KVM: VMX: remove WARN_ON_ONCE in kvm_vcpu_trigger_posted_interrupt

WARN_ON_ONCE(pi_test_sn(&vmx->pi_desc)) in kvm_vcpu_trigger_posted_interrupt()
intends to detect the violation of invariant that VT-d PI notification
event is not suppressed when vcpu is in the guest mode. Because the
two checks for the target vcpu mode and the target suppress field
cannot be performed atomically, the target vcpu mode may change in
between. If that does happen, WARN_ON_ONCE() here may raise false
alarms.

As the previous patch fixed the real invariant breaker, remove this
WARN_ON_ONCE() to avoid false alarms, and document the allowed cases
instead.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Reported-by: "Ramamurthy, Venkatesh" <venkatesh.ramamurthy@intel.com>
Reported-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Fixes: 28b835d ("KVM: Update Posted-Interrupts Descriptor when vCPU is preempted")
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/vmx.c

@@ -5077,21 +5077,30 @@ static inline bool kvm_vcpu_trigger_posted_interrupt(struct kvm_vcpu *vcpu,
 	int pi_vec = nested ? POSTED_INTR_NESTED_VECTOR : POSTED_INTR_VECTOR;
 
 	if (vcpu->mode == IN_GUEST_MODE) {
-		struct vcpu_vmx *vmx = to_vmx(vcpu);
-
 		/*
-		 * Currently, we don't support urgent interrupt,
-		 * all interrupts are recognized as non-urgent
-		 * interrupt, so we cannot post interrupts when
-		 * 'SN' is set.
+		 * The vector of interrupt to be delivered to vcpu had
+		 * been set in PIR before this function.
+		 *
+		 * Following cases will be reached in this block, and
+		 * we always send a notification event in all cases as
+		 * explained below.
+		 *
+		 * Case 1: vcpu keeps in non-root mode. Sending a
+		 * notification event posts the interrupt to vcpu.
+		 *
+		 * Case 2: vcpu exits to root mode and is still
+		 * runnable. PIR will be synced to vIRR before the
+		 * next vcpu entry. Sending a notification event in
+		 * this case has no effect, as vcpu is not in root
+		 * mode.
 		 *
-		 * If the vcpu is in guest mode, it means it is
-		 * running instead of being scheduled out and
-		 * waiting in the run queue, and that's the only
-		 * case when 'SN' is set currently, warning if
-		 * 'SN' is set.
+		 * Case 3: vcpu exits to root mode and is blocked.
+		 * vcpu_block() has already synced PIR to vIRR and
+		 * never blocks vcpu if vIRR is not cleared. Therefore,
+		 * a blocked vcpu here does not wait for any requested
+		 * interrupts in PIR, and sending a notification event
+		 * which has no effect is safe here.
 		 */
-		WARN_ON_ONCE(pi_test_sn(&vmx->pi_desc));
 
 		apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), pi_vec);
 		return true;