KVM: nVMX: fixes for preemption timer migration

Commit 850448f3 ("KVM: nVMX: Fix VMX preemption timer migration",
2020-06-01) accidentally broke nVMX live migration from older version
by changing the userspace ABI.  Restore it and, while at it, ensure
that vmx->nested.has_preemption_timer_deadline is always initialized
according to the KVM_STATE_VMX_PREEMPTION_TIMER_DEADLINE flag.

Cc: Makarand Sonare <makarandsonare@google.com>
Fixes: 850448f3 ("KVM: nVMX: Fix VMX preemption timer migration")
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -4339,14 +4339,15 @@ Errors:
 #define KVM_STATE_VMX_PREEMPTION_TIMER_DEADLINE 0x00000001
 
   struct kvm_vmx_nested_state_hdr {
-	__u32 flags;
 	__u64 vmxon_pa;
 	__u64 vmcs12_pa;
-	__u64 preemption_timer_deadline;
 
 	struct {
 		__u16 flags;
 	} smm;
+
+	__u32 flags;
+	__u64 preemption_timer_deadline;
   };
 
   struct kvm_vmx_nested_state_data {

arch/x86/include/uapi/asm/kvm.h

@@ -408,14 +408,15 @@ struct kvm_vmx_nested_state_data {
 };
 
 struct kvm_vmx_nested_state_hdr {
-	__u32 flags;
 	__u64 vmxon_pa;
 	__u64 vmcs12_pa;
-	__u64 preemption_timer_deadline;
 
 	struct {
 		__u16 flags;
 	} smm;
+
+	__u32 flags;
+	__u64 preemption_timer_deadline;
 };
 
 struct kvm_svm_nested_state_data {

arch/x86/kvm/vmx/nested.c

@@ -6176,6 +6176,7 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
 			goto error_guest_mode;
 	}
 
+	vmx->nested.has_preemption_timer_deadline = false;
 	if (kvm_state->hdr.vmx.flags & KVM_STATE_VMX_PREEMPTION_TIMER_DEADLINE) {
 		vmx->nested.has_preemption_timer_deadline = true;
 		vmx->nested.preemption_timer_deadline =