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Subject: Re: [v4.6-rc1 Regression] x86/topology: Create logical package id
To: Thomas Gleixner <tglx@linutronix.de>,
        Joseph Salisbury <joseph.salisbury@canonical.com>
References: <572CDF53.7040301@canonical.com>
 <alpine.DEB.2.11.1605062046270.3540@nanos>
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From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Date: Fri, 6 May 2016 15:13:17 -0400
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On 05/06/2016 02:48 PM, Thomas Gleixner wrote:
> 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.

Joe, do you have

ed6069b xen/apic: Provide Xen-specific version of cpu_present_to_apicid
APIC op

-boris


>
> 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);
>  
>  	/*