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

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Date:   Thu, 10 Nov 2022 02:11:47 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
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Subject: Re: [PATCH 38/44] KVM: Disable CPU hotplug during hardware enabling

On Thu, 2022-11-10 at 01:33 +0000, Huang, Kai wrote:
> > @@ -9283,7 +9283,13 @@ static int
> > kvm_x86_check_processor_compatibility(struct kvm_x86_init_ops *ops)
> >  	int cpu = smp_processor_id();
> >  	struct cpuinfo_x86 *c = &cpu_data(cpu);
> >  
> > -	WARN_ON(!irqs_disabled());
> > +	/*
> > +	 * 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(!irqs_disabled() && cpu_active(cpu));
> >  
> 
> Also, the logic of:
> 
> 	!irqs_disabled() && cpu_active(cpu)
> 
> is quite weird.
> 
> The original "WARN(!irqs_disabled())" is reasonable because in STARTING
> section
> the IRQ is indeed disabled.
> 
> But this doesn't make sense anymore after we move to ONLINE section, in which
> IRQ has already been enabled (see start_secondary()).  IIUC the WARN_ON()
> doesn't get exploded is purely because there's an additional cpu_active(cpu)
> check.
> 
> So, a more reasonable check should be something like:
> 
> 	WARN_ON(irqs_disabled() || cpu_active(cpu) || !cpu_online(cpu));
> 
> Or we can simply do:
> 
> 	WARN_ON(!cpu_online(cpu) || cpu_active(cpu));
> 
> (because I don't know whether it's possible IRQ can somehow get disabled in
> ONLINE section).
> 
> Btw above is purely based on code analysis, but I haven't done any test.

Hmm.. I wasn't thinking thoroughly.  I forgot CPU compatibility check also
happens on all online cpus when loading KVM.  For this case, IRQ is disabled and
cpu_active() is true.  For the hotplug case, IRQ is enabled but  cpu_active() is
false.

So WARN_ON(!irqs_disabled() && cpu_active(cpu)) looks reasonable.  Sorry for the
noise.  Just needed some time to connect the comment with the code.