Re: [PATCH 1/2] This patch fixes GDBServer's run control for single stepping

[Yao Qi <qiyaoltc at gmail dot com>]

On Fri, Jan 27, 2017 at 6:24 PM, Antoine Tremblay
<antoine.tremblay@ericsson.com> wrote:
>
> Yao Qi writes:
>
>> On Fri, Jan 27, 2017 at 4:06 PM, Antoine Tremblay
>> <antoine.tremblay@ericsson.com> wrote:

>> If emulation is complex, probably
>> we can partially fix this problem by "always using 16-bit thumb instruction
>> if program is out of IT block".
>>
>
> I would be against that since it would leave the feature partially
> working and crashing the program when it fails...
>
> It would also be a regression compared to previous GDBServer.

There won't be any regression.  16-bit thumb breakpoint works pretty well
for any thumb instructions (16-bit and 32-bit) if program is out of IT block.
The 32-bit thumb-2 breakpoint was added for single step IT block.

> Also, IT blocks are a common place to have a breakpoint/tracepoint.
>

We don't change anything when setting breakpoint inside IT block.

>>> I think it would be better to get the current single stepping working
>>> with the stop all threads logic since GDBServer was working like that
>>> when GDB was doing the single stepping anyway. This would fix the current
>>> regression.
>>>
>>> Then work could be done to improve GDBServer to be better at
>>> non-stopping.
>>>
>>> WDYT ?
>>>
>>
>> Sounds like we are applying the ARM linux limitation to a general
>> place.
>> Other software single step targets may write breakpoint in atomic way,
>> so we don't need to stop all threads.  Even in -marm mode, we don't
>> have to stop all threads on inserting breakpoints.
>
> Well ARM is the only software single stepping target, while I agreee
> that we would be affecting general code, I think that since there is
> no software single step breakpoint target that supports atomic
> breakpoint writes at the moment it's normal that the run control
> doesn't support that.

No, ARM Linux ptrace POKETEXT is _atomic_ if the address is 4-byte
aligned, so 32-bit arm breakpoint and 16-bit thumb breakpoint can be
written atomically.  32-bit thumb-2 breakpoint can be written atomically
too if the address is 4-byte aligned.

The only problem we have is inserting a breakpoint on a 2-byte aligned
32-bit thumb-2 instruction inside IT block, we can not use neither 16-bit thumb
breakpoint nor 32-bit thumb breakpoint.  Everything works fine in other
cases.

>
> I don't count -marm as an arch since there no way to check that all the
> program including shared libs etc is -marm, I don't think we could make
> the distinction in GDBServer AFAIK.

We can check with -mthumb.  My hack in last email fixes fails in
schedlock.exp in thumb mode.

-- 
Yao (齐尧)