Re: [PATCH] Fix exception unwinding for ARM Cortex-M

[Adam Renquinha <arenquinha at cimeq dot qc dot ca>]

Hi Fredrik,

>> Seriously though, I was a bit too busy at work to pursue on this and
>> I
>> didn't have a clear idea of where to start after the comments.
>> Moreover,
>> as the patch did "just work" at my workplace, it was a bit difficult
>> to
>> justify spending a lot more time digging information about target
>> descriptions, how to determine what features are present on target,
>> etc.
>> So thanks to push toward the progress of this issue, apparently with
>> your mail subject you successfully brought people that gives more
>> specific guidance to get this done.
>
> I tried to check the status on your submission mid-July 
(https://sourceware.org/ml/gdb/2016-07/msg00011.html),
> but since I got no directions from anyone in the community at that 
time, I decided to try continue myself to get this fixed.
> We work alot with Cortex-M4F on a daily basis, and do really need 
this to be fixed.

We do, too. It isn't fun when you try to debug a firmware using FreeRTOS 
and the FPU on a Cortex-M4, only to see that you cannot backtrace into 
suspended tasks, is it? It took quite a bit of time for us to understand 
where the problem lied... I worked on this bug because I could build 
about anything really easily in my Arch Linux VM, and I know very well 
how to use plain gdb.

>> Indeed, the patch looks really close to what I've done, to the point
>> that I can see that, aside from a small refactoring, the changes
>> between
>> our patches are mainly variable name changes and added comments. You
>> really wrote your patch from scratch? You've got rid of the various
>> calls to `user_reg_map_name_to_regnum', though, which is nice.
>
> Yes I wrote it from scratch, but I agree my patch looks similar to 
yours, as its implementing according to ARM Cortex-M spec,
> I guess it needs to be. But it absolutely helped me to have your 
ideas as starting point to look at, to see where modifications needed to 
be done,
> so I do not mind if you would like to stand as co-author in the 
ChangeLog?
> If you have FSF GDB Assignment, or would like to apply for assignment,
> I can add your name to ChangeLog, its totally fine by me.

That is fine for me too, I don't mind at all. I never had submitted work 
that required an FSF submission, but I plan to do that.

> And I think it would be great if we could try to continue this work 
together, to get the remaining parts for Cortex-M support in GDB.
> The more people who are working on this, the better.

Yes! :)

> I am not so experienced in GDB internals, and also seek guidance how 
to continue go get the last parts in place.

I'm not experienced at all with GDB internals either. I ran a test 
program under `arm-none-eabi-gdb' under plain gdb (a bit of a headache, 
a debugger inside a debugger), and I traced what happened upon 
unwinding, and fixed rather directly what I saw was wrong. I knew that 
what would result from that process would be rather crude, due to my 
inexperience. For instance, target descriptions were news for me.

Guidance on that specific part (handling target description) is needed. 
Anyone?

> My idea is to submit a bug entry in bugzilla on this, one issue for 
the MSP/PSP support, another one for FPCCR, so we can get track of this, 
and someone could be assigned.

Good idea.

>>> Cortex-M has two stack pointers: MSP (Main Stack Pointer) and PSP
>> (Process Stack Pointer).
>>> This is not handled when GDB tries to backtrace in the exception
>> stack unwinder.
>>> Meaning for eg. Cortex-M4F its not possible to get any call-stack
>> backtrace if setting a breakpoint in ISR, and floating-point variable
>> was used.
>>
>> Actually, the info here is inaccurate: FPU context stacking and the
>> MSP/PSP switch are two unrelated concepts. Meaning, you can fix the
>> code
>> to deal with the PSP, but it still won't handle FPU register
>> unstacking.
>
> Correct, my plan now was to split the patch as Yao proposed.
> The the MSP/PSP-fix and FPU-fix would be two different patches.

Yes. Smaller patches will help things moving forward. I wasn't sure at 
the time if beginning with a small patch just covering e.g. magic PC 
values handling would be considered useful, because in itself it fixes 
nothing, but it really is important to handle the MSP/PSP and the FPU 
cases. So I opted for a big comprehensive patch. But indeed, it's easier 
to review, and easier to integrate.

>>> ChangeLog entry should cover what do you change on the function
>> level.
>>> Please read https://sourceware.org/gdb/wiki/ContributionChecklist
>>
>> If only I knew that checklist! I searched for something like this,
>> but
>> maybe it didn't use the correct search terms, I didn't find it.
>
> If you want to stand as co-author in ChangeLog, its totally fine by me,
> just give me a hint if you have the FSF assignment done,
> and I also would be more than happy if you have time to continue your 
great work, and get the last parts of unwinding support for Cortex-M in 
place.

Hmm, if I understand correctly, I should e-mail one of the gdb 
maintainers to get the assignment to fill? You did that before; how it went?

Thanks,


James-Adam Renquinha Henri, Ing. jr
Ingénieur d'application
CIMEQ INC.

Le 2016-08-04 à 03:03, Fredrik Hederstierna a écrit :
> Hi James-Adam,
>
> -----James-Adam Renquinha Henri <arenquinha@cimeq.qc.ca> wrote: -----
> > Hi, I'm back from the dead.
>
> Nice to see you back :)
>
>
> > Seriously though, I was a bit too busy at work to pursue on this and
> > I
> > didn't have a clear idea of where to start after the comments.
> > Moreover,
> > as the patch did "just work" at my workplace, it was a bit difficult
> > to
> > justify spending a lot more time digging information about target
> > descriptions, how to determine what features are present on target,
> > etc.
> > So thanks to push toward the progress of this issue, apparently with
> > your mail subject you successfully brought people that gives more
> > specific guidance to get this done.
>
> I tried to check the status on your submission mid-July (https://sourceware.org/ml/gdb/2016-07/msg00011.html),
> but since I got no directions from anyone in the community at that time, I decided to try continue myself to get this fixed.
> We work alot with Cortex-M4F on a daily basis, and do really need this to be fixed.
>
>
> > Indeed, the patch looks really close to what I've done, to the point
> > that I can see that, aside from a small refactoring, the changes
> > between
> > our patches are mainly variable name changes and added comments. You
> > really wrote your patch from scratch? You've got rid of the various
> > calls to `user_reg_map_name_to_regnum', though, which is nice.
>
> Yes I wrote it from scratch, but I agree my patch looks similar to yours, as its implementing according to ARM Cortex-M spec,
> I guess it needs to be. But it absolutely helped me to have your ideas as starting point to look at, to see where modifications needed to be done,
> so I do not mind if you would like to stand as co-author in the ChangeLog?
> If you have FSF GDB Assignment, or would like to apply for assignment,
> I can add your name to ChangeLog, its totally fine by me.
>
> The important thing for me is to get this fixed, and who gets the actual and final credits I do not mind.
> And I think it would be great if we could try to continue this work together, to get the remaining parts for Cortex-M support in GDB.
> The more people who are working on this, the better.
> I am not so experienced in GDB internals, and also seek guidance how to continue go get the last parts in place.
> I think both to get GDB aware of MSP and PSP, and also FPCCR needs to be a bigger change and are more complicated.
> My idea is to submit a bug entry in bugzilla on this, one issue for the MSP/PSP support, another one for FPCCR, so we can get track of this, and someone could be assigned.
>
>
> > > Cortex-M has two stack pointers: MSP (Main Stack Pointer) and PSP
> > (Process Stack Pointer).
> > > This is not handled when GDB tries to backtrace in the exception
> > stack unwinder.
> > > Meaning for eg. Cortex-M4F its not possible to get any call-stack
> > backtrace if setting a breakpoint in ISR, and floating-point variable
> > was used.
> >
> > Actually, the info here is inaccurate: FPU context stacking and the
> > MSP/PSP switch are two unrelated concepts. Meaning, you can fix the
> > code
> > to deal with the PSP, but it still won't handle FPU register
> > unstacking.
>
> Correct, my plan now was to split the patch as Yao proposed.
> The the MSP/PSP-fix and FPU-fix would be two different patches.
>
>
> > > This patch was inspired by the great work done by James-Adam
> > Renquinha Henri, submitted April this year.
> > Thanks :)
> >
> > > The next thing would then be to also add FPU context control reg
> > FPCCR, which is needed for retrieving info on the FPU lazy stacking.
> > > Though its complicated I think and I will try to investigate an
> > 'arm-m-fpu.xml' profile further, if this is solution perhaps.
> >
> > It indeed is just a bit more complicated. Let me summarize what needs
> > to
> > be done. Have the ARMv7-M Architecture Reference Manual handy, see
> > B1.5.7.
> >
> > - Check if lazy stacking is enabled (FPCCR.LSPEN == 1). If it's not,
> > the
> > case is uncomplicated, registers are stacked as usual
> > - If lazy stacking is active (FPCCR.LSACT == 1), the extended stack
> > has
> > space reserved for the FPU registers (S0-S15, FPSCR), but there are
> > not
> > stacked, they are still in FPU registers unmodified.
> >
> > So that adds more random fetches from "memory" of the target, because
> >
> > these are memory-mapped.
> >
> > ARM gives some example scenarios with chronograms of some important
> > bits
> > with stacking information [1].
>
> Yes, my idea was that GDB needs to be aware of FPCCR, so we need probably an new XML profile feature for this, just as for PSP/MSP.
> But you are correct, its way more complicated. See also ARM Application Note I'm referring to in my patch comments.
>
>
> > > ChangeLog entry should cover what do you change on the function
> > level.
> > > Please read https://sourceware.org/gdb/wiki/ContributionChecklist
> >
> > If only I knew that checklist! I searched for something like this,
> > but
> > maybe it didn't use the correct search terms, I didn't find it.
>
> If you want to stand as co-author in ChangeLog, its totally fine by me,
> just give me a hint if you have the FSF assignment done,
> and I also would be more than happy if you have time to continue your great work, and get the last parts of unwinding support for Cortex-M in place.
>
>
> Thanks, and Kind Regards,
> Fredrik