linux-kernel - Re: [RFC][PATCH 0/2] ftrace: Add access to function arguments for all callbacks

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Message-ID: <20201028230305.zhtpwektsehh54bv@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 28 Oct 2020 16:03:05 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC][PATCH 0/2] ftrace: Add access to function arguments for
 all callbacks

On Wed, Oct 28, 2020 at 06:07:53PM -0400, Steven Rostedt wrote:
> On Wed, 28 Oct 2020 14:33:03 -0700
> Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> > I don't have strong opinion on this feature, but if you want to have it
> > please add a giant disclaimer that this is going to be x86-64 and, may be arm64,
> > feature _forever_. On x86-32 and other architectures there is no way
> > to provide sane regs_get_kernel_argument(regs, n) semantics by blindly
> > saving registers. The kernel needs to know types and calling convention
> > of the architecture. That's the reason bpf side has btf_func_model concept.
> > To make sure that bpf trampoline can support all architectures in the future.
> 
> The trampoline is to only provide the information to get the arguments.
> Yes, it is up to the callback that registered it that knows how to get them.
> 
> What does the bpf trampoline need to get an argument if it already
> knows the type. If it has access to a subset of necessary registers and
> the stack pointer of when the function was called, is it possible to
> figure out the arguments?
> 
> This code is not about a "regs_get_kernel_argument(regs, n)", that was
> only used to show that *that* could be done. Because currently, there's
> *no* way to get those arguments without setting the SAVE_REGS flag,
> which saves way more than necessary. I only used that to test the code
> in a generic way. Ideally, ftrace could use btf to figure out the
> actual arguments of the function if it had a mapping.
> 
> I believe the reason you gave up on using ftrace was because of the
> overhead of the REGS code. If that wasn't the case, and you didn't see
> that overhead, would you have used ftrace?
> 
> In fact, did you look at all to see if you could modify the current
> infrastructure to make it work for you? Or did you just give up to
> write your own from scratch?

I don't understand what you're trying to say.
Are we still discussing your patch or something else?
If your patch then, I said, I'm not against it. I just don't see
a value in it. Since you're agree that regs_get_kernel_argument()
is misleading then what's the point of the patch?
To make mini-SAVE_REGS version? For what kind of use case?