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

[Donglin Peng]

On 2023/3/17 7:21, Russell King (Oracle) wrote:
> On Wed, Mar 15, 2023 at 06:39:10AM -0700, Donglin Peng wrote:
> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > index e24a9820e12f..ad03fc868f34 100644
> > --- a/arch/arm/Kconfig
> > +++ b/arch/arm/Kconfig
> > @@ -99,6 +99,7 @@ config ARM
> >   	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
> >   	select HAVE_FUNCTION_ERROR_INJECTION
> >   	select HAVE_FUNCTION_GRAPH_TRACER
> > +	select HAVE_FUNCTION_GRAPH_RETVAL if HAVE_FUNCTION_GRAPH_TRACER
> >   	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
> >   	select HAVE_GCC_PLUGINS
> >   	select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
> > diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
> > index 3e7bcaca5e07..0151d2ce9958 100644
> > --- a/arch/arm/kernel/entry-ftrace.S
> > +++ b/arch/arm/kernel/entry-ftrace.S
> > @@ -258,6 +258,10 @@ ENDPROC(ftrace_graph_regs_caller)
> >   #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> >   ENTRY(return_to_handler)
> >   	stmdb	sp!, {r0-r3}
> > +#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
> > +	/* Pass the function return value to ftrace_return_to_handler */
> > +	mov	r1, r0
>
> In a similar vein to Peter's comment, do we care about 64-bit return
> values here, because the above only covers 32-bit values.
>
> If we do care about 64-bit values, then we get into EABI/OABI
> stickyness, because on EABI the 64-bit value would have to be passed
> in r2,r3, and OABI would need r1,r2.
>
> it would be better to have the 64-bit argument as the first argument
> to ftrace_return_to_handler() which would eliminate that variability,
> but I don't know what effect that would have for other architectures.
>
> Things get more icky if we want 128-bit values. For EABI, we've
> conveniently just stacked that. For OABI, that would need to be in
> r1-r3 and the final high bits on the stack.
>
> With a 128-bit argument as the first, that would be r0-r3 with the
> existing stack pointer argument stored... on the stack.
>
> So, really it depends what size of return value we want to report.
> Also, please bear in mind that where a function returns a 32-bit
> value, that will be in r0, and r1 will be whatever happened to be
> in it at function exit - there's no defined value for r1.

Thank you. I will document this as a limitation of fgraph return value.
It can just cover most cases at present and I think the r0 is enough.