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From: Sean Christopherson <seanjc@google.com>
To: Chao Gao <chao.gao@intel.com>
Cc: kvm@vger.kernel.org, pbonzini@redhat.com, kevin.tian@intel.com,
	tglx@linutronix.de, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/6] KVM: Do compatibility checks on hotplugged CPUs
Date: Wed, 12 Jan 2022 17:52:08 +0000	[thread overview]
Message-ID: <Yd8VSCghD3OvuGJ7@google.com> (raw)
In-Reply-To: <20220111053205.GD2175@gao-cwp>

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.

  reply	other threads:[~2022-01-12 17:52 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-27  8:15 [PATCH 0/6] Improve KVM's interaction with CPU hotplug Chao Gao
2021-12-27  8:15 ` Chao Gao
2021-12-27  8:15 ` Chao Gao
2021-12-27  8:15 ` Chao Gao
2021-12-27  8:15 ` [PATCH 1/6] KVM: x86: Move check_processor_compatibility from init ops to runtime ops Chao Gao
2022-01-10 23:27   ` Sean Christopherson
2022-01-11  3:36     ` Chao Gao
2022-01-12 17:59       ` Sean Christopherson
2021-12-27  8:15 ` [PATCH 2/6] KVM: x86: Use kvm_x86_ops in kvm_arch_check_processor_compat Chao Gao
2022-01-10 21:10   ` Sean Christopherson
2022-01-11  3:06     ` Chao Gao
2021-12-27  8:15 ` [PATCH 3/6] KVM: Remove opaque from kvm_arch_check_processor_compat Chao Gao
2021-12-27  8:15   ` Chao Gao
2021-12-27  8:15   ` Chao Gao
2021-12-27  8:15   ` Chao Gao
2022-01-10 23:06   ` Sean Christopherson
2022-01-10 23:06     ` Sean Christopherson
2022-01-10 23:06     ` Sean Christopherson
2022-01-10 23:06     ` Sean Christopherson
2022-01-11  3:19     ` Chao Gao
2022-01-11  3:19       ` Chao Gao
2022-01-11  3:19       ` Chao Gao
2022-01-11  3:19       ` Chao Gao
2022-01-12 17:20       ` Sean Christopherson
2022-01-12 17:20         ` Sean Christopherson
2022-01-12 17:20         ` Sean Christopherson
2022-01-12 17:20         ` Sean Christopherson
2022-01-12 17:21         ` Sean Christopherson
2022-01-12 17:21           ` Sean Christopherson
2022-01-12 17:21           ` Sean Christopherson
2022-01-12 17:21           ` Sean Christopherson
2021-12-27  8:15 ` [PATCH 4/6] KVM: Rename and move CPUHP_AP_KVM_STARTING to ONLINE section Chao Gao
2021-12-27  8:15 ` [PATCH 5/6] KVM: x86: Remove WARN_ON in kvm_arch_check_processor_compat Chao Gao
2022-01-10 22:59   ` Sean Christopherson
2022-01-11  2:15     ` Tian, Kevin
2022-01-11 19:48       ` Sean Christopherson
2022-01-12 11:00         ` Chao Gao
2022-01-12 17:35           ` Sean Christopherson
2022-01-17 13:35             ` Chao Gao
2022-01-17 13:46               ` Chao Gao
2022-01-19  0:34                 ` Sean Christopherson
2021-12-27  8:15 ` [PATCH 6/6] KVM: Do compatibility checks on hotplugged CPUs Chao Gao
2022-01-11  0:46   ` Sean Christopherson
2022-01-11  5:32     ` Chao Gao
2022-01-12 17:52       ` Sean Christopherson [this message]
2022-01-12 23:01         ` Jim Mattson

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