From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [PATCH] kvmclock: Ensure time in migration never goes backward Date: Wed, 7 May 2014 22:33:25 -0300 Message-ID: <20140508013325.GA5474@amt.cnet> References: <1399297882-3444-1-git-send-email-agraf@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org To: Alexander Graf Return-path: Received: from mx1.redhat.com ([209.132.183.28]:27954 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751121AbaEHBiK (ORCPT ); Wed, 7 May 2014 21:38:10 -0400 Content-Disposition: inline In-Reply-To: <1399297882-3444-1-git-send-email-agraf@suse.de> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, May 05, 2014 at 03:51:22PM +0200, Alexander Graf wrote: > When we migrate we ask the kernel about its current belief on what the guest > time would be. However, I've seen cases where the kvmclock guest structure > indicates a time more recent than the kvm returned time. > > To make sure we never go backwards, calculate what the guest would have seen > as time at the point of migration and use that value instead of the kernel > returned one when it's more recent. > > While this doesn't fix the underlying issue that the kernel's view of time > is skewed, it allows us to safely migrate guests even from sources that are > known broken. > > Signed-off-by: Alexander Graf OK Alexander better move this logic to the kernel, in KVM_GET_CLOCK. Otherwise every user of KVM_GET_CLOCK would have to apply the workaround.