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

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Message-ID: <20241015183906.19678-B-hca@linux.ibm.com>
Date: Tue, 15 Oct 2024 20:39:06 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: "Masami Hiramatsu (Google)" <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 Tue, Oct 15, 2024 at 10:29:17AM +0900, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> 
> Use ftrace_regs instead of fgraph_ret_regs for tracing return value
> on function_graph tracer because of simplifying the callback interface.
> 
> The CONFIG_HAVE_FUNCTION_GRAPH_RETVAL is also replaced by
> CONFIG_HAVE_FUNCTION_GRAPH_FREGS.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

...

> diff --git a/arch/s390/kernel/mcount.S b/arch/s390/kernel/mcount.S
> index 7e267ef63a7f..a9ca56ea0858 100644
> --- a/arch/s390/kernel/mcount.S
> +++ b/arch/s390/kernel/mcount.S
> @@ -134,14 +134,15 @@ SYM_CODE_END(ftrace_common)
>  SYM_FUNC_START(return_to_handler)
>  	stmg	%r2,%r5,32(%r15)
>  	lgr	%r1,%r15
> -	aghi	%r15,-(STACK_FRAME_OVERHEAD+__FGRAPH_RET_SIZE)
> +	aghi	%r15,-(STACK_FRAME_OVERHEAD+STACK_FRAME_SIZE_FREGS)
>  	stg	%r1,__SF_BACKCHAIN(%r15)
> -	la	%r3,STACK_FRAME_OVERHEAD(%r15)
> -	stg	%r1,__FGRAPH_RET_FP(%r3)
> -	stg	%r2,__FGRAPH_RET_GPR2(%r3)
> -	lgr	%r2,%r3
> +	la	%r4,STACK_FRAME_OVERHEAD(%r15)
> +	stg	%r2,__PT_R2(%r4)
> +	stg	%r3,__PT_R3(%r4)
> +	stg	%r1,__PT_R15(%r4)
> +	lgr	%r2,%r4
>  	brasl	%r14,ftrace_return_to_handler
> -	aghi	%r15,STACK_FRAME_OVERHEAD+__FGRAPH_RET_SIZE
> +	aghi	%r15,STACK_FRAME_SIZE_FREGS
>  	lgr	%r14,%r2
>  	lmg	%r2,%r5,32(%r15)
>  	BR_EX	%r14

Why didn't you simply merge the addon patch which I provided, and
which I tested?
https://lore.kernel.org/all/20240916121656.20933-B-hca@linux.ibm.com

That would make things much simpler... e.g. your new patch is also
writing r3 to fregs, why? The stackframe allocation is also wrong.
I didn't try, but I guess the above code would crash instantly.