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

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Message-ID: <20180105170806.mtylu2zagfxyj3ry@treble>
Date:   Fri, 5 Jan 2018 11:08:06 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "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, Jan 05, 2018 at 05:45:06PM +0100, Borislav Petkov wrote:
> On Fri, Jan 05, 2018 at 04:41:46PM +0000, Woodhouse, David wrote:
> > Nope, alternatives are broken. Only a jmp as the *first* opcode of
> > altinstr gets handled by recompute_jump(), while any subsequent insn is
> > just copied untouched.
> 
> Not broken - simply no one needed it until now. I'm looking into it.
> Looks like the insn decoder might come in handy finally.
> 
> :-)

I seem to recall that we also discussed the need for this for converting
pvops to use alternatives, though the "why" is eluding me at the moment.

-- 
Josh