From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752196AbcAELGS (ORCPT ); Tue, 5 Jan 2016 06:06:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:50880 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751965AbcAELGQ (ORCPT ); Tue, 5 Jan 2016 06:06:16 -0500 Date: Tue, 5 Jan 2016 13:06:10 +0200 From: "Michael S. Tsirkin" To: "Dr. David Alan Gilbert" Cc: Alexander Duyck , Konrad Rzeszutek Wilk , Alexander Duyck , kvm@vger.kernel.org, "linux-pci@vger.kernel.org" , x86@kernel.org, "linux-kernel@vger.kernel.org" , qemu-devel@nongnu.org, Lan Tianyu , Yang Zhang , Alexander Graf , Alex Williamson Subject: Re: [RFC PATCH 0/3] x86: Add support for guest DMA dirty page tracking Message-ID: <20160105130530-mutt-send-email-mst@redhat.com> References: <20151213212557.5410.48577.stgit@localhost.localdomain> <20160104204104.GB17427@char.us.oracle.com> <20160105113317-mutt-send-email-mst@redhat.com> <20160105100104.GA2439@work-vm> <20160105123243-mutt-send-email-mst@redhat.com> <20160105104525.GC2439@work-vm> <20160105125937-mutt-send-email-mst@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160105125937-mutt-send-email-mst@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 05, 2016 at 12:59:54PM +0200, Michael S. Tsirkin wrote: > On Tue, Jan 05, 2016 at 10:45:25AM +0000, Dr. David Alan Gilbert wrote: > > * Michael S. Tsirkin (mst@redhat.com) wrote: > > > On Tue, Jan 05, 2016 at 10:01:04AM +0000, Dr. David Alan Gilbert wrote: > > > > * Michael S. Tsirkin (mst@redhat.com) wrote: > > > > > On Mon, Jan 04, 2016 at 07:11:25PM -0800, Alexander Duyck wrote: > > > > > > >> The two mechanisms referenced above would likely require coordination with > > > > > > >> QEMU and as such are open to discussion. I haven't attempted to address > > > > > > >> them as I am not sure there is a consensus as of yet. My personal > > > > > > >> preference would be to add a vendor-specific configuration block to the > > > > > > >> emulated pci-bridge interfaces created by QEMU that would allow us to > > > > > > >> essentially extend shpc to support guest live migration with pass-through > > > > > > >> devices. > > > > > > > > > > > > > > shpc? > > > > > > > > > > > > That is kind of what I was thinking. We basically need some mechanism > > > > > > to allow for the host to ask the device to quiesce. It has been > > > > > > proposed to possibly even look at something like an ACPI interface > > > > > > since I know ACPI is used by QEMU to manage hot-plug in the standard > > > > > > case. > > > > > > > > > > > > - Alex > > > > > > > > > > > > > > > Start by using hot-unplug for this! > > > > > > > > > > Really use your patch guest side, and write host side > > > > > to allow starting migration with the device, but > > > > > defer completing it. > > > > > > > > > > So > > > > > > > > > > 1.- host tells guest to start tracking memory writes > > > > > 2.- guest acks > > > > > 3.- migration starts > > > > > 4.- most memory is migrated > > > > > 5.- host tells guest to eject device > > > > > 6.- guest acks > > > > > 7.- stop vm and migrate rest of state > > > > > > > > > > > > > > > It will already be a win since hot unplug after migration starts and > > > > > most memory has been migrated is better than hot unplug before migration > > > > > starts. > > > > > > > > > > Then measure downtime and profile. Then we can look at ways > > > > > to quiesce device faster which really means step 5 is replaced > > > > > with "host tells guest to quiesce device and dirty (or just unmap!) > > > > > all memory mapped for write by device". > > > > > > > > > > > > Doing a hot-unplug is going to upset the guests network stacks view > > > > of the world; that's something we don't want to change. > > > > > > > > Dave > > > > > > It might but if you store the IP and restore it quickly > > > after migration e.g. using guest agent, as opposed to DHCP, > > > then it won't. > > > > I thought if you hot-unplug then it will lose any outstanding connections > > on that device. > > > > > It allows calming the device down in a generic way, > > > specific drivers can then implement the fast quiesce. > > > > Except that if it breaks the guest networking it's useless. > > > > Dave > > Is hot unplug useless then? Actually I misunderstood the question, unplug does not have to break guest networking. > > > > > > > > > > > > > -- > > > > > MST > > > > -- > > > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > > -- > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK