Re: [libvirt] [Qemu-devel] [PATCH v2] numa: warn if numa 'mem' option or default RAM splitting between nodes is used.

On Wed, Mar 20, 2019 at 11:26:34AM +0100, Igor Mammedov wrote: Any release prior to the one that changes the use of "mem". IOW, if we changed "mem" in libvirt 5.2.0, then it would break compat with libvirt 5.1.0 from the previous month's release (and of course all versions before 5.1.0 by implication). I don't think this is a mgmt issue. When a new QEMU release comes out it is valid to use it with an existing release of libvirt. You might need new libvirt if you want to use new features from QEMU that didn't exist previously, but existing QEMU features should generally work. With QEMU's deprecation policy, you're not going to be able to use arbitrarily old libvirt as at some point you will hit a version of libvirt that uses the old deprecated approach, instead of the new preferred approach. Whether this is a problem or not depends on the features you are using too. eg if we a CLI arg with a new preferred replacement, & you were never using that CLI arg in the first place, the incompatibility doesn't affect you. QEMU deprecation period is two releases, plus however long in the dev cycle it was deprecated before release. In the best case, libvirt from 12 months ago will have stopped using the deprecated feature. In the worst case, where it is very hard to change libvirt, we might still be using the deprecated feature right up until the end of the deprecation period. That should be the exception & we try not to get into that case as it is painful for users to deploy a new QEMU and find it breaks with their intsalled libvirt. If a feature is completely dropped by QEMU with no replacement, there's nothing libvirt can do to preserve existing VMs that use that feature. Obviously this is painful for users, so QEMU doesn't do that without compelling reason, such as the feature being unfixably broken. This isn't the case with "mem" though - it is an existing feature whose impl is being changed for a different impl. We're just telling apps to change the way they imple the feature from "mem" to "memdev", which breaks live migration compat across whichever version of the app makes the change. In terms of preserving back compat, the distro's hands are tied by what the upstream QEMU does to some extent. If upstream rips out the infra needed to provide the back compat in the distro, they'll have to revert all those upstream changes which can be non-trivial. Considering the distro maintainers are often upstream maintainers too, that's not a net win. The maintainer has saved themselves some work upstream, but created themselves a bigger amount of work downstream. As noted above, with the deprecation feature policy new QEMU is not likely to be compatible with arbitrarily old libvirt, but can usually be expected to be compatible with upto 12 month old libvirt in the best case, unless libvirt is really slow at adapting to deprecation warnings. So the challenge with tieing it to the new QEMU machine type is that machine type is potentially used by a libvirt upto perhaps 12 months old. Somehow the older libvirt has to know to use the new QEMU feature "memdev" that wasn't present required for any of the machine types it knew about when it was first released. This could be solved if QEMU has some machine type based property that indicates whether "memdev" is required for a given machine, but crucially *does not* actually activate that property until several releases later. We're too late for 4.0, so lets consider QEMU 4.1 as the next release of QEMU, which opens for dev in April 2019. QEMU 4.1 could introduce a machine type property "requires-memdev" which defaults to "false" for all existing machine types. It could add a deprecation that says a *future* machine type will report "requires-memdev=true". IOW, "pc-i440fx-4.1" and "pc-i440fx-4.2 must still report "requires-memdev=false", Libvirt 5.4.0 (May 2019) can now add support for "requires-memdev" property. This would be effectively a no-op at time of this libvirt release, since no QEMU would be reporting "requires-memdev=true" for many months to come yet. Now, after 2 QEMU releases with the deprecation wawrning, when the QEMU 5.0.0 dev cycle opens in Jan 2020, the new "pc-i440fx-5.0" machine type can be made to report "requires-memdev=true". IOW, in April 2020 when QEMU 5.0.0 comes out, "mem" would no longer be supported for new machine types. Libvirt at this time would be upto 6.4.0 but that's co-incidental since it would already be doing the right thing since 5.4.0. IOW, this QEMU 5.0.0 would work correctly with libvirt versions in the range 5.4.0 to 6.4.0 (and future). If a user had libvirt < 5.4.0 (ie older than May 2019) nothing would stop them using the "pc-i440fx-5.0" machine type, but libvirt would be liable to use "mem" instead of "memdev" and if that happened they would be unable to live migrate to a host newer libvirt which honours "requires-memdev=true" So in summary the key to being able to tie deprecations to machine type versions, is for QEMU to add a mechanism to report the desired new feature usage approach against the machine type, but then ensure the mechanism continues to report the old approach for 2 more releases. 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 :|