Re: [PATCH v2 1/2] powerpc/ftrace: support CONFIG_FUNCTION_GRAPH_RETVAL
From: Christophe Leroy <hidden>
Date: 2025-08-04 14:50:35
Also in:
linuxppc-dev, lkml
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 <redacted>Reviewed-by: Christophe Leroy <redacted>
quoted hunk ↗ jump to hunk
--- 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 PPCdiff --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 */