linux-kernel - Re: [PATCH v5 02/12] x86/retpoline: Add initial retpoline support

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Message-ID: <1515273680.29312.272.camel@amazon.co.uk>
Date:   Sat, 6 Jan 2018 21:21:20 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>
CC:     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>, <tglx@...utronix.de>,
        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>,
        Thomas Lendacky <Thomas.Lendacky@....com>
Subject: Re: [PATCH v5 02/12] x86/retpoline: Add initial retpoline support

On Sat, 2018-01-06 at 21:16 +0000, Andrew Cooper wrote:
> On 06/01/18 11:49, David Woodhouse wrote:
> > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > index 372ba3f..40e6e54 100644
> > --- a/arch/x86/kernel/cpu/common.c
> > +++ b/arch/x86/kernel/cpu/common.c
> > @@ -904,6 +904,11 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
> >  
> >       setup_force_cpu_bug(X86_BUG_SPECTRE_V1);
> >       setup_force_cpu_bug(X86_BUG_SPECTRE_V2);
> > +#ifdef CONFIG_RETPOLINE
> > +     setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
> > +     if (c->x86_vendor == X86_VENDOR_AMD)
> > +             setup_force_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
> 
> This isn't safe.  It needs to be dependant on finding that LFENCEs are
> actually dispatch serialising.
> 
> In particular, when virtualised, you'll most likely be saddled with the
> hypervisors choice of setting, in which case you need to use retpoline
> as a fallback.

Thanks. I was about to rebase on top of tip/x86/pti which has Tom's
patches to make lfence serialising — which seem to say that if the MSR
isn't available, it *will* be serialising.

I think I'll just refrain from setting X86_FEATURE_RETPOLINE_AMD for
now, and let Tom turn that on in his own time.


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