From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [PATCH] v5: allow userspace to adjust kvmclock offset Date: Mon, 19 Oct 2009 17:51:28 -0200 Message-ID: <20091019195128.GC17781@amt.cnet> References: <1255721316-11777-1-git-send-email-glommer@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, avi@redhat.com To: Glauber Costa Return-path: Content-Disposition: inline In-Reply-To: <1255721316-11777-1-git-send-email-glommer@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Fri, Oct 16, 2009 at 03:28:36PM -0400, 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. > > [ v2: uses a struct with a padding ] > [ v3: provide an ioctl to get clock data too ] > [ v4: used fixed-width signed type for delta ] > [ v5: nitpick fixes, documentation update, and capability bit ] > > Signed-off-by: Glauber Costa Applied, thanks (the ioctl number changed though).