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

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Message-ID: <1515160619.29312.126.camel@amazon.co.uk>
Date:   Fri, 5 Jan 2018 13:56:59 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "Dave Hansen" <dave.hansen@...el.com>,
        Kees Cook <keescook@...gle.com>,
        "Rik van Riel" <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Andy Lutomirski" <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

On Fri, 2018-01-05 at 13:54 +0100, Thomas Gleixner wrote:
> On Thu, 4 Jan 2018, David Woodhouse wrote:
> > diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> > index 07cdd1715705..900fa7016d3f 100644
> > --- a/arch/x86/include/asm/cpufeatures.h
> > +++ b/arch/x86/include/asm/cpufeatures.h
> > @@ -342,5 +342,6 @@
> >  #define X86_BUG_MONITOR                      X86_BUG(12) /* IPI required to wake up remote CPU */
> >  #define X86_BUG_AMD_E400             X86_BUG(13) /* CPU is among the affected by Erratum 400 */
> >  #define X86_BUG_CPU_INSECURE         X86_BUG(14) /* CPU is insecure and needs kernel page table isolation */
> > +#define X86_BUG_NO_RETPOLINE         X86_BUG(15) /* Placeholder: disable retpoline branch thunks */
> 
> I think this is the wrong approach. We have X86_BUG_CPU_INSECURE, which now
> should be renamed to X86_BUG_CPU_MELTDOWN_V3 or something like that. It
> tells the kernel, that the CPU is affected by variant 3.

As it says, it's a placeholder. The actual conditions depend on whether
we decide to use IBRS or not, which also depends on whether we find
IBRS_ATT or whether we're on Skylake+.

The IBRS patch series should be updating it.

> So what we really want is
> 
>    X86_BUG_MELTDOWN_V1/2/3
> 
> which get set when the CPU is affected by a particular variant and then
> have feature flags
> 
>    X86_FEATURE_RETPOLINE
>    X86_FEATURE_IBRS
>    X86_FEATURE_NOSPEC

At some point during this whole painful mess, I had come to the
conclusion that having relocations in altinstr didn't work, and that's
why I had X86_xx_NO_RETPOLINE instead of X86_xx_RETPOLINE. I now think
that something else was wrong when I was testing that, and relocs in
altinstr do work. So sure, X86_FEATURE_RETPOLINE ought to work. I can
change that round, and it's simpler for the IBRS patch set to take it
into account and set it when appropriate.

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