From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:43198 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751572AbcAEKBK (ORCPT ); Tue, 5 Jan 2016 05:01:10 -0500 Date: Tue, 5 Jan 2016 10:01:04 +0000 From: "Dr. David Alan Gilbert" To: "Michael S. Tsirkin" 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: <20160105100104.GA2439@work-vm> References: <20151213212557.5410.48577.stgit@localhost.localdomain> <20160104204104.GB17427@char.us.oracle.com> <20160105113317-mutt-send-email-mst@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20160105113317-mutt-send-email-mst@redhat.com> Sender: linux-pci-owner@vger.kernel.org List-ID: * 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 > > -- > MST -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK