Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 17 Mar 2023 10:49:49 +0800 | From | Donglin Peng <> | Subject | Re: [PATCH v4 1/2] function_graph: Support recording and printing the return value of function |
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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.
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