# Re: \[RFC PATCH v1 06/16\] unwind\_user: Enable archs that define CFA = SP\_callsite + offset

- From: Josh Poimboeuf \<jpoimboe@kernel.org\>
- Date: 2025-07-16 21:32:24 
- List: linux-trace-kernel
- Page: https://ratatoskr.run/linux-trace-kernel/2025/07/3137667
- Thread: https://ratatoskr.run/linux-trace-kernel/2025/07/3129870/t.md

```text
On Thu, Jul 10, 2025 at 06:35:12PM +0200, Jens Remus wrote:
> Most architectures define their CFA as the value of the stack pointer
> (SP) at the call site in the previous frame, as suggested by the DWARF
> standard:
> 
>   CFA = <SP at call site>
> 
> Enable unwinding of user space for architectures, such as s390, which
> define their CFA as the value of the SP at the call site in the previous
> frame with an offset:
> 
>   CFA = <SP at call site> + offset

This is a bit confusing, as the comment and code define it as

    SP = CFA + offset

Should the commit log be updated to match that?

> +++ b/arch/x86/include/asm/unwind_user.h
> @@ -8,6 +8,7 @@
>  	.cfa_off	= (s32)sizeof(long) *  2,				\
>  	.ra_off		= (s32)sizeof(long) * -1,				\
>  	.fp_off		= (s32)sizeof(long) * -2,				\
> +	.sp_val_off	= (s32)0,						\

IIUC, this is similar to ra_off and fp_off in that its an offset from
the CFA.  Can we call it "sp_off"?

-- 
Josh
```