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

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Message-Id: <20241016083313.7198a3abf87e45ddb3dbb671@kernel.org>
Date: Wed, 16 Oct 2024 08:33:13 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Heiko Carstens <hca@...ux.ibm.com>
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, 15 Oct 2024 20:39:06 +0200
Heiko Carstens <hca@...ux.ibm.com> wrote:

> 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.

I thought it is better to focus on replacing fgraph_ret_regs with
ftrace_regs in this patch, but if it is wrong and I should remove
stack_frame, let me fix that.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>