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

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Date:   Wed, 12 Jan 2022 17:52:08 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Gao <chao.gao@...el.com>
Cc:     kvm@...r.kernel.org, pbonzini@...hat.com, kevin.tian@...el.com,
        tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] KVM: Do compatibility checks on hotplugged CPUs

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.