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

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Message-ID: <1516866801.30244.3.camel@infradead.org>
Date:   Thu, 25 Jan 2018 07:53:21 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Dexuan-Linux Cui <dexuan.linux@...il.com>
Cc:     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,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org,
        arjan@...ux.intel.com, decui@...rosoft.com
Subject: Re: tip/master falls off NOP cliff with KPTI under KVM

On Wed, 2018-01-24 at 16:53 -0800, Dexuan-Linux Cui wrote:
> On Wed, Jan 10, 2018 at 2:53 PM, Woodhouse, David <dwmw@...zon.co.uk> wrote:
> > 
> > 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.
> Hi David and all,
> It looks the latest upstream tree did fix the issue.
> Can you please specify the related commit(s) that fixed the issue?
> I need to cherry-pick the fix. I suppose a quick reply from you would save
>  me a lot of time. :-)

Hi Dexuan,

The above commit didn't ever make it into Linus' tree in that form; the
issues were fixed beforehand. So there isn't a subsequent commit that
fixes it. I think it might have just been removing some .align 16 from
nospec-branch.h?

The correct version has been backported to 4.9 and 4.4 releases
already; if you pulled in an early version directly from tip/x86/pti
then I'd recommend you drop it and pull in the real version instead.

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