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

On 03/06/25 2:22 pm, Christophe Leroy wrote:

Le 28/05/2025 à 15:48, Aditya Bodkhe a écrit :

quoted

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From: Aditya Bodkhe <redacted>

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 ?

That's a oversight on my part . This patch necessary changes for 
powerpc32 bit and tested this feature to be working as expected on the same.

quoted

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 <redacted>
---
  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.

Because for powerpc currently for ftrace_regs_get_return_value() this 
function regs_return_value() is getting called and it's implementation 
returns -regs.gpr[3] if is_syscall_success() returns false . This was 
causing fails in ftrace selftests .

Also for ftrace_regs_get_frame_pointer() current implementation was 
always returning 0 hence we had to change that too .

quoted

  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.

Now the way we are setting up the frame we cannot have negative offsets 
, hence we used offsets which are defined already.

quoted

         /*
          * 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 ?

This Frame size is needed because we need pt_regs as part of frame , are 
you saying instead of SWITCH_FRAME_SIZE for 32 bit architecture we 
should just setup a frame with size of pt_regs because ftrace_regs size 
is same as pt_regs on ppc

quoted

+       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 ?

The function ftrace_return_to_handler takes pointer to pt_regs as 
arguement so we are setting that up in stack and passing that pointer.

quoted

  #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