Re: [PATCH v5 1/2] function_graph: Support recording and printing the return value of function

[Russell King (Oracle)]

On Mon, Mar 20, 2023 at 06:16:49AM -0700, Donglin Peng wrote:
> diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
> index 3e7bcaca5e07..ba1986e27af8 100644
> --- a/arch/arm/kernel/entry-ftrace.S
> +++ b/arch/arm/kernel/entry-ftrace.S
> @@ -258,7 +258,15 @@ ENDPROC(ftrace_graph_regs_caller)
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  ENTRY(return_to_handler)
>  	stmdb	sp!, {r0-r3}
> +#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
> +	/*
> +	 * Pass both the function return values in the register r0 and r1
> +	 * to ftrace_return_to_handler
> +	 */
> +	add	r2, sp, #16		@ sp at exit of instrumented routine
> +#else
>  	add	r0, sp, #16		@ sp at exit of instrumented routine
> +#endif
>  	bl	ftrace_return_to_handler
...
> -unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
> +static unsigned long __ftrace_return_to_handler(unsigned long retval_1st,
> +			unsigned long retval_2nd, unsigned long frame_pointer)
...
> +#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
> +unsigned long ftrace_return_to_handler(unsigned long retval_1st,
> +			unsigned long retval_2nd, unsigned long frame_pointer)
> +{
> +	return __ftrace_return_to_handler(retval_1st, retval_2nd, frame_pointer);
> +}
> +#else
> +unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
> +{
> +	return __ftrace_return_to_handler(0, 0, frame_pointer);
> +}
> +#endif
> +

Hi,

To echo Mark's criticism, I also don't like this. I feel it would be
better if ftrace_return_to_handler() always took the same arguments
irrespective of the setting of CONFIG_FUNCTION_GRAPH_RETVAL.

On 32-bit ARM, we have to stack r0-r3 anyway to prevent the call to
ftrace_return_to_handler() corrupting the return value, and these
are the registers we need. So we might as well pass a pointer to
these stacked registers. Whether that's acceptable on other
architectures, I couldn't say.

Thanks.

-- 
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