From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH] v5: allow userspace to adjust kvmclock offset Date: Tue, 20 Oct 2009 08:38:51 +0900 Message-ID: <4ADCF88B.9080402@redhat.com> References: <1255721316-11777-1-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, linux-kernel@vger.kernel.org To: Glauber Costa Return-path: Received: from mx1.redhat.com ([209.132.183.28]:23277 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756045AbZJSXiv (ORCPT ); Mon, 19 Oct 2009 19:38:51 -0400 In-Reply-To: <1255721316-11777-1-git-send-email-glommer@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 10/17/2009 04:28 AM, Glauber Costa wrote: > When we migrate a kvm guest that uses pvclock between two hosts, we may > suffer a large skew. This is because there can be significant differences > between the monotonic clock of the hosts involved. When a new host with > a much larger monotonic time starts running the guest, the view of time > will be significantly impacted. > > Situation is much worse when we do the opposite, and migrate to a host with > a smaller monotonic clock. > > This proposed ioctl will allow userspace to inform us what is the monotonic > clock value in the source host, so we can keep the time skew short, and > more importantly, never goes backwards. Userspace may also need to trigger > the current data, since from the first migration onwards, it won't be > reflected by a simple call to clock_gettime() anymore. > > +struct kvm_clock_data { > + __u64 clock; > + __u64 pad[2]; > +}; > + > The padding is not reusable without a flags word that we can set bits in for extended functionality. Please add one (and a check that it is zero, or -EINVAL). Since it was already applied, please do it as an incremental patch. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.