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

[Christophe Leroy]

Le 28/05/2025 à 15:48, Aditya Bodkhe a écrit :
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> 
> From: Aditya Bodkhe <aditya.b1 at linux.ibm.com>
> 
> 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 powerpc64, ensuring that
> function graph return value tracing is available on this architecture.

Why only powerpc64 ?

I see nothing specific to powerpc64 in your patch, will it work on 
powerpc32 too ?

> 
> 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 <aditya.b1 at linux.ibm.com>
> ---
>   arch/powerpc/Kconfig                     |  1 +
>   arch/powerpc/include/asm/ftrace.h        | 15 +++++++++
>   arch/powerpc/kernel/trace/ftrace_entry.S | 41 ++++++++++++++----------
>   3 files changed, 40 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];
> +}
> +

Why unset and redefine ftrace_regs_get_return_value() and 
ftrace_regs_get_frame_pointer() ? Please explain why the default ones 
can't be used on powerpc.

>   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..eafbfb7584ed 100644
> --- a/arch/powerpc/kernel/trace/ftrace_entry.S
> +++ b/arch/powerpc/kernel/trace/ftrace_entry.S
> @@ -409,23 +409,30 @@ 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

Some of the changes seems to only be renaming and should be done in a 
cleanup/preparatory patch in order to only focus on real necessary 
changes in this patch.

>          /*
>           * 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)

Why do we need such a big frame size just to save two registers ?

> +       stw     r4, GPR4(r1)
> +       stw     r3, GPR3(r1)
> +       addi    r3, r1, SWITCH_FRAME_SIZE
> +       stw     r3, GPR1(r1)
 > +       addi    r3, r1, STACK_INT_FRAME_REGS

Why is this needed ?

>   #endif
> 
>          bl      ftrace_return_to_handler
> @@ -435,15 +442,15 @@ _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 */
> --
> 2.43.5
>