linux-kernel - Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

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Date:   Sat, 6 Jan 2018 10:53:36 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "bp@...e.de" <bp@...e.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "luto@...capital.net" <luto@...capital.net>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

On Fri, 2018-01-05 at 15:50 -0800, Linus Torvalds wrote:
> 
> > +
> > +.macro RETPOLINE_CALL reg:req
> > +       jmp     1113f
> > +1110:  RETPOLINE_JMP \reg
> > +1113:  call    1110b
> > +.endm

(Note that RETPOLINE_CALL is purely internal to nospec-branch.h, used
only from the NOSPEC_CALL macro to make the ALTERNATIVE_2 invocation
les
s ugly than it would have been. But the same applies to NOSPEC_CALL
which is the one that gets used in our .S files including retpoline.S)

> Why do these still force a register name?
> 
> Is it because this is the only user?

By "register name" are you asking why it's invoked as
"NOSPEC_CALL rbx" instead of "NOSPEC_CALL %rbx" with the percent sign?

I think I can probably clean that up now, and turn a lot of %\reg
occurrences into just \reg. The only remaining %\reg would be left in
retpoline.S inside the THUNK macro used from .irp.

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