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

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Message-ID: <20dda91c-46e4-466d-9ebb-7e12fc6f5d28@linux.ibm.com>
Date: Thu, 27 Nov 2025 17:51:47 +0100
From: Jens Remus <jremus@...ux.ibm.com>
To: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        x86@...nel.org, Steven Rostedt <rostedt@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc: 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 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.

#ifndef unwind_user_at_function_start
static inline bool unwind_user_at_function_start(struct pt_regs *regs)
{
	return false;
}
#define unwind_user_at_function_start unwind_user_at_function_start
#endif

If doing so ARCH_INIT_USER_FP_ENTRY_FRAME should be handled similar, so
that archs do not need to provide their own dummy either:

#ifndef ARCH_INIT_USER_FP_ENTRY_FRAME
#define ARCH_INIT_USER_FP_ENTRY_FRAME(ws)
#endif

In that case only ARCH_INIT_USER_FP_FRAME would remain guarded by
!CONFIG_HAVE_UNWIND_USER_FP, so that compile would fail, if enabling
CONFIG_HAVE_UNWIND_USER_FP without providing ARCH_INIT_USER_FP_FRAME:

#ifndef CONFIG_HAVE_UNWIND_USER_FP

#define ARCH_INIT_USER_FP_FRAME(ws)

#endif /* !CONFIG_HAVE_UNWIND_USER_FP */

> +
> +#endif /* !CONFIG_HAVE_UNWIND_USER_FP */
>  
>  int unwind_user(struct unwind_stacktrace *trace, unsigned int max_entries);
>  
Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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