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

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Date:   Thu, 25 Jan 2018 19:00:23 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Dexuan-Linux Cui <dexuan.linux@...il.com>
CC:     Alexey Dobriyan <adobriyan@...il.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "arjan@...ux.intel.com" <arjan@...ux.intel.com>
Subject: RE: tip/master falls off NOP cliff with KPTI under KVM

> From: David Woodhouse [mailto:dwmw2@...radead.org]
> Sent: Wednesday, January 24, 2018 23:53
> 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.

Thanks for the clarification, David!
I'm going to pull the latest version from Linus's master branch.

Thanks,
-- Dexuan