Re: [PATCH v2 1/2] powerpc/ftrace: support CONFIG_FUNCTION_GRAPH_RETVAL

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 22/07/2025 à 10:56, Aditya Bodkhe a écrit :
> commit a1be9ccc57f0 ("function_graph: Support recording and printing the
> return value of function") introduced support for function graph return
> value tracing.
> 
> Additionally, commit a3ed4157b7d8 ("fgraph: Replace fgraph_ret_regs with
> ftrace_regs") further refactored and optimized the implementation,
> making `struct fgraph_ret_regs` unnecessary.
> 
> This patch enables the above modifications for powerpc all, ensuring that
> function graph return value tracing is available on this architecture.
> 
> In this patch we have redefined two functions:
> - 'ftrace_regs_get_return_value()' - the existing implementation on
> ppc returns -ve of return value based on some conditions not
> relevant to our patch.
> - 'ftrace_regs_get_frame_pointer()' - always returns 0 in current code .
> 
> We also allocate stack space to equivalent of 'SWITCH_FRAME_SIZE',
> allowing us to directly use predefined offsets like 'GPR3' and 'GPR4'
> this keeps code clean and consistent with already defined offsets .
> 
> After this patch, v6.14+ kernel can also be built with FPROBE on powerpc
> but there are a few other build and runtime dependencies for FPROBE to
> work properly. The next patch addresses them.
> 
> Signed-off-by: Aditya Bodkhe <adityab1@linux.ibm.com>

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>

> ---
> 
> Changelog:
> v1 -> v2:
> - Added explanation for redefining ftrace_regs_get_return_value()
> and ftrace_regs_get_frame_pointer()
> - Explained why stack space equivalent to 'SWITCH_FRAME_SIZE' is allocated.
> 
>   arch/powerpc/Kconfig                     |  1 +
>   arch/powerpc/include/asm/ftrace.h        | 15 +++++++++
>   arch/powerpc/kernel/trace/ftrace_entry.S | 42 ++++++++++++++----------
>   3 files changed, 41 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index c3e0cc83f120..9163521bc4b9 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -250,6 +250,7 @@ config PPC
>   	select HAVE_FUNCTION_ARG_ACCESS_API
>   	select HAVE_FUNCTION_DESCRIPTORS	if PPC64_ELF_ABI_V1
>   	select HAVE_FUNCTION_ERROR_INJECTION
> +	select HAVE_FUNCTION_GRAPH_FREGS
>   	select HAVE_FUNCTION_GRAPH_TRACER
>   	select HAVE_FUNCTION_TRACER		if !COMPILE_TEST && (PPC64 || (PPC32 && CC_IS_GCC))
>   	select HAVE_GCC_PLUGINS			if GCC_VERSION >= 50200   # plugin support on gcc <= 5.1 is buggy on PPC
> diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
> index 82da7c7a1d12..6ffc9c9cf4e3 100644
> --- a/arch/powerpc/include/asm/ftrace.h
> +++ b/arch/powerpc/include/asm/ftrace.h
> @@ -50,6 +50,21 @@ static __always_inline struct pt_regs *arch_ftrace_get_regs(struct ftrace_regs *
>   		asm volatile("mfmsr %0" : "=r" ((_regs)->msr));	\
>   	} while (0)
>   
> +#undef ftrace_regs_get_return_value
> +static __always_inline unsigned long
> +ftrace_regs_get_return_value(const struct ftrace_regs *fregs)
> +{
> +	return arch_ftrace_regs(fregs)->regs.gpr[3];
> +}
> +#define ftrace_regs_get_return_value ftrace_regs_get_return_value
> +
> +#undef ftrace_regs_get_frame_pointer
> +static __always_inline unsigned long
> +ftrace_regs_get_frame_pointer(const struct ftrace_regs *fregs)
> +{
> +	return arch_ftrace_regs(fregs)->regs.gpr[1];
> +}
> +
>   static __always_inline void
>   ftrace_regs_set_instruction_pointer(struct ftrace_regs *fregs,
>   				    unsigned long ip)
> diff --git a/arch/powerpc/kernel/trace/ftrace_entry.S b/arch/powerpc/kernel/trace/ftrace_entry.S
> index 3565c67fc638..3da14d8271d9 100644
> --- a/arch/powerpc/kernel/trace/ftrace_entry.S
> +++ b/arch/powerpc/kernel/trace/ftrace_entry.S
> @@ -409,23 +409,31 @@ EXPORT_SYMBOL(_mcount)
>   _GLOBAL(return_to_handler)
>   	/* need to save return values */
>   #ifdef CONFIG_PPC64
> -	std	r4,  -32(r1)
> -	std	r3,  -24(r1)
> +	stdu	r1, -SWITCH_FRAME_SIZE(r1)
> +	std	r4, GPR4(r1)
> +	std	r3, GPR3(r1)
> +	/* Save previous stack pointer (r1) */
> +	addi	r3, r1, SWITCH_FRAME_SIZE
> +	std	r3, GPR1(r1)
>   	/* save TOC */
> -	std	r2,  -16(r1)
> -	std	r31, -8(r1)
> +	std	r2, 24(r1)
> +	std	r31, 32(r1)
>   	mr	r31, r1
> -	stdu	r1, -112(r1)
> -
> +	/* pass ftrace_regs/pt_regs to ftrace_return_to_handler */
> +	addi	r3, r1, STACK_INT_FRAME_REGS
>   	/*
>   	 * We might be called from a module.
>   	 * Switch to our TOC to run inside the core kernel.
>   	 */
>   	LOAD_PACA_TOC()
>   #else
> -	stwu	r1, -16(r1)
> -	stw	r3, 8(r1)
> -	stw	r4, 12(r1)
> +	stwu	r1, -SWITCH_FRAME_SIZE(r1)
> +	stw	r4, GPR4(r1)
> +	stw	r3, GPR3(r1)
> +	addi	r3, r1, SWITCH_FRAME_SIZE
> +	stw	r3, GPR1(r1)
> +	/* pass ftrace_regs/pt_regs to ftrace_return_to_handler */
> +	addi	r3, r1, STACK_INT_FRAME_REGS
>   #endif
> 
>   
>   	bl	ftrace_return_to_handler
> @@ -435,15 +443,15 @@ _GLOBAL(return_to_handler)
>   	mtlr	r3
>   
>   #ifdef CONFIG_PPC64
> -	ld	r1, 0(r1)
> -	ld	r4,  -32(r1)
> -	ld	r3,  -24(r1)
> -	ld	r2,  -16(r1)
> -	ld	r31, -8(r1)
> +	ld	r4,  GPR4(r1)
> +	ld	r3,  GPR3(r1)
> +	ld	r2,  24(r1)
> +	ld	r31, 32(r1)
> +	ld	r1,  0(r1)
>   #else
> -	lwz	r3, 8(r1)
> -	lwz	r4, 12(r1)
> -	addi	r1, r1, 16
> +	lwz	r3, GPR3(r1)
> +	lwz	r4, GPR4(r1)
> +	addi	r1, r1, SWITCH_FRAME_SIZE
>   #endif
>   
>   	/* Jump back to real return address */