linux-kernel - Re: [PATCH v9 01/36] tracing: Add a comment about ftrace

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Date: Wed, 24 Apr 2024 15:19:24 +0200
From: Florent Revest <revest@...omium.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Steven Rostedt <rostedt@...dmis.org>, 
	linux-trace-kernel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, bpf <bpf@...r.kernel.org>, 
	Sven Schnelle <svens@...ux.ibm.com>, Alexei Starovoitov <ast@...nel.org>, Jiri Olsa <jolsa@...nel.org>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Alan Maguire <alan.maguire@...cle.com>, Mark Rutland <mark.rutland@....com>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH v9 01/36] tracing: Add a comment about ftrace_regs definition

On Wed, Apr 24, 2024 at 2:23 PM Florent Revest <revest@...omium.org> wrote:
>
> On Mon, Apr 15, 2024 at 2:49 PM Masami Hiramatsu (Google)
> <mhiramat@...nel.org> wrote:
> >
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.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@...nel.org>
> > Acked-by: Mark Rutland <mark.rutland@....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)
>
> Ooc, have we ever considered skipping argument registers that are not
> return value registers in the exit code paths ? For example, why would
> we want to save rdi in a return handler ?
>
> But if we want to avoid the situation of having "sparse ftrace_regs"
> all over again, we'd have to split ftrace_regs into a ftrace_args_regs
> and a ftrace_ret_regs which would make this refactoring even more
> painful, just to skip a few instructions. :|
>
> I don't necessarily think it's worth it, I just wanted to make sure
> this was considered.

Ah, well, I just reached patch 22 and noticed that there you add add:

+ * Basically, ftrace_regs stores the registers related to the context.
+ * On function entry, registers for function parameters and hooking the
+ * function call are stored, and on function exit, registers for function
+ * return value and frame pointers are stored.

So ftrace_regs can be a a sparse structure then. That's fair enough with me! ;)