linux-kernel - Re: [v4.6-rc1 Regression] x86/topology: Create logical package id

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Date:	Sat, 7 May 2016 07:16:05 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Joseph Salisbury <joseph.salisbury@...onical.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Andi Kleen <andi.kleen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Harish Chegondi <harish.chegondi@...el.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Jiri Olsa <jolsa@...hat.com>, Kan Liang <kan.liang@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Stephane Eranian <eranian@...gle.com>,
	Toshi Kani <toshi.kani@...com>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	LKML <linux-kernel@...r.kernel.org>, yingying.zhao@...el.com,
	"Zhang, Xiong Y" <xiong.y.zhang@...el.com>,
	1573231@...s.launchpad.net, 1397880@...s.launchpad.net,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Chen Yucong <slaoub@...il.com>,
	Laura Abbott <labbott@...oraproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [v4.6-rc1 Regression] x86/topology: Create logical package id


* Thomas Gleixner <tglx@...utronix.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