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Tsirkin" To: Stefano Garzarella Subject: Re: [PATCH] vhost-vsock: fix migration issue when seqpacket is supported Message-ID: <20210913094508-mutt-send-email-mst@kernel.org> References: <20210907124935.147164-1-sgarzare@redhat.com> <20210909044603-mutt-send-email-mst@kernel.org> <20210909063623-mutt-send-email-mst@kernel.org> <20210913125142.t34brj4qfvngkfgh@steredhat> MIME-Version: 1.0 In-Reply-To: <20210913125142.t34brj4qfvngkfgh@steredhat> 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-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.398, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable 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: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Eduardo Habkost , Jiang Wang , qemu-stable@nongnu.org, qemu-devel@nongnu.org, Stefan Hajnoczi , Arseny Krasnov Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Sep 13, 2021 at 02:51:42PM +0200, Stefano Garzarella wrote: > On Fri, Sep 10, 2021 at 02:35:53AM -0400, Michael S. Tsirkin wrote: > > On Thu, Sep 09, 2021 at 10:02:12AM +0100, Daniel P. Berrangé wrote: > > > On Thu, Sep 09, 2021 at 04:47:42AM -0400, Michael S. Tsirkin wrote: > > > > On Tue, Sep 07, 2021 at 02:22:24PM +0100, Daniel P. Berrangé wrote: > > > > > On Tue, Sep 07, 2021 at 02:49:35PM +0200, Stefano Garzarella wrote: > > > > > > Commit 1e08fd0a46 ("vhost-vsock: SOCK_SEQPACKET feature bit support") > > > > > > enabled the SEQPACKET feature bit. > > > > > > This commit is released with QEMU 6.1, so if we try to migrate a VM where > > > > > > the host kernel supports SEQPACKET but machine type version is less than > > > > > > 6.1, we get the following errors: > > > > > > > > > > > > Features 0x130000002 unsupported. Allowed features: 0x179000000 > > > > > > Failed to load virtio-vhost_vsock:virtio > > > > > > error while loading state for instance 0x0 of device '0000:00:05.0/virtio-vhost_vsock' > > > > > > load of migration failed: Operation not permitted > > > > > > > > > > > > Let's disable the feature bit for machine types < 6.1, adding a > > > > > > `features` field to VHostVSock to simplify the handling of upcoming > > > > > > features we will support. > > > > > > > > > > IIUC, this will still leave migration broken for anyone migrating > > > > > a >= 6.1 machine type between a kernel that supports SEQPACKET and > > > > > a kernel lacking that, or vica-verca. > > > > > > > > > > If a feature is dependant on a host kernel feature we can't turn > > > > > that on automatically as part of the machine type, as we need > > > > > ABI stability across migration indepdant of kernel version. > > > > > > > > > > > > > > > Regards, > > > > > Daniel > > > > > > > > This is a fundamental problem we have with kernel accelerators. > > > > A higher level solution at management level is needed. > > > > For now yes, we do turn features on by default, > > > > consistent kernels on source and destination are assumed. > > > > For downstreams not a problem at all as they update > > > > userspace and kernel in concert. > > > > > > Even downstream in RHEL that is not actually valid anymore. Container > > > based deployment has killed any assumptions that can be made in this > > > respect. Even if the userspace and kernel are updated in lockstep in > > > a particular RHEL release, you cannot assume the running environment > > > will have a matched pair. > > > > > > Users can be running QEMU userspace from RHEL-8.5 inside a container > > > that has been deployed on a host using a 8.3 kernel. We've even had > > > cases of running QEMU from RHEL-8, on a RHEL-7 host. > > > > > > Regards, > > > Daniel > > > > Is there finally an interest in addressing this then? This would > > involve collecting host features across a cluster and for each host > > figuring out a configuration that works for migration. IIRC a tool was > > proposed for the task (to live alongside e.g. qemu-img). > > Apart from the tool, what if we provide a mechanism for adding/removing > device features at run-time? > After migration we could tell the guest that a feature is no longer > available. > > Maybe it's too complicated, but it would allow us to solve the problem of > migrating between different kernels or, with vDPA, between different devices > that don't support all features. Possible going forward but not supported by the spec at this point, and tricky to do generally. It's possible to do it in a vsock specific way since sockets are currently closed across migration. > > > > As long as we just stick to the machine type the best we can do is > > probably to keep doing what we do now (hope that the two host kernels > > are more or less consistent) as otherwise we'd have to never enable any > > new features in vsock. > > Should we at least merge this patch to allow to migrate a VM between a new > and an old qemu even if the kernel is the same? > > Thanks, > Stefano I'm inclined to do this, yes. -- MST