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

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Message-ID: <20251128101119.GI3245006@noisy.programming.kicks-ass.net>
Date: Fri, 28 Nov 2025 11:11:19 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	x86@...nel.org, Steven Rostedt <rostedt@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Indu Bhagat <indu.bhagat@...cle.com>,
	"Jose E. Marchesi" <jemarch@....org>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Ilya Leoshkevich <iii@...ux.ibm.com>
Subject: Re: [PATCH 2/3] unwind_user/fp: Use dummies instead of ifdef

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@...ux.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.