From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: [PATCH 2/2] KVM: Create debugfs dir and stat files for each VM Date: Thu, 26 Nov 2015 21:47:18 +0100 Message-ID: <56576FD6.40506@de.ibm.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: frankja@linux.vnet.ibm.com, kvm@vger.kernel.org, pbonzini@redhat.com, pmorel@linux.vnet.ibm.com, dan.carpenter@oracle.com, "Kevin's boot bot" To: Tyler Baker Return-path: Received: from e06smtp06.uk.ibm.com ([195.75.94.102]:50885 "EHLO e06smtp06.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753732AbbKZUrX (ORCPT ); Thu, 26 Nov 2015 15:47:23 -0500 Received: from localhost by e06smtp06.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 26 Nov 2015 20:47:21 -0000 Received: from b06cxnps3075.portsmouth.uk.ibm.com (d06relay10.portsmouth.uk.ibm.com [9.149.109.195]) by d06dlp01.portsmouth.uk.ibm.com (Postfix) with ESMTP id A732D17D805A for ; Thu, 26 Nov 2015 20:47:45 +0000 (GMT) Received: from d06av02.portsmouth.uk.ibm.com (d06av02.portsmouth.uk.ibm.com [9.149.37.228]) by b06cxnps3075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id tAQKlJqG3866990 for ; Thu, 26 Nov 2015 20:47:19 GMT Received: from d06av02.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av02.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id tAQKlIwj016895 for ; Thu, 26 Nov 2015 13:47:19 -0700 In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: On 11/26/2015 05:17 PM, Tyler Baker wrote: > Hi Christian, > > The kernelci.org bot recently has been reporting kvm guest boot > failures[1] on various arm64 platforms in next-20151126. The bot > bisected[2] the failures to the commit in -next titled "KVM: Create > debugfs dir and stat files for each VM". I confirmed by reverting this > commit on top of next-20151126 it resolves the boot issue. > > In this test case the host and guest are booted with the same kernel. > The host is booted over nfs, installs qemu (qemu-system arm64 2.4.0), > and launches a guest. The host is booting fine, but when the guest is > launched it errors with "Failed to retrieve host CPU features!". I > checked the host logs, and found an "Unable to handle kernel paging > request" splat[3] which occurs when the guest is attempting to start. > > I scanned the patch in question but nothing obvious jumped out at me, > any thoughts? Not really. Do you have processing running that do read the files in /sys/kernel/debug/kvm/* ? If I read the arm oops message correctly it oopsed inside __srcu_read_lock. there is actually nothing in there that can oops, except the access to the preempt count. I am just guessing right now, but maybe the preempt variable is no longer available (as the process is gone). As long as a debugfs file is open, we hold a reference to the kvm, which holds a reference to the mm, so the mm might be killed after the process. But this is supposed to work, so maybe its something different. An objdump of __srcu_read_lock might help. I will drop it from my tree until we understand the problem Christian