On Mon, 4 Mar 2019 12:39:08 +0000 Daniel P. Berrangé <berrange@xxxxxxxxxx> wrote: > On Mon, Mar 04, 2019 at 01:25:07PM +0100, Igor Mammedov wrote: > > On Mon, 04 Mar 2019 08:13:53 +0100 > > Markus Armbruster <armbru@xxxxxxxxxx> wrote: > > > > > Daniel P. Berrangé <berrange@xxxxxxxxxx> writes: > > > > > > > On Fri, Mar 01, 2019 at 06:33:28PM +0100, Igor Mammedov wrote: > > > >> On Fri, 1 Mar 2019 15:49:47 +0000 > > > >> Daniel P. Berrangé <berrange@xxxxxxxxxx> wrote: > > > >> > > > >> > On Fri, Mar 01, 2019 at 04:42:15PM +0100, Igor Mammedov wrote: > > > >> > > The parameter allows to configure fake NUMA topology where guest > > > >> > > VM simulates NUMA topology but not actually getting a performance > > > >> > > benefits from it. The same or better results could be achieved > > > >> > > using 'memdev' parameter. In light of that any VM that uses NUMA > > > >> > > to get its benefits should use 'memdev' and to allow transition > > > >> > > initial RAM to device based model, deprecate 'mem' parameter as > > > >> > > its ad-hoc partitioning of initial RAM MemoryRegion can't be > > > >> > > translated to memdev based backend transparently to users and in > > > >> > > compatible manner (migration wise). > > > >> > > > > > >> > > That will also allow to clean up a bit our numa code, leaving only > > > >> > > 'memdev' impl. in place and several boards that use node_mem > > > >> > > to generate FDT/ACPI description from it. > > > >> > > > > >> > Can you confirm that the 'mem' and 'memdev' parameters to -numa > > > >> > are 100% live migration compatible in both directions ? Libvirt > > > >> > would need this to be the case in order to use the 'memdev' syntax > > > >> > instead. > > > >> Unfortunately they are not migration compatible in any direction, > > > >> if it where possible to translate them to each other I'd alias 'mem' > > > >> to 'memdev' without deprecation. The former sends over only one > > > >> MemoryRegion to target, while the later sends over several (one per > > > >> memdev). > > > > > > > > If we can't migration from one to the other, then we can not deprecate > > > > the existing 'mem' syntax. Even if libvirt were to provide a config > > > > option to let apps opt-in to the new syntax, we need to be able to > > > > support live migration of existing running VMs indefinitely. Effectively > > > > this means we need the to keep 'mem' support forever, or at least such > > > > a long time that it effectively means forever. > > > > > > > > So I think this patch has to be dropped & replaced with one that > > > > simply documents that memdev syntax is preferred. > > > > > > We have this habit of postulating absolutes like "can not deprecate" > > > instead of engaging with the tradeoffs. We need to kick it. > > > > > > So let's have an actual look at the tradeoffs. > > > > > > We don't actually "support live migration of existing running VMs > > > indefinitely". > > > > > > We support live migration to any newer version of QEMU that still > > > supports the machine type. > > > > > > We support live migration to any older version of QEMU that already > > > supports the machine type and all the devices the machine uses. > > > > > > Aside: "support" is really an honest best effort here. If you rely on > > > it, use a downstream that puts in the (substantial!) QA work real > > > support takes. > > > > > > Feature deprecation is not a contract to drop the feature after two > > > releases, or even five. It's a formal notice that users of the feature > > > should transition to its replacement in an orderly manner. > > > > > > If I understand Igor correctly, all users should transition away from > > > outdated NUMA configurations at least for new VMs in an orderly manner. > > Yes, we can postpone removing options until there are machines type > > versions that were capable to use it (unfortunate but probably > > unavoidable unless there is a migration trick to make transition > > transparent) but that should not stop us from disabling broken > > options on new machine types at least. > > > > This series can serve as formal notice with follow up disabling of > > deprecated options for new machine types. (As Thomas noted, just warnings > > do not work and users continue to use broken features regardless whether > > they are don't know about issues or aware of it [*]) > > > > Hence suggested deprecation approach and enforced rejection of legacy > > numa options for new machine types in 2 releases so users would stop > > using them eventually. > > When we deprecate something, we need to have a way for apps to use the > new alternative approach *at the same time*. So even if we only want to > deprecate for new machine types, we still have to first solve the problem > of how mgmt apps will introspect QEMU to learn which machine types expect > the new options. I'm not aware any mechanism to introspect machine type options (existing or something being developed). Are/were there any ideas about it that were discussed in the past? Aside from developing a new mechanism what are alternative approaches? I mean when we delete deprecated CLI option, how it's solved on libvirt side currently? For example I don't see anything introspection related when we have been removing deprecated options recently. More exact question specific to this series usecase, how libvirt decides when to use -numa node,memdev or not currently? > > Regards, > Daniel -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
