linux-kernel - Re: [PATCH v2 09/13] powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH

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Message-ID: <5c7b5334-6071-f131-a509-9a49ca3d628c@csgroup.eu>
Date:   Tue, 15 Feb 2022 15:51:58 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Ingo Molnar <mingo@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>
Subject: Re: [PATCH v2 09/13] powerpc/ftrace: Implement
 CONFIG_DYNAMIC_FTRACE_WITH_ARGS

+ 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@...roup.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@...roup.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 ?
>>
>> 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 ?
> 

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.

Christophe