Re: [PATCH v2 27/43] KVM: VMX: Move Posted Interrupt ndst computation out of write loop

On Thu, Oct 28, 2021, Maxim Levitsky wrote:
> On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> > Hoist the CPU => APIC ID conversion for the Posted Interrupt descriptor
> > out of the loop to write the descriptor, preemption is disabled so the
> > CPU won't change, and if the APIC ID changes KVM has bigger problems.
> > 
> > No functional change intended.
> 
> Is preemption always disabled in vmx_vcpu_pi_load? vmx_vcpu_pi_load is called
> from vmx_vcpu_load, which is called indirectly from vcpu_load which is called
> from many ioctls, which userspace does. In these places I don't think that
> preemption is disabled.

Preemption is disabled in vcpu_load() by the get_cpu().  The "cpu" param that's
passed around the vcpu_load() stack is also why I think it's ok to _not_ assert
that preemption is disabled in vmx_vcpu_pi_load(); if preemption is enabled,
"cpu" is unstable and thus the entire "load" operation is busted.


#define get_cpu()		({ preempt_disable(); __smp_processor_id(); })
#define put_cpu()		preempt_enable()


void vcpu_load(struct kvm_vcpu *vcpu)
{
	int cpu = get_cpu();

	__this_cpu_write(kvm_running_vcpu, vcpu);
	preempt_notifier_register(&vcpu->preempt_notifier);
	kvm_arch_vcpu_load(vcpu, cpu);
	put_cpu();
}
EXPORT_SYMBOL_GPL(vcpu_load);