KVM: nVMX: WARN on attempt to switch the currently loaded VMCS

WARN if KVM attempts to switch to the currently loaded VMCS.  Now that
nested_vmx_free_vcpu() doesn't blindly call vmx_switch_vmcs(), all paths
that lead to vmx_switch_vmcs() are implicitly guarded by guest vs. host
mode, e.g. KVM should never emulate VMX instructions when guest mode is
active, and nested_vmx_vmexit() should never be called when host mode is
active.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923184452.980-8-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -258,7 +258,7 @@ static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs)
 	struct loaded_vmcs *prev;
 	int cpu;
 
-	if (vmx->loaded_vmcs == vmcs)
+	if (WARN_ON_ONCE(vmx->loaded_vmcs == vmcs))
 		return;
 
 	cpu = get_cpu();