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

[Aditya Bodkhe <adityab1@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 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>
---

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 */
-- 
2.50.0