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

[Peter Zijlstra]

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@xxxxxxxxxxxxx> 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.