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Tsirkin" To: Peter Xu Cc: Akihiko Odaki , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Thomas Huth , Yuri Benditovich , eduardo@habkost.net, marcel.apfelbaum@gmail.com, philmd@linaro.org, wangyanan55@huawei.com, dmitry.fleytman@gmail.com, jasowang@redhat.com, sriram.yagnaraman@est.tech, sw@weilnetz.de, qemu-devel@nongnu.org, yan@daynix.com, Fabiano Rosas , devel@lists.libvirt.org Subject: Re: [PATCH v2 4/4] virtio-net: Add support for USO features Message-ID: <20240802123127-mutt-send-email-mst@kernel.org> References: <39a8bb8b-4191-4f41-aaf7-06df24bf3280@daynix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.124, 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, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Fri, Aug 02, 2024 at 12:26:22PM -0400, Peter Xu wrote: > And that's why I was thinking (where I totally agree with you on this) that > whether we should settle a short term plan first to be on the safe side > that we start with migration always being compatible, then we figure the > other approach. We have two big issues around migration compatibility we never solved: - some guest visible behaviour depends on a package outside of qemu: as that package can change, so can qemu behaviour - sometimes we change guest visible behaviour and only discover this after the release: fixing that breaks migration to one version, not fixing breaks migration to another These, to me, look similar enough that I feel we should look at them together from QAPI POV. Both issues sometimes can have work-arounds, enabling these would be nice. Also, both issues have a clean solution, which can come in two flavors: 1. basic: detecting incompatibility and not starting qemu on destination (or failing migration, possibly early, which I consider a less clean solution). 2. advanced: ability to go from a set of configurations to a flag making them compatible. -- MST