linux-kernel - Re: [PATCH v4 1/2] function_graph: Support recording and printing the return value of function

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Message-ID: <20230315145757.GE2006103@hirez.programming.kicks-ass.net>
Date:   Wed, 15 Mar 2023 15:57:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Donglin Peng <pengdonglin@...gfor.com.cn>, mhiramat@...nel.org,
        linux@...linux.org.uk, mark.rutland@....com, will@...nel.org,
        catalin.marinas@....com, palmer@...belt.com,
        paul.walmsley@...ive.com, tglx@...utronix.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, mingo@...hat.com,
        xiehuan09@...il.com, dinghui@...gfor.com.cn,
        huangcun@...gfor.com.cn, dolinux.peng@...il.com,
        linux-trace-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] function_graph: Support recording and printing
 the return value of function

On Wed, Mar 15, 2023 at 10:13:48AM -0400, Steven Rostedt wrote:
> On Wed, 15 Mar 2023 14:49:11 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
> > > index 1265ad519249..35ac9c58dc77 100644
> > > --- a/arch/x86/kernel/ftrace_64.S
> > > +++ b/arch/x86/kernel/ftrace_64.S
> > > @@ -348,6 +348,10 @@ SYM_CODE_START(return_to_handler)
> > >  	movq %rax, (%rsp)
> > >  	movq %rdx, 8(%rsp)
> > >  	movq %rbp, %rdi
> > > +#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
> > > +	/* Pass the function return value to ftrace_return_to_handler */
> > > +	movq %rax, %rsi
> > > +#endif
> > >  
> > >  	call ftrace_return_to_handler  
> > 
> > What about the case of double register return values (when the value
> > is returned in the A,D pair) ?
> 
> Is there anything that does that in 64 bit kernels?

Note sure; but I have a patch series that introduces cmpxchg128 and
friends. Most of the actual functions are __always_inline, but still,
the moment a compiler decides to break out a subfunction on a u128
boundary we're in luck.