Re: [PATCH 6/6] KVM: Do compatibility checks on hotplugged CPUs

From: Sean Christopherson
Date: Wed Jan 12 2022 - 12:52:17 EST


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.