From: David Blaikie <dblaikie@gmail.com>
To: Simon Marchi <simon.marchi@polymtl.ca>
Cc: Tim Newsome <tim@sifive.com>, gdb <gdb@sourceware.org>
Subject: Re: Remote query for structure layout
Date: Tue, 30 Mar 2021 20:27:40 -0700 [thread overview]
Message-ID: <CAENS6EuJwV4sxM6NwdffQ5bpPqkPrdk62qD_PNo0Jiy3pQqv=Q@mail.gmail.com> (raw)
In-Reply-To: <1c8fec3c-0d7e-d133-bf88-adaf764473f1@polymtl.ca>
(let me know if I'm just being an unhelpful bystander here - I clearly
don't have a lot of context)
On Tue, Mar 30, 2021 at 5:16 PM Simon Marchi <simon.marchi@polymtl.ca> wrote:
>
> On 2021-03-30 6:44 p.m., David Blaikie wrote:
> > If it's "just" some user-code, is there a variable of the desired type being declared around the function call?
>
> AFAIK, this type wouldn't be used by the FreeRTOS code at all, so no.
> Again, here's my understanding, hopefully it's close enough to the
> reality.
>
> When a trap occurs, FreeRTOS saves the current task's register values on
> the task's stack, using some arch-specific assembly code. For example,
> for RISC-V:
>
> https://github.com/FreeRTOS/FreeRTOS-Kernel/blob/534eba66ce4a5bda45d5edeeb81ac5a3cf6d0df8/portable/GCC/RISC-V/portASM.S#L121
Ah, OK. So this is part of signal handling - so it's not part of the
code that's compiled into the user's program, for instance... not even
in some system library linked in, necessarily, I guess?
> The way these registers are pushed is an implementation detail of
> FreeRTOS. And we can imagine that it can vary depending on the
> compile-time FreeRTOS configuration,
I guess in the worst case it could be totally dynamic - it could pick
a different layout each time.
(guess a side question: How's this different from other systems? I
don't know how other/more common systems handle registers during
signals)
> which OpenOCD doesn't know about.
>
> To get register values of scheduled out tasks, OpenOCD needs to
> interpret these register values from the tasks' stacks.
>
> So Tim's suggestion is: have FreeRTOS declare a structure that has the
> exact layout as the saved registers on the stack:
>
> struct freertos_saved_regs {
> int x1;
> int x5;
> int x6;
> ...
> };
>
> Consumers could read that structure's layout from the DWARF info, and
> read the register values based on that. That would be a lot more robust
> than hard-coding in the consumers how FreeRTOS stores things.
If practical experience has shown the hard-coding is not
stable/reliable (than FreeRTOS does change its strategy from time to
time - but it's always constant for any given build of the FreeRTOS) I
guess.
But I'm not sure where FreeRTOS would expose this structure to user
code - it's not like there's a system library header that user code
must include...
> However, since that struct would never actually be used by FreeRTOS'
> code, the compiler won't emit it. Hence the need to find a way to force
> the compiler to include it in the DWARF.
>
> Does that clarify the situation?
Somewhat - any lack of understanding is just my ignorance in this
field/area in general, to be clear. (I'm also not a core gdb
developer, so I'm not the sort of person you have to convince of
anything - just a curious bystander trying to understand/maybe offer
some insightful suggestions (I predominantly work on LLVM's debug info
emission, so that's my background/connection))
- Dave
next prev parent reply other threads:[~2021-03-31 3:27 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.6.1616932808.1485743.gdb@sourceware.org>
2021-03-30 20:35 ` Tim Newsome
2021-03-30 21:33 ` Simon Marchi
2021-03-30 21:41 ` David Blaikie
2021-03-30 21:49 ` Simon Marchi
2021-03-30 22:03 ` David Blaikie
2021-03-30 22:38 ` Simon Marchi
2021-03-30 22:44 ` David Blaikie
2021-03-31 0:16 ` Simon Marchi
2021-03-31 3:27 ` David Blaikie [this message]
2021-03-31 13:00 ` Simon Marchi
2021-04-02 22:05 ` David Blaikie
2021-04-03 19:39 ` Simon Marchi
2021-03-31 14:06 ` Tim Newsome
2021-03-28 11:06 Thomas Weißschuh
2021-03-29 16:05 ` Simon Marchi
2021-03-29 17:33 ` Thomas Weißschuh
2021-03-29 19:42 ` Simon Marchi
2021-03-29 20:02 ` Philippe Waroquiers
2021-03-29 20:10 ` Simon Marchi
2021-03-29 20:20 ` Philippe Waroquiers
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