KVM: VMX: Invoke NMI handler via indirect call instead of INTn

Rework NMI VM-Exit handling to invoke the kernel handler by function
call instead of INTn.  INTn microcode is relatively expensive, and
aligning the IRQ and NMI handling will make it easier to update KVM
should some newfangled method for invoking the handlers come along.

Suggested-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200915191505.10355-3-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -6325,40 +6325,40 @@ static void vmx_apicv_post_state_restore(struct kvm_vcpu *vcpu)
 
 void vmx_do_interrupt_nmi_irqoff(unsigned long entry);
 
+static void handle_interrupt_nmi_irqoff(struct kvm_vcpu *vcpu, u32 intr_info)
+{
+	unsigned int vector = intr_info & INTR_INFO_VECTOR_MASK;
+	gate_desc *desc = (gate_desc *)host_idt_base + vector;
+
+	kvm_before_interrupt(vcpu);
+	vmx_do_interrupt_nmi_irqoff(gate_offset(desc));
+	kvm_after_interrupt(vcpu);
+}
+
 static void handle_exception_nmi_irqoff(struct vcpu_vmx *vmx)
 {
 	u32 intr_info = vmx_get_intr_info(&vmx->vcpu);
 
 	/* if exit due to PF check for async PF */
-	if (is_page_fault(intr_info)) {
+	if (is_page_fault(intr_info))
 		vmx->vcpu.arch.apf.host_apf_flags = kvm_read_and_reset_apf_flags();
 	/* Handle machine checks before interrupts are enabled */
-	} else if (is_machine_check(intr_info)) {
+	else if (is_machine_check(intr_info))
 		kvm_machine_check();
 	/* We need to handle NMIs before interrupts are enabled */
-	} else if (is_nmi(intr_info)) {
-		kvm_before_interrupt(&vmx->vcpu);
-		asm("int $2");
-		kvm_after_interrupt(&vmx->vcpu);
-	}
+	else if (is_nmi(intr_info))
+		handle_interrupt_nmi_irqoff(&vmx->vcpu, intr_info);
 }
 
 static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
 {
-	unsigned int vector;
-	gate_desc *desc;
 	u32 intr_info = vmx_get_intr_info(vcpu);
 
 	if (WARN_ONCE(!is_external_intr(intr_info),
 	    "KVM: unexpected VM-Exit interrupt info: 0x%x", intr_info))
 		return;
 
-	vector = intr_info & INTR_INFO_VECTOR_MASK;
-	desc = (gate_desc *)host_idt_base + vector;
-
-	kvm_before_interrupt(vcpu);
-	vmx_do_interrupt_nmi_irqoff(gate_offset(desc));
-	kvm_after_interrupt(vcpu);
+	handle_interrupt_nmi_irqoff(vcpu, intr_info);
 }
 
 static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)