From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zachary Amsden Subject: Re: [PATCH v2 2/2] turn off kvmclock when resetting cpu Date: Wed, 05 May 2010 10:33:52 -1000 Message-ID: <4BE1D630.5090600@redhat.com> References: <1273087161-32428-1-git-send-email-glommer@redhat.com> <1273087161-32428-2-git-send-email-glommer@redhat.com> <1273087161-32428-3-git-send-email-glommer@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, avi@redhat.com, mtosatti@redhat.com To: Glauber Costa Return-path: Received: from mx1.redhat.com ([209.132.183.28]:42517 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753302Ab0EEUd5 (ORCPT ); Wed, 5 May 2010 16:33:57 -0400 Received: from int-mx02.intmail.prod.int.phx2.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) by mx1.redhat.com (8.13.8/8.13.8) with ESMTP id o45KXvCb026500 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Wed, 5 May 2010 16:33:57 -0400 In-Reply-To: <1273087161-32428-3-git-send-email-glommer@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 05/05/2010 09:19 AM, Glauber Costa wrote: > Currently, in the linux kernel, we reset kvmclock if we are rebooting > into a crash kernel through kexec. The rationale, is that a new kernel > won't follow the same memory addresses, and the memory where kvmclock is > located in the first kernel, will be something else in the second one. > > We don't do it in normal reboots, because the second kernel ends up > registering kvmclock again, which has the effect of turning off the > first instance. > > This is, however, totally wrong. This assumes we're booting into > a kernel that also has kvmclock enabled. If by some reason we reboot > into something that doesn't do kvmclock including but not limited to: > * rebooting into an older kernel without kvmclock support, > * rebooting with no-kvmclock, > * rebootint into another O.S, > > we'll simply have the hypervisor writting into a random memory position > into the guest. Neat, uh? > > Moreover, I believe the fix belongs in qemu, since it is the entity > more prepared to detect all kinds of reboots (by means of a cpu_reset), > not to mention the presence of misbehaving guests, that can forget > to turn kvmclock off. > > It is also necessary to reset other msrs, so this patch resets > everything that kvm exports through its MSR list. > Does that include the TSC? Zach