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

Thiago Jung Bauermann via Gdb-patches
2023-02-04T15:21:59Z

Luis Machado <luis.machado@arm.com> writes:

> On 2/2/23 03:47, Simon Marchi wrote:
>> On 2/1/23 21:54, Thiago Jung Bauermann wrote:
>>> 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?
>>
>
> It should be fine because the first time we fetch a process target
> description, it is eventually obtained from the first and only thread.
> So the SVE vector length should be correct.
>
> Any subsequent attempts to use the process' target description (the
> first one we obtained), after further stops, may end up using an
> incorrect description.
>
> I think this is handled correctly by the target architecture target
> hook though. But there are other places where this is potentially
> incorrect.
>
> For example...
>
> - When using gcore to dump a core file, GDB only dumps a single target
> description. While this might be correct for a target with a fixed
> target description or a AArch64 target that doesn't support SVE, it
> likely won't be correctly for one AArch64 target supporting SVE if its
> threads changed vector length mid-execution. Either we emit target
> description notes by thread, or we don't emit a target description
> note for those cases.
>
> - When loading the above/older gcore core files back, GDB will use a
> potentially incorrect target description. If we decide to emit
> per-thread target descriptions, it should be fine. Otherwise we may
> need to have a "thread architecture" hook for core files as well.
>
> - The remote has no concept of a thread architecture (Thiago is
> addressing this with this patch series).
>
> - AArch64 frames may have slightly different vg values, which means
> their gdbarches are different as well.
>
> Given the differences between two gdbarches are small, we mostly get
> away with it. But if there are further differences (different hooks,
> for example), I fear we may run into a situation where we use an
> incorrect gdbarch to call a particular hook.

Indeed, good points! Thank you for bringing them up. I can address core
file dumping/loading after this series.

Regarding frames with different vg values, it's important to be aware of
this discrepancy but IMHO it makes sense to work on it when it becomes
a problem...

--
Thiago