Re: [libvirt] [RFC] Vhost-user backends cross-version migration support

Hi, Few months ago, Michael reported a problem about migrating VMs relying on vhost-user between hosts supporting different backend versions: - Message-Id: <20161011173526-mutt-send-email-mst(a)kernel.org&gt; - https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg03026.html The goal of this thread is to draft a proposal based on the outcomes of discussions with contributors of the different parties (DPDK/OVS /libvirt/...).

Problem statement: ================== When migrating a VM from one host to another, the interfaces exposed by QEMU must stay unchanged in order to guarantee a successful migration. In the case of vhost-user interface, parameters like supported Virtio feature set, max number of queues, max vring sizes,... must remain compatible. Indeed, the frontend not being re-initialized, no renegotiation happens at migration time. For example, we have a VM that runs on host A, which has its vhost-user backend advertising VIRTIO_F_RING_INDIRECT_DESC feature. Since the Guest also support this feature, it is successfully negotiated, and guest transmit packets using indirect descriptor tables, that the backend knows to handle. At some point, the VM is being migrated to host B, which runs an older version of the backend not supporting this VIRTIO_F_RING_INDIRECT_DESC feature. The migration would break, because the Guest still have the VIRTIO_F_RING_INDIRECT_DESC bit sets, and the virtqueue contains some decriptors pointing to indirect tables, that backend B doesn't know to handle. This is just an example about Virtio features compatibility, but other backend implementation details could cause other failures. What we need is to be able to query the destination host's backend to ensure migration is possible. Also, we would need to query this statically, even before the VM is started, to be sure it could be migrated elsewhere for any reason.

Solution 3: Libvirt queries OVS for vhost backend version string: *OK* ====================================================================== The idea is to have a table of supported versions, associated to key/value pairs. Libvirt could query the list of supported versions strings for each hosts, and select the first common one among all hosts. Then, libvirt would ask OVS to probe the vhost-user interfaces in the selected version (compatibility mode). For example host A runs OVS-2.7, and host B OVS-2.6. Host A's OVS-2.7 has an OVS-2.6 compatibility mode (e.g. with indirect descriptors disabled), which should be selected at vhost-user interface probe time. Advantage of doing so is that libvirt does not need any update if new keys are introduced (i.e. it does not need to know how the new keys have to be handled), all these checks remain in OVS's vhost-user implementation. Ideally, we would support per vhost-user interface compatibility mode, which may have an impact also on DPDK API, as the Virtio feature update API is global, and not per port. - Implementation: ----------------- Goal here is just to illustrate this proposal, I'm sure you will have good suggestion to improve it. In OVS vhost-user library, we would introduce a new structure, for example (neither compiled nor tested): struct vhostuser_compat { char *version; uint64_t virtio_features; uint32_t max_rx_queue_sz; uint32_t max_nr_queues; }; *version* field is the compatibility version string. It could be something like: "upstream.ovs-dpdk.v2.6" In case for example Fedora adds some more patches to its package that would break migration to upstream version, it could have a dedicated compatibility string: "fc26.ovs-dpdk.v2.6". In case OVS-v2.7 does not break compatibility with previous OVS-v2.6 version, then no need to create a new compatibility entry, just keep v2.6 one. *virtio_features* field is the Virtio features set for a given compatibility version. When an OVS tag is to be created, it would be associated to a DPDK version. The Virtio features for these version would be stored in this field. It would allow to upgrade the DPDK package for example from v16.07 to v16.11 without breaking migration. In case the distribution wants to benefit from latests Virtio features, it would have to create a new entry to ensure migration won't be broken. *max_rx_queue_sz* *max_nr_queues* fields are just here for example, don't think this is needed today. I just want to illustrate that we have to anticipate other parameters than the Virtio feature set, even if not necessary at the moment. We create a table with different compatibility versions in OVS vhost-user lib: static struct vhostuser_compat vu_compat[] = { { .version = "upstream.ovs-dpdk.v2.7", .virtio_features = 0x12045694, .max_rx_queue_sz = 512, }, { .version = "upstream.ovs-dpdk.v2.6", .virtio_features = 0x10045694, .max_rx_queue_sz = 1024, }, } At some time during installation, or system init, the table would be parsed, and compatibility version strings would be stored into the OVS database, or a new tool would be created to list these strings. Before launching the VM, libvirt will query the version strings for each hosts using for example the JSON RPC API of OVS (maybe not the best solution, looking forward for your comments on this). Libvirt would then select the first common supported version, and insert this string into the vhost-user interfaces parameters in the OVS DBs of each host. When the vhost-user connection is initiated, OVS would know in which compatibility mode to init the interface, for example by restricting the support Virtio features of the interface. Do you think this is reasonable? Or maybe you have alternative ideas that would be best fit to ensure successful migration?