netdev - Re: [RFC bpf-next 00/16] bpf: Speed up trampoline attach

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Date:   Tue, 27 Oct 2020 09:14:01 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, Daniel Xu <dxu@...uu.xyz>,
        Jesper Brouer <jbrouer@...hat.com>,
        Toke Høiland-Jørgensen 
        <toke@...hat.com>, Viktor Malik <vmalik@...hat.com>
Subject: Re: [RFC bpf-next 00/16] bpf: Speed up trampoline attach

On Mon, 26 Oct 2020 21:30:14 -0700
Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> > Direct calls wasn't added so that bpf and ftrace could co-exist, it was
> > that for certain cases, bpf wanted a faster way to access arguments,
> > because it still worked with ftrace, but the saving of regs was too
> > strenuous.  
> 
> Direct calls in ftrace were done so that ftrace and trampoline can co-exist.
> There is no other use for it.

What does that even mean? And I'm guessing when you say "trampoline"
you mean a "bpf trampoline" because "trampoline" is used for a lot more
than bpf, and bpf does not own that term.

Do you mean, "direct calls in ftrace were done so that bpf trampolines
could work". Remember, ftrace has a lot of users, and it must remain
backward compatible.

-- Steve