On Wed, Apr 21, 2021 at 11:29:45AM +0200, Vitaly Kuznetsov wrote: > Daniel P. Berrangé <berra...@redhat.com> writes: > > > On Wed, Apr 21, 2021 at 10:38:06AM +0200, Vitaly Kuznetsov wrote: > >> Eduardo Habkost <ehabk...@redhat.com> writes: > >> > >> > On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: > >> >> * Paolo Bonzini (pbonz...@redhat.com) wrote: > >> >> > On 06/04/21 13:42, Vitaly Kuznetsov wrote: > >> >> > > older machine types are still available (I disable it for <= 5.1 > >> >> > > but we > >> >> > > can consider disabling it for 5.2 too). The feature is upstream > >> >> > > since > >> >> > > Linux 5.8, I know that QEMU supports much older kernels but this > >> >> > > doesn't > >> >> > > probably mean that we can't enable new KVM PV features unless all > >> >> > > supported kernels have it, we'd have to wait many years otherwise. > >> >> > > >> >> > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL > >> >> > 7, > >> >> > though that will go away in 6.1. > >> >> > > >> >> > We should take the occasion of dropping RHEL7 to be clearer about > >> >> > which > >> >> > kernels are supported. > >> >> > >> >> It would be nice to be able to define sets of KVM functonality that we > >> >> can either start given machine types with, or provide a separate switch > >> >> to limit kvm functionality back to some defined point. We do trip over > >> >> the same things pretty regularly when accidentally turning on new > >> >> features. > >> > > >> > The same idea can apply to the hyperv=on stuff Vitaly is working > >> > on. Maybe we should consider making a generic version of the > >> > s390x FeatGroup code, use it to define convenient sets of KVM and > >> > hyperv features. > >> > >> True, the more I look at PV features enablement, the more I think that > >> we're missing something important in the logic. All machine types we > >> have are generally suposed to work with the oldest supported kernel so > >> we should wait many years before enabling some of the new PV features > >> (KVM or Hyper-V) by default. > >> > >> This also links to our parallel discussion regarding migration > >> policies. Currently, we can't enable PV features by default based on > >> their availability on the host because of migration, the set may differ > >> on the destination host. What if we introduce (and maybe even switch to > >> it by default) something like > >> > >> -migratable opportunistic (stupid name, I know) > >> > >> which would allow to enable all features supported by the source host > >> and then somehow checking that the destination host has them all. This > >> would effectively mean that it is possible to migrate a VM to a > >> same-or-newer software (both kernel an QEMU) but not the other way > >> around. This may be a reasonable choice. > > > > I don't think this is usable in pratice. Any large cloud or data center > > mgmt app using QEMU relies on migration, so can't opportunistically > > use arbitrary new features. They can only use features in the oldest > > kernel their deployment cares about. This can be newer than the oldest > > that QEMU supports, but still older than the newest that exists. > > > > ie we have situation where: > > > > - QEMU upstream minimum host is version 7 > > - Latest possible host is version 45 > > - A particular deployment has a mixture of hosts at version 24 and 37 > > > > "-migratable opportunistic" would let QEMU use features from version 37 > > despite the deployment needing compatibility with host version 24 still. > > > > True; I was not really thinking about 'big' clouds/data centers, these > should have enough resources to carefully set all the required features > and not rely on the 'default'. My thoughts were around using migration > for host upgrade on smaller (several hosts) deployments and in this case > it's probably fairly reasonable to require to start with the oldest host > and upgrade them all if getting new features is one of the upgrade goals.
> > It is almost as if we need to have a way to explicitly express a minimum > > required host version that VM requires compatibility with, so deployments > > can set their own baseline that is newer than QEMU minimum. > > Yes, maybe, but setting the baseline is also a non-trivial task: > e.g. how would users know which PV features they can enable without > going through Linux kernel logs or just trying them on the oldest kernel > they need? This should probably be solved by some upper layer management > app which would collect feature sets from all hosts and come up with a > common subset. I'm not sure if this is done by some tools already. I specifically didn't talk in terms of features, because the problem you describe is unreasonable to push onto applications. Rather QEMU could express host baseline - "host-v1" - features A and B - "host-v2" - features A, B and C - "host-v3" - features A, B, C, D, E and f The mgmt app / admin only has to know which QEMU host baselines their hosts support. Essentially this could be viewed as separating the host kernel dependant bits out of the machine type, into a separate configuration axis. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
