On Thu, Oct 28, 2021, Maxim Levitsky wrote:

On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:

quoted

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);