Date:   Thu, 16 Mar 2023 23:21:07 +0000
From:   "Russell King (Oracle)" <linux@armlinux.org.uk>
To:     Donglin Peng <pengdonglin@sangfor.com.cn>
Cc:     mhiramat@kernel.org, rostedt@goodmis.org, mark.rutland@arm.com,
        will@kernel.org, catalin.marinas@arm.com, palmer@dabbelt.com,
        paul.walmsley@sifive.com, tglx@linutronix.de,
        dave.hansen@linux.intel.com, x86@kernel.org, mingo@redhat.com,
        xiehuan09@gmail.com, dinghui@sangfor.com.cn,
        huangcun@sangfor.com.cn, dolinux.peng@gmail.com,
        linux-trace-kernel@vger.kernel.org,
        linux-riscv@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/2] function_graph: Support recording and printing
 the return value of function
Message-ID: <ZBOkY8FkqgoBfzQ2@shell.armlinux.org.uk>
References: <20230315133911.958741-1-pengdonglin@sangfor.com.cn>
 <20230315133911.958741-2-pengdonglin@sangfor.com.cn>
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Sender: Russell King (Oracle) <linux@armlinux.org.uk>
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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.

-- 
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