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Tsirkin" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: Re: [RFC v3] VFIO Migration Message-ID: <20201116072805-mutt-send-email-mst@kernel.org> References: <20201110095349.GA1082456@stefanha-x1.localdomain> <64fb6a41-fbfa-994c-9619-4df41ac97fde@redhat.com> <20201111143615.GA1421166@stefanha-x1.localdomain> <20201111154850.GG906488@redhat.com> <20201116111524.GD96297@stefanha-x1.localdomain> <20201116114125.GE104771@redhat.com> <20201116065906-mutt-send-email-mst@kernel.org> <20201116120518.GH104771@redhat.com> MIME-Version: 1.0 In-Reply-To: <20201116120518.GH104771@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/11/15 22:35:17 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: John G Johnson , "Tian, Kevin" , Yan Zhao , quintela@redhat.com, Jason Wang , "Zeng, Xin" , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Kirti Wankhede , Thanos Makatos , Alex Williamson , Gerd Hoffmann , Stefan Hajnoczi , Felipe Franciosi , Christophe de Dinechin , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Nov 16, 2020 at 12:05:18PM +0000, Daniel P. Berrangé wrote: > On Mon, Nov 16, 2020 at 07:03:03AM -0500, Michael S. Tsirkin wrote: > > On Mon, Nov 16, 2020 at 11:41:25AM +0000, Daniel P. Berrangé wrote: > > > > It is possible to simplify the problem, but we'll lose freedom. For > > > > example, hard coding knowledge of the device implementation into the > > > > management tool eliminates the need for a general migration checking > > > > algorithm. Or we might be able to simplify it by explicitly not > > > > supporting cross-device implementation migration (although that would > > > > place stricter rules on what a new version of an existing device can > > > > change in order to preserve migration compatibility). > > > > > > Is migrating between 2 different vendors' impls of the same core > > > device spec really a thing that's needed ? > > > > If there's intent to have this supercede vhost-user then certainly. > > Same I'm guessing for NVMe. > > > > > > > > I have doubts that these trade-offs can be made without losing support > > > > for use cases that are necessary. > > > > > > >From my POV, the key goal is that it should be possible to migrate > > > between two hosts without needing to check every single possible > > > config parameter that the device supports. It should only be neccessary > > > to check the parameters that are actually changed from their default > > > values. Then there just needs to be some simple string parameter that > > > encodes a particular set of devices, akin to the versioned machine > > > type. > > > > > > Applications that want to migration between cross-vendor device impls > > > could opt-in to checking every single little parameter, but most can > > > just stick with a much simplified view where they only have to check > > > the parameters that they've actually overriden/exposed. > > > > It's a problem even for a single vendor. And we have lots of experience > > telling us it's a messy, difficult one. Just punting and saying > > vendors will do the right thing will not lead to quality > > implementations. > > I'm not suggesting we punt on the problem. I'm saying that checking for > migration compatibility should not need to be made more complex than what > we already do for QEMU. The core problem being tackled is essentially the > same in both cases. > > Regards, > Daniel There's a difference: in case of QEMU versions are release based. At release time a new version is generated. So QEMU upstream ships version X and Red Hat ships Y at a different time and they are not compatible. This won't work for devices: same device needs to work with both upstream and Red Hat and migrate upstream-upstream and Red Hat-Red Hat (though not upstream-Red Hat). > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|