Re: [PATCH v16 0/4] perf: Support the deferred unwinding infrastructure

[Jens Remus <jremus@linux.ibm.com>]

Hello Peter,

very nice!

On 10/24/2025 4:51 PM, Peter Zijlstra wrote:

> Subject: unwind_user/x86: Teach FP unwind about start of function
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Fri Oct 24 12:31:10 CEST 2025
> 
> When userspace is interrupted at the start of a function, before we
> get a chance to complete the frame, unwind will miss one caller.
> 
> X86 has a uprobe specific fixup for this, add bits to the generic
> unwinder to support this.
> 
> Suggested-by: Jens Remus <jremus@linux.ibm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> +++ b/kernel/unwind/user.c

> +static int unwind_user_next_fp(struct unwind_user_state *state)
> +{
> +	struct pt_regs *regs = task_pt_regs(current);
> +
> +	const struct unwind_user_frame fp_frame = {
> +		ARCH_INIT_USER_FP_FRAME(state->ws)
> +	};
> +	const struct unwind_user_frame fp_entry_frame = {
> +		ARCH_INIT_USER_FP_ENTRY_FRAME(state->ws)
> +	};
> +
> +	if (state->topmost && unwind_user_at_function_start(regs))
> +		return unwind_user_next_common(state, &fp_entry_frame);

IIUC this will cause kernel/unwind/user.c to fail compile on
architectures that will support HAVE_UNWIND_USER_SFRAME but not
HAVE_UNWIND_USER_FP (such as s390), and thus do not need to implement
unwind_user_at_function_start().

Either s390 would need to supply a dummy unwind_user_at_function_start()
or the unwind user sframe series needs to address this and supply
a dummy one if FP is not enabled, so that the code compiles with only
SFRAME enabled.

What do you think?

> +
> +	return unwind_user_next_common(state, &fp_frame);
> +}
Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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