linux-kernel - Re: [PATCH v5 05/12] x86/retpoline/ftrace: Convert ftrace assembler indirect jumps

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Date:   Sat, 6 Jan 2018 20:53:48 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v5 05/12] x86/retpoline/ftrace: Convert ftrace assembler
 indirect jumps

On Sat, 6 Jan 2018, Linus Torvalds wrote:

> On Sat, Jan 6, 2018 at 3:49 AM, David Woodhouse <dwmw@...zon.co.uk> wrote:
> >
> > -       call    *ftrace_trace_function
> > +       movl    ftrace_trace_function, %ecx
> > +       NOSPEC_CALL %ecx
> 
> Can't we just do
> 
>     NOSPEC_CALL ftrace_trace_function
> 
> now?
> 
> [ Goes off and looks ]
> 
> Oh. The AMD lfence version wants a register. Oh well.

The register load could be put into the macro itself, though we need to
supply a scratch register

     NOSPEC_CALL ftrace_trace_function scratch_reg=%ecx

Whether thats much better, I don't know.

Thanks,

	tglx