On Thu, 2021-10-28 at 16:12 +0000, Sean Christopherson wrote:
> 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.
Yes, I even knew about the get_cpu() behavier which indeed has to disable preemption.
But I didn't notice call to it, when I wrote this mail! Later I did notice it but it was
too late. Sometimes sending all the review mails at once at the end does make sense after all,
I guess.
Best regards,
Maxim Levitsky
>
>
> #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);
>
