On Fri, Nov 18, 2022 at 10:52:32AM +0100, Jiri Denemark wrote:
* Qemu already provide an option 'enforce' to validate if features with which vm is started is exactly same as one provided and nothing is silently dropped.
Right, but it's not enough. In addition to removed features libvirt also checks for unexpectedly added features. And you really need to do both. Because if you ask for -cpu Model,feat1=on,feat2=on,enforce and QEMU says everything is fine, the guest might see more than what you asked. For example, if a feature is enabled only if a host supports it you may or may not get it without any complains from QEMU. But if you get it you really need to explicitly ask for it during migration, otherwise the feature can just silently disappear. Of course, this would be a really bad behavior from QEMU, but that does not mean it can't happen (I think SVM is a bit problematic in this way) and the whole point of libvirt's checks is to prevent this kind of issues.
Hi Jiri, I'm not following this very well. I think you're saying that qemu has had bugs previously where features get silently enabled, and it's libvirt's job/role to paper over those issues? Do you have some specific cases of this? thanks john