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Message-ID: <1e79fee0-2845-498f-bd16-4589edf19df7@linux.ibm.com>
Date: Fri, 13 Jun 2025 13:20:17 +0530
From: Aditya Bodkhe <adityab1@...ux.ibm.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
linuxppc-dev@...ts.ozlabs.org
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
maddy@...ux.ibm.com, mpe@...erman.id.au, npiggin@...il.com,
rostedt@...dmis.org, mhiramat@...nel.org, mark.rutland@....com,
Aditya Bodkhe <aditya.b1@...ux.ibm.com>
Subject: 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 :
>> [Vous ne recevez pas souvent de courriers de adityab1@...ux.ibm.com.
>> Découvrez pourquoi ceci est important à
>> https://aka.ms/LearnAboutSenderIdentification ]
>>
>> From: Aditya Bodkhe <aditya.b1@...ux.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 ?
>
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.
>>
>> 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@...ux.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.
>
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 .
>> 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.
>> /*
>> * 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
>> + 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.
>> #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
>>
>
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