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[79.51.2.59]) by smtp.gmail.com with ESMTPSA id n10sm4712697ejk.86.2021.09.14.03.42.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Sep 2021 03:42:11 -0700 (PDT) Date: Tue, 14 Sep 2021 12:42:09 +0200 From: Stefano Garzarella To: "Michael S. Tsirkin" Subject: Re: [PATCH] vhost-vsock: fix migration issue when seqpacket is supported Message-ID: <20210914104209.knw76yfzs2547rrb@steredhat> References: <20210907124935.147164-1-sgarzare@redhat.com> <20210909044603-mutt-send-email-mst@kernel.org> <20210909063623-mutt-send-email-mst@kernel.org> <20210913125142.t34brj4qfvngkfgh@steredhat> <20210913094508-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: <20210913094508-mutt-send-email-mst@kernel.org> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=sgarzare@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=sgarzare@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 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_MSPIKE_H2=-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: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= , 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 09:46:48AM -0400, Michael S. Tsirkin wrote: >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. Yep, I see. > > >> > >> > 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. > If you haven't queued it yet, I'd like to send a v2 using an `on,off,auto` property: `auto` would be the actual behavior when enabled, `on` instead requires that the kernel supports the feature otherwise there is an error. Thanks, Stefano