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Wed, 1 Apr 2020 18:34:52 +0000 (UTC) Received: from w520.home (ovpn-112-162.phx2.redhat.com [10.3.112.162]) by smtp.corp.redhat.com (Postfix) with ESMTP id DD13CD768F; Wed, 1 Apr 2020 18:34:49 +0000 (UTC) Date: Wed, 1 Apr 2020 12:34:49 -0600 From: Alex Williamson To: Yan Zhao Subject: Re: [PATCH v16 QEMU 00/16] Add migration support for VFIO devices Message-ID: <20200401123449.62e1a729@w520.home> In-Reply-To: <20200401064153.GF6631@joy-OptiPlex-7040> References: <1585084154-29461-1-git-send-email-kwankhede@nvidia.com> <20200331123424.3c28b30a@w520.home> <20200401064153.GF6631@joy-OptiPlex-7040> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.61 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Zhengxiao.zx@Alibaba-inc.com" , "Tian, Kevin" , "Liu, Yi L" , "cjia@nvidia.com" , "eskultet@redhat.com" , "Yang, Ziye" , "qemu-devel@nongnu.org" , "cohuck@redhat.com" , "shuangtai.tst@alibaba-inc.com" , "dgilbert@redhat.com" , "Wang, Zhi A" , "mlevitsk@redhat.com" , "pasic@linux.ibm.com" , "aik@ozlabs.ru" , Kirti Wankhede , "eauger@redhat.com" , "felipe@nutanix.com" , "jonathan.davies@nutanix.com" , "Liu, Changpeng" , "Ken.Xue@amd.com" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, 1 Apr 2020 02:41:54 -0400 Yan Zhao wrote: > On Wed, Apr 01, 2020 at 02:34:24AM +0800, Alex Williamson wrote: > > On Wed, 25 Mar 2020 02:38:58 +0530 > > Kirti Wankhede wrote: > > > > > Hi, > > > > > > This Patch set adds migration support for VFIO devices in QEMU. > > > > Hi Kirti, > > > > Do you have any migration data you can share to show that this solution > > is viable and useful? I was chatting with Dave Gilbert and there still > > seems to be a concern that we actually have a real-world practical > > solution. We know this is inefficient with QEMU today, vendor pinned > > memory will get copied multiple times if we're lucky. If we're not > > lucky we may be copying all of guest RAM repeatedly. There are known > > inefficiencies with vIOMMU, etc. QEMU could learn new heuristics to > > account for some of this and we could potentially report different > > bitmaps in different phases through vfio, but let's make sure that > > there are useful cases enabled by this first implementation. > > > > With a reasonably sized VM, running a reasonable graphics demo or > > workload, can we achieve reasonably live migration? What kind of > > downtime do we achieve and what is the working set size of the pinned > > memory? Intel folks, if you've been able to port to this or similar > > code base, please report your results as well, open source consumers > > are arguably even more important. Thanks, > > > hi Alex > we're in the process of porting to this code, and now it's able to > migrate successfully without dirty pages. > > when there're dirty pages, we met several issues. > one of them is reported here > (https://lists.gnu.org/archive/html/qemu-devel/2020-04/msg00004.html). > dirty pages for some regions are not able to be collected correctly, > especially for memory range from 3G to 4G. > > even without this bug, qemu still got stuck in middle before > reaching stop-and-copy phase and cannot be killed by admin. > still in debugging of this problem. Thanks, Yan. So it seems we have various bugs, known limitations, and we haven't actually proven that this implementation provides a useful feature, at least for the open source consumer. This doesn't give me much confidence to consider the kernel portion ready for v5.7 given how late we are already :-\ Thanks, Alex