KVM: VMX: Add a helper for NMI handling

Add a helper to handles NMI exit.

TDX handles the NMI exit the same as VMX case.  Add a helper to share the
code with TDX, expose the helper in common.h.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Co-developed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-ID: <20250222014757.897978-15-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: Sean Christopherson

arch/x86/kvm/vmx/common.h

@@ -177,4 +177,6 @@ static inline void __vmx_deliver_posted_interrupt(struct kvm_vcpu *vcpu,
 	kvm_vcpu_trigger_posted_interrupt(vcpu, POSTED_INTR_VECTOR);
 }
 
+noinstr void vmx_handle_nmi(struct kvm_vcpu *vcpu);
+
 #endif /* __KVM_X86_VMX_COMMON_H */

arch/x86/kvm/vmx/vmx.c

@@ -7214,6 +7214,20 @@ static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu,
 	}
 }
 
+noinstr void vmx_handle_nmi(struct kvm_vcpu *vcpu)
+{
+	if ((u16)vmx_get_exit_reason(vcpu).basic != EXIT_REASON_EXCEPTION_NMI ||
+	    !is_nmi(vmx_get_intr_info(vcpu)))
+		return;
+
+	kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
+	if (cpu_feature_enabled(X86_FEATURE_FRED))
+		fred_entry_from_kvm(EVENT_TYPE_NMI, NMI_VECTOR);
+	else
+		vmx_do_nmi_irqoff();
+	kvm_after_interrupt(vcpu);
+}
+
 static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 					unsigned int flags)
 {
@@ -7257,15 +7271,7 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 	if (likely(!vmx_get_exit_reason(vcpu).failed_vmentry))
 		vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
 
-	if ((u16)vmx_get_exit_reason(vcpu).basic == EXIT_REASON_EXCEPTION_NMI &&
-	    is_nmi(vmx_get_intr_info(vcpu))) {
-		kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
-		if (cpu_feature_enabled(X86_FEATURE_FRED))
-			fred_entry_from_kvm(EVENT_TYPE_NMI, NMI_VECTOR);
-		else
-			vmx_do_nmi_irqoff();
-		kvm_after_interrupt(vcpu);
-	}
+	vmx_handle_nmi(vcpu);
 
 out:
 	guest_state_exit_irqoff();