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Date: Sat, 6 Jan 2018 20:53:48 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
cc: David Woodhouse <dwmw@amazon.co.uk>,
        Andi Kleen <ak@linux.intel.com>, Paul Turner <pjt@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
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        Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Subject: Re: [PATCH v5 05/12] x86/retpoline/ftrace: Convert ftrace assembler
 indirect jumps
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On Sat, 6 Jan 2018, Linus Torvalds wrote:

> On Sat, Jan 6, 2018 at 3:49 AM, David Woodhouse <dwmw@amazon.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