KVM: SVM: Don't rewrite guest ICR on AVIC IPI virtualization failure

Don't bother rewriting the ICR value into the vAPIC page on an AVIC IPI
virtualization failure, the access is a trap, i.e. the value has already
been written to the vAPIC page.  The one caveat is if hardware left the
BUSY flag set (which appears to happen somewhat arbitrarily), in which
case go through the "nodecode" APIC-write path in order to clear the BUSY
flag.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220204214205.3306634-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -1291,6 +1291,7 @@ void kvm_apic_send_ipi(struct kvm_lapic *apic, u32 icr_low, u32 icr_high)
 
 	kvm_irq_delivery_to_apic(apic->vcpu->kvm, apic, &irq, NULL);
 }
+EXPORT_SYMBOL_GPL(kvm_apic_send_ipi);
 
 static u32 apic_get_tmcct(struct kvm_lapic *apic)
 {

arch/x86/kvm/svm/avic.c

@@ -324,18 +324,18 @@ int avic_incomplete_ipi_interception(struct kvm_vcpu *vcpu)
 	switch (id) {
 	case AVIC_IPI_FAILURE_INVALID_INT_TYPE:
 		/*
-		 * AVIC hardware handles the generation of
-		 * IPIs when the specified Message Type is Fixed
-		 * (also known as fixed delivery mode) and
-		 * the Trigger Mode is edge-triggered. The hardware
-		 * also supports self and broadcast delivery modes
-		 * specified via the Destination Shorthand(DSH)
-		 * field of the ICRL. Logical and physical APIC ID
-		 * formats are supported. All other IPI types cause
-		 * a #VMEXIT, which needs to emulated.
+		 * Emulate IPIs that are not handled by AVIC hardware, which
+		 * only virtualizes Fixed, Edge-Triggered INTRs.  The exit is
+		 * a trap, e.g. ICR holds the correct value and RIP has been
+		 * advanced, KVM is responsible only for emulating the IPI.
+		 * Sadly, hardware may sometimes leave the BUSY flag set, in
+		 * which case KVM needs to emulate the ICR write as well in
+		 * order to clear the BUSY flag.
 		 */
-		kvm_lapic_reg_write(apic, APIC_ICR2, icrh);
-		kvm_lapic_reg_write(apic, APIC_ICR, icrl);
+		if (icrl & APIC_ICR_BUSY)
+			kvm_apic_write_nodecode(vcpu, APIC_ICR);
+		else
+			kvm_apic_send_ipi(apic, icrl, icrh);
 		break;
 	case AVIC_IPI_FAILURE_TARGET_NOT_RUNNING:
 		/*