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

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
	Jiri Kosina <jikos@kernel.org>,
	Joe Lawrence <joe.lawrence@redhat.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Miroslav Benes <mbenes@suse.cz>, Ingo Molnar <mingo@redhat.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
	"live-patching@vger.kernel.org" <live-patching@vger.kernel.org>
Subject: Re: [PATCH v2 09/13] powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS
Date: Tue, 15 Feb 2022 14:28:04 +0000	[thread overview]
Message-ID: <6dc50f09-4d14-afa2-d2a1-34b72b880edf@csgroup.eu> (raw)
In-Reply-To: <1644930705.g64na2kgvd.naveen@linux.ibm.com>

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 ?
> 
> 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 ?

Thanks
Christophe