Re: [PATCH v2 35/43] KVM: SVM: Signal AVIC doorbell iff vCPU is in guest mode

On Wed, Oct 27, 2021, Paolo Bonzini wrote:
> On 27/10/21 17:06, Sean Christopherson wrote:
> > > Does this still need to check the "running" flag?  That should be a strict
> > > superset of vcpu->mode = IN_GUEST_MODE.
> > 
> > No.  Signalling the doorbell when "running" is set but the vCPU is not in the
> > guest is just an expensive nop.  So even if KVM were to rework its handling of
> > "running" to set the flag immediately before VMRUN and clear it immediately after,
> > keying off IN_GUEST_MODE and not "running" would not be wrong, just sub-optimal.
> > 
> > I doubt KVM will ever make the "running" flag super precise, because keeping the
> > flag set when the vCPU is loaded avoids VM-Exits on other vCPUs due to undelivered
> > IPIs.
> 
> Right, so should we drop the "if (running)" check in this patch, at the same
> time as it's adding the IN_GUEST_MODE check?

LOL, I think we have a Three^WTwo Stooges routine going on.  This patch does
remove avic_vcpu_is_running() and replaces it with the vcpu->mode check.  Or am
I completely misunderstanding what your referring to?

-       if (avic_vcpu_is_running(vcpu)) {
+       /*
+        * Signal the doorbell to tell hardware to inject the IRQ if the vCPU
+        * is in the guest.  If the vCPU is not in the guest, hardware will
+        * automatically process AVIC interrupts at VMRUN.
+        */
+       if (vcpu->mode = IN_GUEST_MODE) {
                int cpu = READ_ONCE(vcpu->cpu);