Re: [PATCH 16/17] powerpc/ftrace: Add support for -fpatchable-function-entry

[Naveen N Rao <naveen@kernel.org>]

Christophe Leroy wrote:
> 
> 
> Le 19/06/2023 à 11:47, Naveen N Rao a écrit :
>> GCC v13.1 updated support for -fpatchable-function-entry on ppc64le to
>> emit nops after the local entry point, rather than before it. This
>> allows us to use this in the kernel for ftrace purposes. A new script is
>> added under arch/powerpc/tools/ to help detect if nops are emitted after
>> the function local entry point, or before the global entry point.
>> 
>> With -fpatchable-function-entry, we no longer have the profiling
>> instructions generated at function entry, so we only need to validate
>> the presence of two nops at the ftrace location in ftrace_init_nop(). We
>> patch the preceding instruction with 'mflr r0' to match the
>> -mprofile-kernel ABI for subsequent ftrace use.
>> 
>> This changes the profiling instructions used on ppc32. The default -pg
>> option emits an additional 'stw' instruction after 'mflr r0' and before
>> the branch to _mcount 'bl _mcount'. This is very similar to the original
>> -mprofile-kernel implementation on ppc64le, where an additional 'std'
>> instruction was used to save LR to its save location in the caller's
>> stackframe. Subsequently, this additional store was removed in later
>> compiler versions for performance reasons. The same reasons apply for
>> ppc32 so we only patch in a 'mflr r0'.
>> 
>> Signed-off-by: Naveen N Rao <naveen@kernel.org>
> 
> Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> 
> Nit below
> 
>> ---
>>   arch/powerpc/Kconfig                          | 14 +++++++---
>>   arch/powerpc/Makefile                         |  5 ++++
>>   arch/powerpc/include/asm/ftrace.h             |  6 +++--
>>   arch/powerpc/include/asm/vermagic.h           |  4 ++-
>>   arch/powerpc/kernel/module_64.c               |  2 +-
>>   arch/powerpc/kernel/trace/ftrace.c            | 14 ++++++++--
>>   arch/powerpc/kernel/trace/ftrace_entry.S      |  2 ++
>>   .../gcc-check-fpatchable-function-entry.sh    | 26 +++++++++++++++++++
>>   8 files changed, 64 insertions(+), 9 deletions(-)
>>   create mode 100755 arch/powerpc/tools/gcc-check-fpatchable-function-entry.sh
>> 
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index bff5820b7cda14..9352d8e68152e1 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -187,6 +187,7 @@ config PPC
>>   	select DYNAMIC_FTRACE			if FUNCTION_TRACER
>>   	select EDAC_ATOMIC_SCRUB
>>   	select EDAC_SUPPORT
>> +	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY if ARCH_USING_PATCHABLE_FUNCTION_ENTRY
>>   	select GENERIC_ATOMIC64			if PPC32
>>   	select GENERIC_CLOCKEVENTS_BROADCAST	if SMP
>>   	select GENERIC_CMOS_UPDATE
>> @@ -227,8 +228,8 @@ config PPC
>>   	select HAVE_DEBUG_KMEMLEAK
>>   	select HAVE_DEBUG_STACKOVERFLOW
>>   	select HAVE_DYNAMIC_FTRACE
>> -	select HAVE_DYNAMIC_FTRACE_WITH_ARGS	if MPROFILE_KERNEL || PPC32
>> -	select HAVE_DYNAMIC_FTRACE_WITH_REGS	if MPROFILE_KERNEL || PPC32
>> +	select HAVE_DYNAMIC_FTRACE_WITH_ARGS	if ARCH_USING_PATCHABLE_FUNCTION_ENTRY || MPROFILE_KERNEL || PPC32
>> +	select HAVE_DYNAMIC_FTRACE_WITH_REGS	if ARCH_USING_PATCHABLE_FUNCTION_ENTRY || MPROFILE_KERNEL || PPC32
> 
> ARCH_USING_PATCHABLE_FUNCTION_ENTRY defaults to y if PPC32, so you can 
> remove PPC32 from the condition here.

ARCH_USING_PATCHABLE_FUNCTION_ENTRY also depends on compiler support for 
-fpatchable-function-entry, and gcc v5.x doesn't support that. So, we 
need to retain PPC32 here.

- Naveen

> 
>>   	select HAVE_EBPF_JIT
>>   	select HAVE_EFFICIENT_UNALIGNED_ACCESS
>>   	select HAVE_FAST_GUP
>> @@ -256,7 +257,7 @@ config PPC
>>   	select HAVE_MOD_ARCH_SPECIFIC
>>   	select HAVE_NMI				if PERF_EVENTS || (PPC64 && PPC_BOOK3S)
>>   	select HAVE_OPTPROBES
>> -	select HAVE_OBJTOOL			if PPC32 || MPROFILE_KERNEL
>> +	select HAVE_OBJTOOL			if ARCH_USING_PATCHABLE_FUNCTION_ENTRY || MPROFILE_KERNEL || PPC32
> 
> Same
> 
>>   	select HAVE_OBJTOOL_MCOUNT		if HAVE_OBJTOOL
>>   	select HAVE_PERF_EVENTS
>>   	select HAVE_PERF_EVENTS_NMI		if PPC64
>> @@ -550,6 +551,13 @@ config MPROFILE_KERNEL
>>   	depends on PPC64 && CPU_LITTLE_ENDIAN && FUNCTION_TRACER
>>   	def_bool $(success,$(srctree)/arch/powerpc/tools/gcc-check-mprofile-kernel.sh $(CC) -I$(srctree)/include -D__KERNEL__)
>>   
>> +config ARCH_USING_PATCHABLE_FUNCTION_ENTRY
>> +	depends on FUNCTION_TRACER && (PPC32 || PPC64_ELF_ABI_V2)
>> +	depends on $(cc-option,-fpatchable-function-entry=2)
>> +	def_bool y if PPC32
>> +	def_bool $(success,$(srctree)/arch/powerpc/tools/gcc-check-fpatchable-function-entry.sh $(CC) -mlittle-endian) if PPC64 && CPU_LITTLE_ENDIAN
>> +	def_bool $(success,$(srctree)/arch/powerpc/tools/gcc-check-fpatchable-function-entry.sh $(CC) -mbig-endian) if PPC64 && CPU_BIG_ENDIAN
>> +
...