linux-kernel - Re: [PATCH v16 04/18] function_graph: Replace fgraph_ret_regs with ftrace

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Message-ID: <20241016083323.16801-A-hca@linux.ibm.com>
Date: Wed, 16 Oct 2024 10:33:23 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Florent Revest <revest@...omium.org>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Alan Maguire <alan.maguire@...cle.com>,
        Mark Rutland <mark.rutland@....com>, linux-arch@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v16 04/18] function_graph: Replace fgraph_ret_regs with
 ftrace_regs

On Wed, Oct 16, 2024 at 08:47:20AM +0900, Masami Hiramatsu wrote:
> On Tue, 15 Oct 2024 20:39:06 +0200
> Heiko Carstens <hca@...ux.ibm.com> wrote:
> 
> > That would make things much simpler... e.g. your new patch is also
> > writing r3 to fregs, why? 
> 
> BTW, according to the document [1], r3 is for "return value 1", isn't it
> used usually?
> 
> [1] https://www.kernel.org/doc/Documentation/s390/Debugging390.txt

That is true for the 32 bit ABI, but not for the 64 bit ABI which we
care about. Besides other this is also the reason why I removed the
above file five years ago: f62f7dcbf023 ("Documentation/s390: remove
outdated debugging390 documentation").

If you really want to understand the 64 bit s390 ABI then you need to
look at https://github.com/IBM/s390x-abi .

A PDF file of the latest release is available at
https://github.com/IBM/s390x-abi/releases/download/v1.6.1/lzsabi_s390x.pdf

See section "1.2.5. Return Values" for return value handling.

All of that said, I would appreciate if you would just merge the
provided patch, unless there is a reason for not doing that. Chances
are that I missed something with all the recent fregs vs ptregs
changes.