On Tue, Jan 11, 2022, Chao Gao wrote: > On Tue, Jan 11, 2022 at 12:46:52AM +0000, Sean Christopherson wrote: > >This has a fairly big flaw in that it prevents KVM from creating VMs even if the > >offending CPU is offlined. That seems like a very reasonable thing to do, e.g. > >admin sees that hotplugging a CPU broke KVM and removes the CPU to remedy the > >problem. And if KVM is built-in, reloading KVM to wipe hardware_incompatible > >after offlining the CPU isn't an option. ... > >That said, I'm not convinced that continuing with the hotplug in this scenario > >is ever the right thing to do. Either the CPU being hotplugged really is a different > >CPU, or it's literally broken. In both cases, odds are very, very good that running > >on the dodgy CPU will hose the kernel sooner or later, i.e. KVM's compatibility checks > >are just the canary in the coal mine. > > Ok. Then here are two options: > 1. KVM always prevents incompatible CPUs from being brought up regardless of running VMs > 2. make "disabling KVM on incompatible CPUs" an opt-in feature. > > Which one do you think is better? IMO, #1. It's simpler to implement and document, and is less likely to surprise the user. We can always pivot to #2 _if_ anyone requests the ability to dynamically disable KVM in order to bring up heterogenous CPUs and has a reasonable, sane use case for doing so. But that's a big "if" as I would be very surprised if it's even possible to encounter such a setup without a hardware bug, firmware bug, and/or user error.
