and linux_get_hwcap Thiago Jung Bauermann
2023-02-01  9:07   ` Luis Machado
2023-02-01 10:54   ` Andrew Burgess
2023-02-01 16:01     ` Simon Marchi
2023-02-01 19:33       ` Thiago Jung Bauermann
2023-02-01 19:53         ` Simon Marchi
2023-02-01 21:55           ` Thiago Jung Bauermann
2023-02-06 19:54   ` Pedro Alves
2023-02-06 20:16     ` Simon Marchi
2023-02-07 15:19       ` Pedro Alves
2023-02-07 21:47         ` Thiago Jung Bauermann
2023-02-09  1:31           ` Simon Marchi
2023-02-10  3:54             ` Thiago Jung Bauermann
2023-02-07 22:28     ` Thiago Jung Bauermann
2023-01-30  4:45 ` [PATCH v3 3/8] gdbserver/linux-aarch64: Factor out function to get aarch64_features Thiago Jung Bauermann
2023-02-01  8:59   ` Luis Machado
2023-02-01 16:04     ` Simon Marchi
2023-02-01 22:13       ` Thiago Jung Bauermann
2023-01-30  4:45 ` [PATCH v3 4/8] gdbserver/linux-aarch64: When thread stops, update its target description Thiago Jung Bauermann
2023-02-01  9:05   ` Luis Machado
2023-02-01 11:06   ` Andrew Burgess
2023-02-01 16:21     ` Simon Marchi
2023-02-01 16:32       ` Luis Machado
2023-02-02  2:54         ` Thiago Jung Bauermann
2023-02-02  3:47           ` Simon Marchi
2023-02-03  3:47             ` Thiago Jung Bauermann [this message]
2023-02-03 11:13               ` Luis Machado
2023-02-04 15:26                 ` Thiago Jung Bauermann
2023-02-03 11:11             ` Luis Machado
2023-02-04 15:21               ` Thiago Jung Bauermann
2023-02-06  9:07                 ` Luis Machado
2023-02-06 12:15                   ` Thiago Jung Bauermann
2023-02-06 20:29                 ` Pedro Alves
2023-02-07  8:11                   ` Luis Machado
2023-02-07 14:39                     ` Thiago Jung Bauermann
2023-02-03 10:57           ` Luis Machado
2023-02-04  6:18             ` Thiago Jung Bauermann
2023-02-06 20:26           ` Pedro Alves
2023-02-07 21:06             ` Thiago Jung Bauermann
2023-02-09  2:46               ` Simon Marchi
2023-02-10  3:29                 ` Thiago Jung Bauermann
2023-02-10 14:56                   ` Luis Machado
2023-02-10 15:04                     ` Tom Tromey
2023-02-10 15:28                       ` Luis Machado
2023-02-10 17:26                       ` Thiago Jung Bauermann
2023-02-10 21:01                         ` Simon Marchi
2023-01-30  4:45 ` [PATCH v3 5/8] gdbserver: Transmit target description ID in thread list and stop reply Thiago Jung Bauermann
2023-01-30 12:52   ` Eli Zaretskii
2023-01-30 14:05     ` Thiago Jung Bauermann
2023-02-01  9:39   ` Luis Machado
2023-02-01 12:07   ` Andrew Burgess
2023-02-01 13:03     ` Eli Zaretskii
2023-02-01 17:37     ` Simon Marchi
2023-02-02 20:36       ` Thiago Jung Bauermann
2023-02-02 20:56         ` Simon Marchi
2023-02-01 20:46     ` Simon Marchi
2023-02-02 21:43       ` Thiago Jung Bauermann
2023-02-01 14:51   ` Andrew Burgess
2023-02-01 17:03     ` Simon Marchi
2023-02-02 19:52       ` Thiago Jung Bauermann
2023-02-02 20:51         ` Simon Marchi
2023-02-03  2:44           ` Thiago Jung Bauermann
2023-02-03 16:29             ` Andrew Burgess
2023-02-04  6:08               ` Thiago Jung Bauermann
2023-02-03 11:22       ` Luis Machado
2023-02-03 12:50         ` Simon Marchi
2023-01-30  4:45 ` [PATCH v3 6/8] gdb/remote: Parse tdesc field in stop reply and threads list XML Thiago Jung Bauermann
2023-02-01  9:52   ` Luis Machado
2023-02-05  0:06     ` Thiago Jung Bauermann
2023-02-06  9:10       ` Luis Machado
2023-02-01 14:32   ` Andrew Burgess
2023-02-01 19:50     ` Simon Marchi
2023-02-01 20:16       ` Simon Marchi
2023-02-03 11:27         ` Luis Machado
2023-02-03 13:19           ` Simon Marchi
2023-02-03 16:33             ` Andrew Burgess
2023-01-30  4:45 ` [PATCH v3 7/8] gdb/aarch64: Detect vector length changes when debugging remotely Thiago Jung Bauermann
2023-02-01  9:58   ` Luis Machado
2023-02-01 15:26   ` Andrew Burgess
2023-02-01 20:20     ` Simon Marchi
2023-02-03 11:31       ` Luis Machado
2023-02-03 16:38       ` Andrew Burgess
2023-02-03 19:07         ` Simon Marchi
2023-01-30  4:45 ` [PATCH v3 8/8] gdb/testsuite: Add test to exercise multi-threaded AArch64 SVE inferiors Thiago Jung Bauermann
2023-02-01 10:10   ` Luis Machado
2023-02-06 19:11 ` [PATCH v3 0/8] gdbserver improvements for AArch64 SVE support Pedro Alves
2023-02-06 20:05   ` Simon Marchi
2023-02-06 21:06     ` Pedro Alves
2023-02-07 13:49       ` Simon Marchi

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