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

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Date:   Thu, 25 Jan 2018 20:13:04 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dexuan Cui <decui@...rosoft.com>
cc:     David Woodhouse <dwmw2@...radead.org>,
        Dexuan-Linux Cui <dexuan.linux@...il.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "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

On Thu, 25 Jan 2018, Dexuan Cui wrote:
> > 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.

The simpler solution is to pull from tip x86/pti which is still 4.14 based
and contains only the stuff which has to do with these issues. It's mostly
in Linus tree already except for the topmost 3 patches.

Thanks,

	tglx