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Date: Sat, 7 May 2016 07:16:05 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Joseph Salisbury <joseph.salisbury@canonical.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
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        Harish Chegondi <harish.chegondi@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>, Kan Liang <kan.liang@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Luis R. Rodriguez" <mcgrof@suse.com>,
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        "Zhang, Xiong Y" <xiong.y.zhang@intel.com>, 1573231@bugs.launchpad.net,
        1397880@bugs.launchpad.net, Ingo Molnar <mingo@redhat.com>,
        x86@kernel.org, Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Chen Yucong <slaoub@gmail.com>,
        Laura Abbott <labbott@fedoraproject.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        David Vrabel <david.vrabel@citrix.com>
Subject: Re: [v4.6-rc1 Regression] x86/topology: Create logical package id
Message-ID: <20160507051605.GA13962@gmail.com>
References: <572CDF53.7040301@canonical.com>
 <alpine.DEB.2.11.1605062046270.3540@nanos>
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* Thomas Gleixner <tglx@linutronix.de> wrote:

> > I was hoping to get your feedback, since you are the patch author.  Do
> > you think gathering any additional data will help diagnose this issue,
> > or would it be best to submit a revert request?
> 
> Yuck. That dies with a divide error. And that looks like XEN is supplying crap
> data in the CPUID.
> 
> Does the patch below cure the issue?
> 
> Thanks,
> 
>         tglx
> 
> 8<---------------
> 
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -332,6 +332,11 @@ static void __init smp_init_package_map(
>  	 * primary cores.
>  	 */
>  	ncpus = boot_cpu_data.x86_max_cores;
> +	if (!ncpus) {
> +		pr_warn("x86_max_cores == zero !?!?");
> +		ncpus = 1;
> +	}
> +

Even if the underlying Xen bug is fixed, I think we should add this protective 
measure to defend against future CPUID mishaps.

Thanks,

	Ingo