On Tue, Jan 19, 2021 at 02:22:05PM +0000, David Edmondson wrote: > Currently "-cpu -feature,+feature" will disable -feature, which seems > contrary to the intention of the user. Fix this such that the later > flag wins. There are no changes to the interaction of +/- and =on/=off.
The -feature/+feature syntax is the legacy way of configuring features, with feature=on|off being the preferred, since that matches the general QEMU standard for boolean properties. Your proposed change in ordering of + vs - makes conceptual sense, but it is none the less a semantic change in behaviour that may well cause breakage for existing deployed VMs. This impacts guest ABI so could particularly cause live migration problems. IOW, we should have implemented it the way you propose in the first place, but I don't think it is safe to change it now, unless you can tie that new semantic to a machine type version. Before we consider that though, Paolo has just deprecated many of the legacy approaches for boolean properties in this: https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg04341.html I'm inclined to say that we just follow on from that and finally deprecate the +feature/-feature CPU syntax which we're already considering legacy. This would remove the need to care about changing its behaviour 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 :|
