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Date:   Mon, 20 Aug 2018 15:02:31 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Jinpu Wang <jinpu.wang@profitbricks.com>
cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable <stable@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, konrad.wilk@oracle.com,
        Peter Zijlstra <peterz@infradead.org>,
        David Woodhouse <dwmw@amazon.co.uk>
Subject: Re: [BUG]smt sysfs dir missing on 4.4.148 and 4.14.63
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On Mon, 20 Aug 2018, Jinpu Wang wrote:

> On Sat, Aug 18, 2018 at 2:50 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > On Fri, 17 Aug 2018, Greg Kroah-Hartman wrote:
> > > On Fri, Aug 17, 2018 at 02:39:00PM +0200, Jinpu Wang wrote:
> > > >
> > > > I found the problem, CONFIG_HOTPLUG_SMT is enable due to CONFIG_SMP,
> > > > but I did explicitly enable CONFIG_HOTPLUG_CPU.
> > > >
> > > > That's why the smt dir is missing, and kernel panic when I boot with
> > > > 'nosmt' or 'l1tf=full' probably also other combination.
> > > >
> > > > With CONFIG_HOTPLUG_CPU=y, it works fine so far.
> > >
> > > So do we need a new configuration dependancy here to resolve this
> > > problem?  Why is this not an issue in Linus's tree?
> >
> > That's probably the same problem on Linus tree. Have not tested it yet
> > though. I'll have a look.
> >
> > Thanks,
> >
> >         tglx
> >
> Yeah, it's the same problem on Linus tree.
> 
> Probably makes sense to do the following, I can sent a formal patch if
> you think it's right thing to do:
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index b0312f8..8a52f0b 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -188,7 +188,7 @@ config X86
>         select HAVE_SYSCALL_TRACEPOINTS
>         select HAVE_UNSTABLE_SCHED_CLOCK
>         select HAVE_USER_RETURN_NOTIFIER
> -       select HOTPLUG_SMT                      if SMP
> +       select HOTPLUG_SMT                      if SMP && HOTPLUG_CPU

That works, but we might as well force 'select HOTPLUG_CPU if SMP' so the
L1TF mitigations are functional. Sigh....

Thanks,

	tglx