Re: [PATCH 2/3] unwind_user/fp: Use dummies instead of ifdef

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Nov 27, 2025 at 05:51:47PM +0100, Jens Remus wrote:
> On 11/25/2025 5:43 PM, Jens Remus wrote:
> > This simplifies the code.   unwind_user_next_fp() does not need to
> > return -EINVAL if config option HAVE_UNWIND_USER_FP is disabled, as
> > unwind_user_start() will then not select this unwind method and
> > unwind_user_next() will therefore not call it.
> > 
> > Note that enabling the config option HAVE_UNWIND_USER_FP without
> > defining ARCH_INIT_USER_FP_FRAME, ARCH_INIT_USER_FP_ENTRY_FRAME, and
> > unwind_user_at_function_start() will result in a compile error, which
> > is helpful when implementing support for unwind user fp in an
> > architecture.
> > 
> > Signed-off-by: Jens Remus <jremus@linux.ibm.com>
> 
> > diff --git a/include/linux/unwind_user.h b/include/linux/unwind_user.h
> 
> > @@ -5,9 +5,17 @@
> >  #include <linux/unwind_user_types.h>
> >  #include <asm/unwind_user.h>
> >  
> > -#ifndef ARCH_INIT_USER_FP_FRAME
> > - #define ARCH_INIT_USER_FP_FRAME
> > -#endif
> > +#ifndef CONFIG_HAVE_UNWIND_USER_FP
> > +
> > +#define ARCH_INIT_USER_FP_FRAME
> > +#define ARCH_INIT_USER_FP_ENTRY_FRAME
> 
> Will fix this as follows in the next version:
> 
> #define ARCH_INIT_USER_FP_FRAME(ws)
> #define ARCH_INIT_USER_FP_ENTRY_FRAME(ws)
> 
> > +
> > +static inline bool unwind_user_at_function_start(struct pt_regs *regs)
> > +{
> > +	return false;
> > +}
> 
> Would it be better to provide a generic dummy implementation (see below)
> or should each arch implement that if it cannot tell whether the topmost
> frame is at function start? If so, would it move from linux/unwind_user.h
> to asm-generic/unwind_user.h?  Either way it would need to be outside of
> the !CONFIG_HAVE_UNWIND_USER_FP guard.

I suppose a common fallback would be fine.