linux-kernel - Re: tip/master falls off NOP cliff with KPTI under KVM

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Date:   Wed, 10 Jan 2018 22:53:43 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Alexey Dobriyan <adobriyan@...il.com>, <pbonzini@...hat.com>,
        <rkrcmar@...hat.com>, <tglx@...utronix.de>, <mingo@...hat.com>,
        <hpa@...or.com>, <dave.hansen@...ux.intel.com>
CC:     <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <x86@...nel.org>, <arjan@...ux.intel.com>
Subject: Re: tip/master falls off NOP cliff with KPTI under KVM

On Thu, 2018-01-11 at 01:34 +0300, Alexey Dobriyan wrote:
> 
> Bisection points to
> 
>         f3433c1010c6af61c9897f0f0447f81b991feac1 is the first bad commit
>         commit f3433c1010c6af61c9897f0f0447f81b991feac1
>         Author: David Woodhouse <dwmw@...zon.co.uk>
>         Date:   Tue Jan 9 14:43:11 2018 +0000
> 
>             x86/retpoline/entry: Convert entry assembler indirect jumps

Thanks. We've fixed the underlying problem with the alternatives
mechanism, *and* changed the retpoline code not to actually rely on
said fix.

> RETPOLINE is enabled but build system is reporting that compiler doesn't
> support it (Gentoo 6.4.0 p1.1)
> 
> Disabling CONFIG_RETPOLINE fixes boot.
> 
> And build system reports that "system may be insecure" even if RETPOLINE
> is disabled.

That's odd. The warning is *inside* 'ifdef CONFIG_RETPOLINE'...
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