linux-kernel - Re: [PATCH] x86/alternatives: Fix optimize

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Message-ID: <20180110211636.nagyyyjkkqd4pkvu@treble>
Date:   Wed, 10 Jan 2018 15:16:36 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <andi@...stfloor.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Jiri Kosina <jikos@...nel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/alternatives: Fix optimize_nops() checking

On Wed, Jan 10, 2018 at 01:11:48PM -0800, Linus Torvalds wrote:
> On Wed, Jan 10, 2018 at 1:08 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> >
> > There are a few of the form 'call *somefunc'.
> >
> > The existing code handles them not by virtue of the relocs, as I said,
> > but by a simple delta of the old and new location of the instruction.
> >
> > But it only does so for the *first* instruction of the altinstr, if it
> > happens to be a (4-byte?) branch.
> 
> Ugh. That's nasty.
> 
> Wouldn't it be much better to simply do it as part of relocation instead?

Not possible, the relocations are applied during vmlinux linking.

-- 
Josh