Re: [PATCH v2 09/13] powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Christophe Leroy wrote:
> + S390 people
> 
> Le 15/02/2022 à 15:28, Christophe Leroy a écrit :
>> 
>> 
>> Le 15/02/2022 à 14:36, Naveen N. Rao a écrit :
>>> Michael Ellerman wrote:
>>>> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>>>>> Le 14/02/2022 à 16:25, Naveen N. Rao a écrit :
>>>>>> Christophe Leroy wrote:
>>>>>>> Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS. It accelerates the call
>>>>>>> of livepatching.
>>>>>>>
>>>>>>> Also note that powerpc being the last one to convert to
>>>>>>> CONFIG_DYNAMIC_FTRACE_WITH_ARGS, it will now be possible to remove
>>>>>>> klp_arch_set_pc() on all architectures.
>>>>>>>
>>>>>>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>>>>>>> ---
>>>>>>>  arch/powerpc/Kconfig                 |  1 +
>>>>>>>  arch/powerpc/include/asm/ftrace.h    | 17 +++++++++++++++++
>>>>>>>  arch/powerpc/include/asm/livepatch.h |  4 +---
>>>>>>>  3 files changed, 19 insertions(+), 3 deletions(-)
>>>>>>>
>>>>>>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>>>>>>> index cdac2115eb00..e2b1792b2aae 100644
>>>>>>> --- a/arch/powerpc/Kconfig
>>>>>>> +++ b/arch/powerpc/Kconfig
>>>>>>> @@ -210,6 +210,7 @@ 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_EBPF_JIT
>>>>>>>      select HAVE_EFFICIENT_UNALIGNED_ACCESS    if 
>>>>>>> !(CPU_LITTLE_ENDIAN && POWER7_CPU)
>>>>>>> diff --git a/arch/powerpc/include/asm/ftrace.h 
>>>>>>> b/arch/powerpc/include/asm/ftrace.h
>>>>>>> index b3f6184f77ea..45c3d6f11daa 100644
>>>>>>> --- a/arch/powerpc/include/asm/ftrace.h
>>>>>>> +++ b/arch/powerpc/include/asm/ftrace.h
>>>>>>> @@ -22,6 +22,23 @@ static inline unsigned long 
>>>>>>> ftrace_call_adjust(unsigned long addr)
>>>>>>>  struct dyn_arch_ftrace {
>>>>>>>      struct module *mod;
>>>>>>>  };
>>>>>>> +
>>>>>>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
>>>>>>> +struct ftrace_regs {
>>>>>>> +    struct pt_regs regs;
>>>>>>> +};
>>>>>>> +
>>>>>>> +static __always_inline struct pt_regs 
>>>>>>> *arch_ftrace_get_regs(struct ftrace_regs *fregs)
>>>>>>> +{
>>>>>>> +    return &fregs->regs;
>>>>>>> +}
>>>>>>
>>>>>> I think this is wrong. We need to differentiate between 
>>>>>> ftrace_caller() and ftrace_regs_caller() here, and only return 
>>>>>> pt_regs if coming in through ftrace_regs_caller() (i.e., 
>>>>>> FL_SAVE_REGS is set).
>>>>>
>>>>> Not sure I follow you.
>>>>>
>>>>> This is based on 5740a7c71ab6 ("s390/ftrace: add 
>>>>> HAVE_DYNAMIC_FTRACE_WITH_ARGS support")
>>>>>
>>>>> It's all the point of HAVE_DYNAMIC_FTRACE_WITH_ARGS, have the regs 
>>>>> also with ftrace_caller().
>>>>>
>>>>> Sure you only have the params, but that's the same on s390, so what 
>>>>> did I miss ?

Steven has explained the rationale for this in his other response:
https://lore.kernel.org/all/20220215093849.556d5444@gandalf.local.home/

>>>
>>> It looks like s390 is special since it apparently saves all registers 
>>> even for ftrace_caller: 
>>> https://lore.kernel.org/all/YbipdU5X4HNDWIni@osiris/
>> 
>> It is not what I understand from their code, see 
>> https://elixir.bootlin.com/linux/v5.17-rc3/source/arch/s390/kernel/mcount.S#L37 
>> 
>> 
>> They have a common macro called with argument 'allregs' which is set to 
>> 0 for ftrace_caller() and 1 for ftrace_regs_caller().
>> When allregs == 1, the macro seems to save more.
>> 
>> But ok, I can do like x86, but I need a trick to know whether 
>> FL_SAVE_REGS is set or not, like they do with fregs->regs.cs
>> Any idea what the condition can be for powerpc ?

We'll need to explicitly zero-out something in pt_regs in 
ftrace_caller(). We can probably use regs->msr since we don't expect it 
to be zero when saved from ftrace_regs_caller().

>> 
> 
> Finally, it looks like this change is done  via commit 894979689d3a 
> ("s390/ftrace: provide separate ftrace_caller/ftrace_regs_caller 
> implementations") four hours the same day after the implementation of 
> arch_ftrace_get_regs()
> 
> They may have forgotten to change arch_ftrace_get_regs() which was added 
> in commit 5740a7c71ab6 ("s390/ftrace: add HAVE_DYNAMIC_FTRACE_WITH_ARGS 
> support") with the assumption that ftrace_caller and ftrace_regs_caller 
> where identical.

Indeed, good find!


Thanks,
Naveen