linux-kernel - Re: [PATCH 38/44] KVM: Disable CPU hotplug during hardware enabling

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Date:   Thu, 17 Nov 2022 15:16:15 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "borntraeger@...ux.ibm.com" <borntraeger@...ux.ibm.com>,
        "Yao, Yuan" <yuan.yao@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
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Subject: Re: [PATCH 38/44] KVM: Disable CPU hotplug during hardware enabling

On Thu, Nov 17, 2022, Huang, Kai wrote:
> On Wed, 2022-11-16 at 17:11 +0000, Sean Christopherson wrote:
> > static int kvm_x86_check_processor_compatibility(void)
> > {
> >         int cpu = smp_processor_id();
> >         struct cpuinfo_x86 *c = &cpu_data(cpu);
> > 
> >         /*
> >          * Compatibility checks are done when loading KVM and when enabling
> >          * hardware, e.g. during CPU hotplug, to ensure all online CPUs are
> >          * compatible, i.e. KVM should never perform a compatibility check on
> >          * an offline CPU.
> >          */
> >         WARN_ON(!cpu_online(cpu));
> 
> Looks good to me.  Perhaps this also can be removed, though.

Hmm, it's a bit superfluous, but I think it could fire if KVM messed up CPU
hotplug again, e.g. if the for_each_online_cpu() => IPI raced with CPU unplug.

> And IMHO the removing of WARN_ON(!irq_disabled()) should be folded to the patch
> "[PATCH 37/44] KVM: Rename and move CPUHP_AP_KVM_STARTING to ONLINE section". 
> Because moving from STARTING section to ONLINE section changes the IRQ status
> when the compatibility check is called.

Yep, that's what I have coded up, just smushed it all together here.