linux-kernel - Re: [PATCH v2 00/11] unwind, perf: sframe user space unwinding, deferred perf callchains

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Message-ID: <20241027172430.h6xvamrauxmwccx7@treble.attlocal.net>
Date: Sun, 27 Oct 2024 10:24:30 -0700
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, x86@...nel.org,
	Heiko Carstens <hca@...ux.ibm.com>,
	Ilya Leoshkevich <iii@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v2 00/11] unwind, perf: sframe user space unwinding,
 deferred perf callchains

On Wed, Oct 23, 2024 at 03:22:35PM +0200, Jens Remus wrote:
> We are looking forward to implement support for unwinding of user space
> using SFrame in kernel/perf on s390. One major concern is that your x86
> implementation currently relies on a fallback to unwinding using frame
> pointer. On s390 unwinding using frame pointer is unsupported, because
> of lack of proper s390x ABI [1] specification and compiler support. In
> theory there would be a s390-specific alternative of unwinding using
> backchain (compiler option -mbackchain), but this has limitations and
> there is currently no distribution where user space is built with
> backchain.
> 
> How much of an issue would it be if s390 could not provide an unwinding
> fallback implementation? Do you see the possibility to get away without?

No problem, I'll make the fallback dependent on CONFIG_HAVE_UNWIND_USER_FP.

> For s390 support of unwinding using SFrame we would need to make a few
> changes to your generic unwinding framework in the kernel:

Also no problem, I'm sure there will need to be tweaks going forward.

Thanks for looking at it!  v3 will be posted soon.

-- 
Josh