Path: csiph.com!aioe.org!bofh.it!news.nic.it!robomod
From: Ingo Molnar <mingo@kernel.org>
Newsgroups: linux.kernel
Subject: Re: [v4.6-rc1 Regression] x86/topology: Create logical package id
Date: Sat, 07 May 2016 07:20:03 +0200
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References: <rvSw3-1EJ-39@gated-at.bofh.it> <rvT8K-20P-11@gated-at.bofh.it>
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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