linux-kernel - Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines

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Date:   Fri, 22 Apr 2022 11:45:41 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Wang ShaoBo <bobo.shaobowang@...wei.com>, cj.chengjian@...wei.com,
        huawei.libin@...wei.com, xiexiuqi@...wei.com, liwei391@...wei.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        catalin.marinas@....com, will@...nel.org, zengshun.wu@...look.com,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically
 allocated trampolines

On Fri, 22 Apr 2022 11:12:39 +0100
Mark Rutland <mark.rutland@....com> wrote:

> As an aside, I'd also love to remove the REGS/!REGs distinction, and always
> save a minimum amount of state (like ARGS, but never saving a full pt_regs),
> since on arm64 the extra state stored for the REGS case isn't useful (and we
> can't reliably capture all of the pt_regs state anyway, so bits of it are made
> up or not filled in).

Note, the reason for the addition of REGS was a requirement of kprobes.
Because before ftrace, kprobes would be triggered at the start of a
function by a breakpoint that would load in all the regs. And for backward
compatibility, Masami wanted to make sure that kprobes coming from ftrace
had all the regs just like it had when coming from a breakpoint.

IIUC, kprobes is the only reason we have the "regs" variant (all other use
cases could get by with the ARGS version).

-- Steve