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Date: Thu, 23 May 2024 19:10:31 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>, Florent Revest
 <revest@chromium.org>, linux-trace-kernel@vger.kernel.org, LKML
 <linux-kernel@vger.kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>,
 bpf <bpf@vger.kernel.org>, Sven Schnelle <svens@linux.ibm.com>, Alexei
 Starovoitov <ast@kernel.org>, Jiri Olsa <jolsa@kernel.org>, Arnaldo
 Carvalho de Melo <acme@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>,
 Alan Maguire <alan.maguire@oracle.com>, Mark Rutland
 <mark.rutland@arm.com>, Peter Zijlstra <peterz@infradead.org>, Thomas
 Gleixner <tglx@linutronix.de>, Guo Ren <guoren@kernel.org>
Subject: Re: [PATCH v10 01/36] tracing: Add a comment about ftrace_regs
 definition
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On Tue,  7 May 2024 23:08:12 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> To clarify what will be expected on ftrace_regs, add a comment to the
> architecture independent definition of the ftrace_regs.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Acked-by: Mark Rutland <mark.rutland@arm.com>
> ---
>  Changes in v8:
>   - Update that the saved registers depends on the context.
>  Changes in v3:
>   - Add instruction pointer
>  Changes in v2:
>   - newly added.
> ---
>  include/linux/ftrace.h |   26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 54d53f345d14..b81f1afa82a1 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -118,6 +118,32 @@ extern int ftrace_enabled;
>  
>  #ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
>  
> +/**
> + * ftrace_regs - ftrace partial/optimal register set
> + *
> + * ftrace_regs represents a group of registers which is used at the
> + * function entry and exit. There are three types of registers.
> + *
> + * - Registers for passing the parameters to callee, including the stack
> + *   pointer. (e.g. rcx, rdx, rdi, rsi, r8, r9 and rsp on x86_64)
> + * - Registers for passing the return values to caller.
> + *   (e.g. rax and rdx on x86_64)
> + * - Registers for hooking the function call and return including the
> + *   frame pointer (the frame pointer is architecture/config dependent)
> + *   (e.g. rip, rbp and rsp for x86_64)
> + *
> + * Also, architecture dependent fields can be used for internal process.
> + * (e.g. orig_ax on x86_64)
> + *
> + * On the function entry, those registers will be restored except for
> + * the stack pointer, so that user can change the function parameters
> + * and instruction pointer (e.g. live patching.)
> + * On the function exit, only registers which is used for return values
> + * are restored.

I wonder if we should also add a note about some architectures in some
circumstances may store all pt_regs in ftrace_regs. For example, if an
architecture supports FTRACE_WITH_REGS, it may pass the pt_regs within the
ftrace_regs. If that is the case, then ftrace_get_regs() called on it will
return a pointer to a valid pt_regs, or NULL if it is not supported or the
ftrace_regs does not have a all the registers.

-- Steve


> + *
> + * NOTE: user *must not* access regs directly, only do it via APIs, because
> + * the member can be changed according to the architecture.
> + */
>  struct ftrace_regs {
>  	struct pt_regs		regs;
>  };