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

["Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Fri, Feb 03, 2017 at 03:11:10PM +0100, Maxime Coquelin wrote:
> Hi,
> 
> On 02/01/2017 09:35 AM, Maxime Coquelin wrote:
> > 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-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> >  - 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/...).
> 
> Thanks the first feedback. It seems to converge that this is Nova's
> role, but not Libvirt one to manage these versions from management tool
> layer.


I think the conclusion is not that it should go up the stack.  I think
this will just get broken all the time.  No one understands versions and
stuff. Even QEMU developers get confused and break compatibility once in
a while.

My conclusion is that doing it from OVS side is wrong.  Migration is not
an OVS thing, it's a QEMU thing, and libvirt abstracts QEMU.    People
just want migration to work, ok? It's our job to do it, we do not really
need a "make things work" flag.

If libvirt does not want to use the vhost-user protocol (which sounds
reasonable, it's rather complex) how about qemu providing a small
utility to query the port?  We could output json or whatever.

This can help with MTU as well.

And maybe it will help with nowait support - if someone uses the utility
to dump backend config once, QEMU can later start the device without
feature queries.


> This change has has no impact from OVS perspective, same requirements
> apply. I am interested on OVS contributors feedback on the below design
> proposal.
> 
> Especially, I would like to have your opinion on the best way for OVS to
> expose its supported versions:
> - Static file generated at build time from version table described below
> - Entries in the OVS DB
> - Dedicated tool listing strings from the version table described below
> 
> For selecting the right version of the vhost-user backend, do you agree
> it should be done via a new parameter of the ovs-vsctl add-port command
> for dpdkvhostuser ports?
> 
> 
> > 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?
> 
> Thanks,
> Maxime