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Date:   Mon, 15 Aug 2022 17:02:03 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Jiri Olsa <olsajiri@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, bpf <bpf@vger.kernel.org>,
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        Hao Luo <haoluo@google.com>,
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        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [RFC] ftrace: Add support to keep some functions out of ftrace
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On Mon, Aug 15, 2022 at 07:45:16AM -0700, Alexei Starovoitov wrote:
> On Mon, Aug 15, 2022 at 7:33 AM Peter Zijlstra <peterz@infradead.org> wrote:

> > It is in full control of the 'call __fentry__'. Absolute full NAK on you
> > trying to make it otherwise.
> 
> Don't mix 'call fentry' generated by the compiler with nop5 inserted
> by macroses or JITs.

Looking at:

 https://lore.kernel.org/all/20191211123017.13212-3-bjorn.topel@gmail.com/

this seems to want to prod at the __fentry__ sites.

> > > Soon we will have nop5 in the middle of the function.
> > > ftrace must not touch it.
> >
> > How are you generating that NOP and what for?
> 
> We're generating nop5-s in JITed code to further
> attach to.

Ftrace doesn't know about those; so how can it break that?

Likewise it doesn't know about the static_branch/static_call NOPs and
nothing is broken.

Ftrace only knows about the __fentry__ sites, and it *does* own them.
Are you saying ftrace is writing to a code location not a __fentry__
site?