From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eduardo Habkost Subject: Re: TSC deadline timer in guests vs. migration? Date: Tue, 5 Jul 2016 10:04:22 -0300 Message-ID: <20160705130422.GO4131@thinpad.lan.raisama.net> References: <759376f0-e0bf-e53a-99e4-598bf14547e3@redhat.com> <20160704193005.GA28201@amt.cnet> <20160704194506.GE4131@thinpad.lan.raisama.net> <20160705125140.GA17664@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Paolo Bonzini , David Matlack , Peter Hornyack , KVM list To: Marcelo Tosatti Return-path: Received: from mx1.redhat.com ([209.132.183.28]:46769 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755123AbcGENEZ (ORCPT ); Tue, 5 Jul 2016 09:04:25 -0400 Content-Disposition: inline In-Reply-To: <20160705125140.GA17664@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, Jul 05, 2016 at 09:51:41AM -0300, Marcelo Tosatti wrote: > On Mon, Jul 04, 2016 at 04:45:06PM -0300, Eduardo Habkost wrote: > > On Mon, Jul 04, 2016 at 04:30:08PM -0300, Marcelo Tosatti wrote: > > > On Mon, Jul 04, 2016 at 01:01:42PM +0200, Paolo Bonzini wrote: > > > > Can bad things happen if a guest using the TSC deadline timer is > > > > migrated? The guest doesn't re-calibrate the TSC after migration, and > > > > the TSC frequency can and will change unless your data center is > > > > perfectly homogeneous. > > > > > > It can fire earlier if the destination runs at a higher frequency. > > > It will fire past the configured time if the destination runs at a slower frequency. > > > > > > Suppose the first case is worse. > > > > > > Should convert the expiration time to nanoseconds i suppose, and then > > > convert back on the destination. > > > > This won't make any difference if the guest sets up a new timer > > after migration (but using the old TSC frequency), will it? > > It does, because the timer setup traps to the host, where you can > convert it to the proper value: To convert it to the proper value, you need to know what's the TSC frequency the guest is assuming. How would you do that? -- Eduardo