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Subject: Re: [PATCH 3/3] MIPS: ftrace: Add DYNAMIC_FTRACE_WITH_REGS support
To:     Steven Rostedt <rostedt@goodmis.org>
References: <1612080878-5426-1-git-send-email-hejinyang@loongson.cn>
 <1612080878-5426-3-git-send-email-hejinyang@loongson.cn>
 <20210201095612.62e62544@gandalf.local.home>
Cc:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Ingo Molnar <mingo@redhat.com>,
        Wu Zhangjin <wuzhangjin@gmail.com>, linux-mips@vger.kernel.org,
        linux-kernel@vger.kernel.org, Huacai Chen <chenhuacai@kernel.org>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Tiezhu Yang <yangtiezhu@loongson.cn>
From:   Jinyang He <hejinyang@loongson.cn>
Message-ID: <01a8f2e1-b549-b2ce-37fd-7cc4bb516107@loongson.cn>
Date:   Tue, 2 Feb 2021 20:21:48 +0800
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On 02/01/2021 10:56 PM, Steven Rostedt wrote:

> On Sun, 31 Jan 2021 16:14:38 +0800
> Jinyang He <hejinyang@loongson.cn> wrote:
>
>> In the past, we have always used the address of _mcount as the address of
>> ftrace_caller. It reduces one ftrace_modify_code operation when do ftrace
>> on modules on 64Bit platform in this way. In order to provide
>> DYNAMIC_FTRACE_WITH_REGS, we have to take _mcount out of ftrace_caller and
>> add a new definition of _mcount. It is necessary to modify 2 instructions.
>> Also add the definition of ftrace_regs_caller. ftrace_regs_caller will
>> store and restore more registers. Of course, some functions in ftrace.c
>> also need to consider ftrace_regs_caller. Modify these functions and add
>> the related code of ftrace_regs_caller.
> Note, while you are making these changes, you may want to look at the new
> feature of ftrace that has HAVE_DYNAMIC_FTRACE_WITH_ARGS.
>
> I noticed that with x86 (and probably all other archs), you need to save
> the arguments before calling the ftrace callbacks in the ftrace trampoline.
> If done properly, this means that the callbacks should be able to access
> the function arguments. What happens then, it structures the arguments in a
> way that if the function was called with "WITH_REGS" set, its the full
> pt_regs structure. Otherwise, it's a partial structure called "ftrace_regs".
>
>
> See arch/x86/include/asm/ftrace.h for ftrace_regs.
>
> Then the ftrace_regs is passed to the callback instead of pt_regs (for all
> callbacks!).
>
> If a callback has the REGS flag set (ftrace_caller_regs), then to get the
> pt_regs, it needs to call:
>
> 	struct pt_regs *regs = arch_ftrace_get_regs(ftrace_regs);
>
> Where arch_ftrace_get_regs() returns the full pt_regs if the callback was
> called from a ftrace_caller_regs trampoline, otherwise it must return NULL.
>
> The reason to return NULL is that we don't want callbacks using pt_regs,
> thinking it's fully populated when it is not.
>
> But if HAVE_DYNAMIC_FTRACE_ARGS is set, then all ftrace callbacks
> (regardless of REGS flag being set) has access to the arguments from the
> ftrace_regs.
>
> -- Steve
Hi, Steve,

Thank you for your comment. It really helps a lot. It's time to learn more!


Thanks, :-)
Jinyang