linux-kernel - Re: [RFC][PATCH] ftrace/x86: Remove mcount support

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Date:   Thu, 9 May 2019 22:03:46 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        live-patching@...r.kernel.org, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC][PATCH] ftrace/x86: Remove mcount support

On Thu, 9 May 2019, Steven Rostedt wrote:

> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> There's two methods of enabling function tracing in Linux on x86. One is
> with just "gcc -pg" and the other is "gcc -pg -mfentry". The former will use
> calls to a special function "mcount" after the frame is set up in all C
> functions. The latter will add calls to a special function called "fentry"
> as the very first instruction of all C functions.
> 
> At compile time, there is a check to see if gcc supports, -mfentry, and if
> it does, it will use that, because it is more versatile and less error prone
> for function tracing.
> 
> Starting with v4.19, the minimum gcc supported to build the Linux kernel,
> was raised to version 4.6. That also happens to be the first gcc version to
> support -mfentry. Since on x86, using gcc versions from 4.6 and beyond will
> unconditionally enable the -mfentry, it will no longer use mcount as the
> method for inserting calls into the C functions of the kernel. This means
> that there is no point in continuing to maintain mcount in x86.
> 
> Remove support for using mcount. This makes the code less complex, and will
> also allow it to be simplified in the future.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Thanks; the fact that mcount happens only after the prologue has already 
happened makes it unusuable anyway for anything nontrivial.

	Acked-by: Jiri Kosina <jkosina@...e.cz>

-- 
Jiri Kosina
SUSE Labs