Re: [PATCH v12 02/14] unwind_user: Add frame pointer support

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 3 Jul 2025 18:21:10 +0200
Jens Remus <jremus@linux.ibm.com> wrote:

> >> 	/* stack going in wrong direction? */
> >> 	if (sp <= state->sp - topmost)
> >> 		goto done;  
> > 
> > How do you calculate "topmost" then?
> > 
> > Is it another field you add to "state"?  
> 
> Correct.  It is a boolean set to true in unwind_user_start() and set to
> false in unwind_user_next() when updating the state.

So it's subtracting 1 or zero? So that the topmost can be equal. Well,
that would need a bit of commenting.

> 
> I assume most architectures need above change, as their SP at function
> entry should be equal to the SP at call site (unlike x86-64 due to CALL).
> 
> s390 also needs this information to allow restoring of FP/RA saved in
> other registers (instead of on the stack) only for the topmost frame.
> For any other frame arbitrary register contents would not be available,
> as user unwind only unwinds SP, FP, and RA.
> 
> I would post my s390 sframe support patches as RFC once you have
> provided a merged sframe branch as discussed in:
> https://lore.kernel.org/all/20250702124737.565934b5@batman.local.home/

I did have a merge branch on my repo. But I was hoping to see your code
so that I can add this to this patch before having to post again. But
now I'm posting without this change, as I don't want to screw it up. I
think I know what it it looks like, but it would be better to see what
you did to make sure what I envision is correct.

Oh well. I'll post v13 without it.

-- Steve