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

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Date:   Fri, 17 Mar 2023 10:49:49 +0800
From:   Donglin Peng <pengdonglin@...gfor.com.cn>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     mhiramat@...nel.org, rostedt@...dmis.org, 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 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.