On Wed, Jan 12, 2022 at 9:54 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > 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. How quickly we forget the Woodcrest B/G fiasco.
