KVM: VMX: Optimize handling of VM-Entry failures in vmx_vcpu_run()

Mark the VM-Fail, VM-Exit on VM-Enter, and #MC on VM-Enter paths as
'unlikely' so as to improve code generation so that it favors successful
VM-Enter.  The performance of successful VM-Enter is for more important,
irrespective of whether or not success is actually likely.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200410174703.1138-2-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -6724,11 +6724,16 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
 	vmx->nested.nested_run_pending = 0;
 	vmx->idt_vectoring_info = 0;
 
-	vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
-	if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
+	if (unlikely(vmx->fail)) {
+		vmx->exit_reason = 0xdead;
+		return;
+	}
+
+	vmx->exit_reason = vmcs_read32(VM_EXIT_REASON);
+	if (unlikely((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY))
 		kvm_machine_check();
 
-	if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
+	if (unlikely(vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
 		return;
 
 	vmx->loaded_vmcs->launched = 1;