linux-kernel - Re: [RFC PATCH v1 07/16] unwind_user: Enable archs that do not necessarily save RA

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Message-ID: <fb9ee560-d449-4d46-9fb1-19780ff28e65@linux.ibm.com>
Date: Fri, 18 Jul 2025 10:28:32 +0200
From: Jens Remus <jremus@...ux.ibm.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        bpf@...r.kernel.org, x86@...nel.org,
        Steven Rostedt <rostedt@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Masami Hiramatsu
 <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrii Nakryiko <andrii@...nel.org>,
        Indu Bhagat <indu.bhagat@...cle.com>,
        "Jose E. Marchesi" <jemarch@....org>,
        Beau Belgrave <beaub@...ux.microsoft.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Florian Weimer <fweimer@...hat.com>,
        Sam James <sam@...too.org>
Subject: Re: [RFC PATCH v1 07/16] unwind_user: Enable archs that do not
 necessarily save RA

On 17.07.2025 13:09, Jens Remus wrote:
> On 17.07.2025 01:01, Josh Poimboeuf wrote:
>> On Thu, Jul 10, 2025 at 06:35:13PM +0200, Jens Remus wrote:
>>> +++ b/arch/Kconfig
>>> @@ -450,6 +450,11 @@ config HAVE_UNWIND_USER_SFRAME
>>>  	bool
>>>  	select UNWIND_USER
>>>  
>>> +config HAVE_USER_RA_REG
>>> +	bool
>>> +	help
>>> +	  The arch passes the return address (RA) in user space in a register.
>>
>> How about "HAVE_UNWIND_USER_RA_REG" so it matches the existing
>> namespace?
> 
> Ok.  I am open to any improvements.

Thinking about this again I realized that the config option actually
serves two purposes:

1. Enable code (e.g. unwind user) to determine the presence of the new
   user_return_address().  That is where I derived the name from.
2. Enable unwind user (sframe) to behave differently, if an architecture
   has/uses a RA register (unlike x86, which solely uses the stack).

I think the primary notion is that an architecture has/uses a register
for the return address and thus provides user_return_address().  What
consumers such as unwind user do with that info is secondary.

Thoughts?

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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