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Re: [libvirt] [RFC v2] [OVS/NOVA] Vhost-user backends cross-version migration support



On Wed, Aug 30, 2017 at 04:25:10PM -0300, Eduardo Habkost wrote:
> On Wed, Aug 30, 2017 at 05:23:39PM +0300, Michael S. Tsirkin wrote:
> > On Wed, Aug 30, 2017 at 10:17:27AM -0300, Eduardo Habkost wrote:
> > > I'm CCing libvir-list and qemu-devel because I would like to get
> > > feedback from libvirt and QEMU developers too.
> > > 
> > > On Tue, Aug 08, 2017 at 10:49:21PM +0300, Michael S. Tsirkin wrote:
> > > > On Tue, Jul 18, 2017 at 03:42:08PM +0200, Maxime Coquelin wrote:
> > > > > This is an revival from a thread I initiated earlier this year [0], that
> > > > > I had to postpone due to other priorities.
> > > > > 
> > > > > First, I'd like to thanks reviewers of my first proposal, this new
> > > > > version tries to address the comments made:
> > > > >  1.This is Nova's role and not Libvirt's to query hosts supported
> > > > > compatibility modes and to select one, since Nova adds the vhost-user
> > > > > ports and has visibility on other hosts. Hence I remove libvirt ML and
> > > > > add Openstack one in the recipient list.
> > > > >  2. By default, the compatibility version selected is the most recent
> > > > > one, except if the admin selects on older compat version.
> > > > > 
> > > > > The goal of this thread is to draft a solution based on the outcomes
> > > > > of discussions with contributors of the different parties (DPDK/OVS
> > > > > /Nova/...).
> > > > > 
> > > > > I'm really interested on feedback from OVS & Nova contributors,
> > > > > as my experience with these projects is rather limited.
> > > > > 
> > > > > 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
> > > > > re-negotiation 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. (e.g.
> > > > > configurable queues sizes)
> > > > > 
> > > > > What we need is to be able to query the destination host's backend to
> > > > > ensure migration is possible before it is initiated.
> > > > 
> > > > This remided me strongly of the issues around the virtual CPU modeling
> > > > in KVM, see
> > > > https://wiki.qemu.org/index.php/Features/CPUModels#Querying_host_capabilities
> > > > 
> > > > QEMU recently gained query-cpu-model-expansion to allow capability queries.
> > > > 
> > > > Cc Eduardo accordingly. Eduardo, could you please take a look -
> > > > how is the problem solved on the KVM/VCPU side? Do the above
> > > > problem and solution for vhost look similar?
> > > 
> > > (Sorry for taking so long to reply)
> > > 
> > > CPU configuration in QEMU has the additional problem of features
> > > depending on host hardware and kernel capabilities (not just QEMU
> > > software capabilities).  Do you have vhost-user features that
> > > depend on the host kernel or hardware too, or all of them just
> > > depend on the vhost-user backend software?
> > 
> > vhost-net features depend on host kernel.
> > 
> > > If it depends only on software, a solution similar to how
> > > machine-types work in QEMU sound enough.  If features depend on
> > > host kernel or host hardware too, it is a bit more complex: it
> > > means you need an interface to find out if each configurable
> > > feature/version is really available on the host.
> > > 
> > > (In the case of CPU models, we started with an interface that
> > > reported which CPU models were runnable on the host.  But as
> > > libvirt allows enabling/disabling individual CPU features, the
> > > interface had to be extended to report which CPU features were
> > > available/unavailable on the host.)
> > > 
> > >                           * * *
> > > 
> > > Now, there's one thing that seems very different here: the
> > > guest-visible interface is not defined only by QEMU, but also by
> > > the vhost-user backend.  Is that correct?
> > 
> > Not exactly. As long as there are no bugs it's defined by QEMU but
> > depends on backend capabilities. Bugs in a backend could be guest
> > visible - same as kvm really.
> 
> I'm a bit confused here.
> 
> I will try to enumerate the steps involved in the process, for
> clarity:
> 1) Querying which features are available on a host;
> 2) Choosing a reasonable default based on what's available on the
>    relevant host(s), before starting a VM;
> 3) Actually configuring what will be seen by the guest, based on
>    (1), (2) (and optionally user input/configuration).
> 
> Above you say that (1) on vhost-net depend on host kernel too.
> That's OK.
> 
> I also understand that (2) can't be done by libvirt and QEMU
> alone, because they don't have information about the vhost-user
> backend before the VM is configured.  That's OK too.
> 
> However, I don't see the data flow of the configuration step (3)
> clearly.  If the guest ABI is only defined by QEMU, does that
> mean configuring the guest-visible features would always be done
> through libvirt+QEMU?
> 
> In other words, would the corresponding
> vhostuser_compat.virtio_features value (or other knobs that
> affect guest ABI) always flow this way:
>   OVS -> libvirt -> QEMU -> vhost-user-backend -> guest
> and not directly this way:
>   OVS -> vhost-user-backend -> guest
> ?

Barring bugs, after init it works like this

libvirt -----------------------+
                               |
                               v
OVS -> vhost-user-backend <-> QEMU -> guest

On device init qemu queries all features, enables
the subset configured by libvirt and also supplies
them (with possible modifications) to guest.


> 
> > 
> > > This means QEMU won't fully control the resulting guest ABI
> > > anymore.  I would really prefer if we could keep libvirt+QEMU in
> > > control of the guest ABI as usual, making QEMU configure all the
> > > guest-visible vhost-user features.  But I understand this would
> > > require additional interfaces between QEMU and libvirt, and
> > > extending the libvirt APIs.
> > > 
> > > So, if QEMU is really not going to control the resulting guest
> > > ABI completely, can we at least provide a mechanism which QEMU
> > > can use to ask vhost-user for guest ABI details on migration, and
> > > block migration if vhost-user was misconfigured on the
> > > destination host when migrating?
> > > 
> > > 
> > > > 
> > > > > The below proposal has been drafted based on how Qemu manages machine types:
> > > > > 
> > > > > Proposal
> > > > > ========
> > > > > 
> > > > > The idea is to have a table of supported version strings in OVS,
> > > > > associated to key/value pairs. Nova or any other management tool could
> > > > > query OVS for the list of supported versions strings for each hosts.
> > > > > By default, the latest compatibility version will be selected, but the
> > > > > admin can select manually an older compatibility mode in order to ensure
> > > > > successful migration to an older destination host.
> > > > > 
> > > > > Then, Nova would add OVS's vhost-user port with adding the selected
> > > > > version (compatibility mode) as an extra parameter.
> > > > > 
> > > > > Before starting the VM migration, Nova will ensure both source and
> > > > > destination hosts' vhost-user interfaces run in the same compatibility
> > > > > modes, and will prevent it if this is not the case.
> > > > > 
> > > > > 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 port add
> > > > > time to ensure migration will succeed to host B.
> > > > > 
> > > > > Advantage of doing so is that Nova 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 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, or a
> > > > > config file packaged with OVS stores the list of compatibiliy versions.
> > > > > 
> > > > > Before launching the VM, Nova will query the version strings for the
> > > > > host so that the admin can select an older compatibility mode. If none
> > > > > selected by the admin, then the most recent one will be used by default,
> > > > > and passed to the OVS's add-port command as parameter. Note that if no
> > > > > compatibility mode is passed to the add-port command, the most recent
> > > > > one is selected by OVS as default.
> > > > > 
> > > > > 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.
> > > > > 
> > > > > Cheers,
> > > > > Maxime
> > > > > 
> > > > > [0]:
> > > > >  https://mail.openvswitch.org/pipermail/ovs-dev/2017-February/328257.html
> > > > >  <b2a5501c-7df7-ad2a-002f-d731c445a502 at redhat.com>
> > > 
> > > -- 
> > > Eduardo
> 
> -- 
> Eduardo


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