Re: [libvirt] [PATCH 6/8] virQEMUCapsSetFromNodes: Skip setting deprecated flags

On 3/7/19 1:07 PM, Daniel P. Berrangé wrote: > On Thu, Mar 07, 2019 at 12:52:46PM +0100, Erik Skultety wrote: > > > However, looking at the bigger picture is this a safe thing to do? I mean, > > > imagine the following scenario: > > > > > > 1) say there is capability X that affects certain part of cmd line. And > > > assume that those two possibilities are incompatible. If cmd line is > > > generated one way then migration to a qemu which has cmd line generated the > > > other way fails. > > > > > > 2) in release R we deprecate X and thus do not format it in <capabilities/> > > > in status XML. > > > > > > 3) user starts a domain D. > > > > > > 4) user saves D into a file > > > > > > 5) sysadmin downgrades libvirt to R-1 > > > > Do we even support downgrade this way? I know we migrate to older version but > > isn't that different? Okay, forget about save+restore. Let's just keep the domain D running and then downgrade libvirt. Now, instead of X affecting cmd line let it affect how we talk to qemu on monitor. Since R assumed X always being there then X wouldn't be recorded in status XML. Later when R-1 daemon is starting and parsing the status XML it won't find X and thus might make wrong decissions. For instance, if X would be some capability that affects the way we generate PCI addresses for guest devices, then this would really harm the user. Worse, let just start the domain with R-1, then upgrade to R to test it, call an API over D (at which point the status XML is regenerated, now without X stored in it), find that it's not working so roll back to R-1 and puff. The capability is gone.