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Date: Fri, 6 May 2016 20:48:16 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Joseph Salisbury <joseph.salisbury@canonical.com>
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Subject: Re: [v4.6-rc1 Regression] x86/topology: Create logical package id
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On Fri, 6 May 2016, Joseph Salisbury wrote:
> A kernel bug report was opened against Ubuntu [0].  After a kernel
> bisect, it was found that reverting the following commit resolved this bug:
> 
> commit 1f12e32f4cd5243ae46d8b933181be0d022c6793
> Author: Thomas Gleixner <tglx@linutronix.de>
> Date:   Mon Feb 22 22:19:15 2016 +0000
> 
>     x86/topology: Create logical package id
> 
> To build successfully with this commit reverted, I also had to revert
> commits: e7ee3e8,2d4de83,87f01cc and 33c3cc7.
> 
> The regression was introduced as of v4.6-rc1.
>     
> 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;
+	}
+
 	__max_logical_packages = DIV_ROUND_UP(total_cpus, ncpus);
 
 	/*