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Date:   Tue, 1 Aug 2023 20:40:54 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        linux-trace-kernel@vger.kernel.org,
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        Arnaldo Carvalho de Melo <acme@kernel.org>,
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        Florent Revest <revest@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a
 struct/union
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On Wed, 2 Aug 2023 09:21:46 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> > Then use kprobes. When I asked Masami what the difference between fprobes
> > and kprobes was, he told me that it would be that it would no longer rely
> > on the slower FTRACE_WITH_REGS. But currently, it still does.  
> 
> kprobes needs to keep using pt_regs because software-breakpoint exception
> handler gets that. And fprobe is used for bpf multi-kprobe interface,
> but I think it can be optional.
> 
> So until user-land tool supports the ftrace_regs, you can just disable
> using fprobes if CONFIG_DYNAMIC_FTRACE_WITH_REGS=n

I'm confused. I asked about the difference between kprobes on ftrace
and fprobes, and you said it was to get rid of the requirement of
FTRACE_WITH_REGS.

 https://lore.kernel.org/all/20230120205535.98998636329ca4d5f8325bc3@kernel.org/

> 
> Then you can safely use 
> 
> struct pt_regs *regs = ftrace_get_regs(fregs);
> 
> I think we can just replace the CONFIG_FPROBE ifdefs with
> CONFIG_DYNAMIC_FTRACE_WITH_REGS in kernel/trace/bpf_trace.c
> And that will be the first version of using ftrace_regs in fprobe.

But it is still slow. The FTRACE_WITH_REGS gives us the full pt_regs
and saves all registers including flags, which is a very slow operation
(and noticeable in profilers).

And this still doesn't work on arm64.

Maybe we can add a ftrace_partial_regs(fregs) that returns a
partially filled pt_regs, and the caller that uses this obviously knows
its partial (as it's in the name). But this doesn't quite help out arm64
because unlike x86, struct ftrace_regs does not contain an address
compatibility with pt_regs fields. It would need to do a copy.

 ftrace_partial_regs(fregs, &regs) ?

-- Steve