Re: [PATCH v3 2/8] KVM: Register cpuhp and syscore callbacks when enabling hardware

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 14, 2024, Paolo Bonzini wrote:
> On 6/8/24 02:06, Sean Christopherson wrote:
> > Register KVM's cpuhp and syscore callback when enabling virtualization
> > in hardware instead of registering the callbacks during initialization,
> > and let the CPU up/down framework invoke the inner enable/disable
> > functions.  Registering the callbacks during initialization makes things
> > more complex than they need to be, as KVM needs to be very careful about
> > handling races between enabling CPUs being onlined/offlined and hardware
> > being enabled/disabled.
> > 
> > Intel TDX support will require KVM to enable virtualization during KVM
> > initialization, i.e. will add another wrinkle to things, at which point
> > sorting out the potential races with kvm_usage_count would become even
> > more complex.
> > 
> > Note, using the cpuhp framework has a subtle behavioral change: enabling
> > will be done serially across all CPUs, whereas KVM currently sends an IPI
> > to all CPUs in parallel.  While serializing virtualization enabling could
> > create undesirable latency, the issue is limited to creation of KVM's
> > first VM,
> 
> Isn't that "limited to when kvm_usage_count goes from 0 to 1", so every time
> a VM is started if you never run two?

Yes, "first" isn't the correct word/phrase.