KVM: VMX: Drop pointless PI.NDST update when blocking

Don't update Posted Interrupt's NDST, a.k.a. the target pCPU, in the
pre-block path, as NDST is guaranteed to be up-to-date.  The comment
about the vCPU being preempted during the update is simply wrong, as the
update path runs with IRQs disabled (from before snapshotting vcpu->cpu,
until after the update completes).

Since commit 8b306e2f ("KVM: VMX: avoid double list add with VT-d
posted interrupts", 2017-09-27) The vCPU can get preempted _before_
the update starts, but not during.  And if the vCPU is preempted before,
vmx_vcpu_pi_load() is responsible for updating NDST when the vCPU is
scheduled back in.  In that case, the check against the wakeup vector in
vmx_vcpu_pi_load() cannot be true as that would require the notification
vector to have been set to the wakeup vector _before_ blocking.

Opportunistically switch to using vcpu->cpu for the list/lock lookups,
which do not need pre_pcpu since the same commit.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-25-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/posted_intr.c

@@ -135,7 +135,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
  * - Store the vCPU to the wakeup list, so when interrupts happen
  *   we can find the right vCPU to wake up.
  * - Change the Posted-interrupt descriptor as below:
- *      'NDST' <-- vcpu->pre_pcpu
  *      'NV' <-- POSTED_INTR_WAKEUP_VECTOR
  * - If 'ON' is set during this process, which means at least one
  *   interrupt is posted for this vCPU, we cannot block it, in
@@ -144,7 +143,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
  */
 int pi_pre_block(struct kvm_vcpu *vcpu)
 {
-	unsigned int dest;
 	struct pi_desc old, new;
 	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
 
@@ -156,10 +154,10 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
 	local_irq_disable();
 
 	vcpu->pre_pcpu = vcpu->cpu;
-	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
+	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));
 	list_add_tail(&vcpu->blocked_vcpu_list,
-		      &per_cpu(blocked_vcpu_on_cpu, vcpu->pre_pcpu));
-	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
+		      &per_cpu(blocked_vcpu_on_cpu, vcpu->cpu));
+	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));
 
 	do {
 		old.control = new.control = pi_desc->control;
@@ -168,21 +166,6 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
 		     "Warning: SN field of posted-interrupts "
 		     "is set before blocking\n");
 
-		/*
-		 * Since vCPU can be preempted during this process,
-		 * vcpu->cpu could be different with pre_pcpu, we
-		 * need to set pre_pcpu as the destination of wakeup
-		 * notification event, then we can find the right vCPU
-		 * to wakeup in wakeup handler if interrupts happen
-		 * when the vCPU is in blocked state.
-		 */
-		dest = cpu_physical_id(vcpu->pre_pcpu);
-
-		if (x2apic_mode)
-			new.ndst = dest;
-		else
-			new.ndst = (dest << 8) & 0xFF00;
-
 		/* set 'NV' to 'wakeup vector' */
 		new.nv = POSTED_INTR_WAKEUP_VECTOR;
 	} while (cmpxchg64(&pi_desc->control, old.control,